HSC Chemistry Topic 1
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Andrew Nguyen
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HSC topic 1 Production Of Materials 1. Fossil fuels provide both energy and raw materials such as ethylene, for the production of other substances.
Construct word and balanced formulae equations of chemical reactions as they are encountered Basic reactions to remember:
1. Element+ oxygen element oxide 4Na (s) + O (g)
2NaO (s)
2. Element + hydrogen element hydride N (g) + 3H (g) 2NH (g) 3. Metallic Oxide + water NaO (s) + HO (l)
hydroxide
2NaOH (aq)
4. Non metallic oxide + water acid SO (g) + HO (l) HSO (aq) 5. Metal + water hydroxide + hydrogen gas 2K(s) + 2HO (l) 2KOH (aq) + H (g) 6. Metal + acid salt + water Mg (s) + 2HCl (aq) MgCl (aq) + H (g) 7. Acid + base salt +water HNO (aq) + NaOH (aq)
NaNO (aq) + HO (l)
8. Acid + Carbonate salt + carbon dioxide + water HCl (aq) + CaCO (s)
CaCl (aq) + CO (g) + HO (l)
9. Carbonate + heat oxide + carbon dioxide CaCO (s) + heat CaO (s) + CO (g) 10. Metallic oxide + heat
metal + oxygen
2HgO (s) +heat 2Hg (s) +O (g)
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11. Hydroxide +heat water + oxide Mg(OH) (g) + heat HO (l) + MgO (s) 12. Metallic Oxide + acid MnO (s) + 2HCl (aq)
salt + water
MnCl (aq) + HO (l)
13. Non metallic oxide + base salt + water CO (g) + 2 NaOH (aq) NaCO (aq) + HO (l) 14. Carbon dioxide + water+ UV rays glucose + oxygen 6CO + 6HO (l) +UV CHO(s) + 6O (g) Complete combustion: Hydrocarbon + oxygen
water + carbon dioxide
Displacement reactions:
Y + X (anion) y
X + Y (anion); where Y > X on activity series.
ene/alk ane ane reactions: Alk ene/a
± Cracking of pentane:
pentane
ethylene + propane
C5H12 (g)
C2H4 (g) + C3H8 (g)
± Hydrogenation of ethylene:
ethylene + hydrogen
C2H4 (g) + H2 (g)
ethane C2H6 (g)
± Hydration of ethylene:
ethylene + water
C2H4 (g) + H2O (l)
ethanol C2H5OH (l)
specifically, Chlorination) of ethylene : ± Halogenation (more specifically,
ethylene + chlorine
C2H4 (g) + Cl2 (g)
1,2-dichloroethane 1,2-dichloroethane C2H4Cl2 (l)
± Hydrohalogenation (more specifically, Hydrofluorination) of ethylene:
ethylene + hydrogen fluoride
fluoroethane
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11. Hydroxide +heat water + oxide Mg(OH) (g) + heat HO (l) + MgO (s) 12. Metallic Oxide + acid MnO (s) + 2HCl (aq)
salt + water
MnCl (aq) + HO (l)
13. Non metallic oxide + base salt + water CO (g) + 2 NaOH (aq) NaCO (aq) + HO (l) 14. Carbon dioxide + water+ UV rays glucose + oxygen 6CO + 6HO (l) +UV CHO(s) + 6O (g) Complete combustion: Hydrocarbon + oxygen
water + carbon dioxide
Displacement reactions:
Y + X (anion) y
X + Y (anion); where Y > X on activity series.
ene/alk ane ane reactions: Alk ene/a
± Cracking of pentane:
pentane
ethylene + propane
C5H12 (g)
C2H4 (g) + C3H8 (g)
± Hydrogenation of ethylene:
ethylene + hydrogen
C2H4 (g) + H2 (g)
ethane C2H6 (g)
± Hydration of ethylene:
ethylene + water
C2H4 (g) + H2O (l)
ethanol C2H5OH (l)
specifically, Chlorination) of ethylene : ± Halogenation (more specifically,
ethylene + chlorine
C2H4 (g) + Cl2 (g)
1,2-dichloroethane 1,2-dichloroethane C2H4Cl2 (l)
± Hydrohalogenation (more specifically, Hydrofluorination) of ethylene:
ethylene + hydrogen fluoride
fluoroethane
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±
y
C2H4 (g) + HFl (g)
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C2H5Fl (g)
Reaction of cyclohexene with
bromine water:
cyclohexene + bromine + water
C6H10 (l) + Br2 (aq) + H2O (l)
2-bromo-1-cyclohexanol 2-bromo-1-cyclohexanol + hydrogen bromide C6H10BrOH (l) + HBr (aq)
ermentation and other ethano l-based reactions: F ermentation
± Dehydration of ethanol:
ethanol
ethylene + water
C2H5OH (l)
C2H4 (g) + H2O (l)
± Combustion of ethanol:
ethanol + oxygen
C2H5OH (l) + 3O2 (g)
carbon dioxide + water 2CO2 (g) + 3H2O (g)
± Fermentation of glucose:
y
glucose
ethanol + carbon dioxide
C6H12O6 (aq)
2C2H5OH (aq) + 2CO2 (g)
lectrochemistry: ectrochemistry: E l
± Displacement of copper from solution due to zinc :
zinc + copper sulfate
Zn (s) + CuSO4 (aq)
zinc sulfate + copper ZnSO4 (aq) +
Cu (s)
± Ionic equation of this reaction:
zinc + copper(II) ion + sulfate ion
Zn
2+
2-
2+
+ Cu + SO4
Zn
zinc(II) ion + sulfate ion + copper 2-
+ SO4 + Cu
± Net ionic equation of this reaction:
zinc + copper(II) ion
Zn (s) +
2+
Cu
zinc(II) ion + copper 2+
Zn
(aq)
(aq)
+ Cu (s)
± Half-equations of this equation: 2+
Zn
Zn
Cu + 2e
2+
¯
¯
+ 2e
Cu
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Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum Petroleum (crude oil) is a complex mixture of hydrocarbons consisting mainly of alkanes and cycloalkanes, with smaller quantities of unsaturated hydrocarbons including alkenes. For the crude oil to be used as a fuel or as a raw material for the petrochemical industry, it must be refined. To refine the crude oil, the process used is called fractional distillation which is separated based on different boiling pts of the hydrocarbons. Ethylene (CH) is one of the most useful substances in the petrochemical industry. To obtain ethylene we must crack these larg e hydrocarbon chains formed from the fractional distillation of crude oil. Cracking is the process of brea king large hydrocarbon molecules into smaller length chains, using heat or a catalyst. Each fraction of the hydrocarbon has a different volatility, Boiling Pt and Mr. The proportions of different fractions obtained by the fractional distillation of petroleum usually do not match the demands of the market. There is greater demand for some fractions than for others. However, the ethylene produced by the crude oil is no t enough to match the global g lobal demand for the substance. Therefore other products formed from the fractional distillation of crude oil must be cracked to produce ethylene.
E.g. The cracking of pentane into ethylene and propane : Thermal cracking of petroleum fractions
This method of breaking down larger fractions of hydrocarbons is a non catalytic process in which a mixture of alkanes with steam is passed through very hot metal tubes (700-1000°C) and at just above atmospheric pressure to decompose the alkanes completely into small alkanes such as ethylene, propene and butene. Some hydrogen gas may be produced as well. E.g.1.
CH (g) 4CH (g) + CH (g) + H (g)
E.g.2.
CH (g) CH=CH (g) + H (g)
However, thermal cracking had many disadvantages. 1. It was very expensive because of the energy required to maintain these high temperatures. 2. It was also difficult to control the production of the r esultant products as there are many different places where the breaking of bonds could occur. Production Of Materials
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Catalytic cracking of petroleum fractions
This process involves the use of a catalyst to break down feedstock into smaller hydrocarbon molecules. The main catalysts for the catalytic cracking of petroleum fractions are a group of silicate minerals called zeolites. Zeolites are crystalline substances composed of aluminium, silicon and oxygen. The catalyst is usually in the form of a fine powder that is circulated with the feedstock in the catalytic cracker. Zeolite crystals have a three dimensional network structure containing a l arge number of tiny pores or channels. The reactant molecules are adsorbed in these pores where their reactions are catalysed. It is possible to synthesis zeolites with pores of different sizes to control the end products formed. Note : The zeolite crystal allows the reaction to occur at a much lower temperature range.
This image shows a portion of the zeolite catalyst.
Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products An alkane and an alkene with comparable molecular weights will have similar physical properties because both are non-polar substances and display the same type of intermolecular forces namely dispersion forces. Alkenes however are much more reactive than alkanes due to the presence of the covalent double bond in its structure. Ethylene is an alkene and can undergo additional reactions by opening out its double bond for two other atoms in a diatomic molecule to be added in.
There are many types of addition reactions :
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Hydrogenation: Hydrogen is reacted with ethylene, using a platinum catalyst at 150°C. The product is
ethane.
Hydration: Ethylene is reacted with water, using dilute phosphoric acid as a catalyst, to produce
ethanol. This is an industrially important reaction.
Halogenation: Reactive molecules from the halogen group (Fl 2, Cl 2 and Br2) can all react with ethylene.
EG: Chlorine molecule reacting with ethylene forms 1,2-dichloroethane.
Hydrohalogenation: In this reaction, a hydrohalogen (such as HCl or HFl) and ethylene react to form a
halo-ethane. EG: HFl reacting with ethylene forms fluoroethane.
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Identify that ethylene serves as a monomer from which polymers are made A monomer is a small versatile molecule which consists of a carbon double bond that is capable of linking up with other such molecules to form a macromolecule. A polymer is a natural occurring or synthetic compound consisting of large molecules made up of a linked series of repeated simple monomers. Polyethylene is an additional polymer because when its monomer (ethylene) goes under the process of hydrogenation, no additional molecules are produced (e.g. water) there is no gain or loss of electrons, the double bond just simply opens and monomers att ach.
From this diagram of polyethylene, we can clearly see that no additional molecules are produced. Outline the steps in the production of polyethylene as an example of a commercially and industrially important polymer Polyethylene is used both commercially and industrially. There are two methods for its production: High Pressure Method: In this process, ethylene is subjected to high pressures and temperatures of
300°C. A molecule, called the initiator, is introduced, usually a peroxide. The peroxide starts off a chain reaction, creating the polyethylene macromolecule. This process creates branched chains of polyethylene that cannot be packed together tightly. Thus branched polyethylene is called low-density polyethylene (LPDE). Ziegler- Natta Process: This process uses only a few at mospheres of pressure and temperatures of about
60°C. An ionic catalyst is used : it is a mixture of titanium (III) chloride and a trialkylaluminium compound. This process creates unbranched chains of polyethylene that can be packed together very densely. Thus unbranched polyethylene is called high-density polyethylene (HDPE).
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The steps taken to produce LDPE and HDPE are the same, but the initiator molecule is different. Initiation
The initiator molecule (peroxide radical- an oxygen compound with a free electron) is added to the ethylene container. The initiator reacts with one ethylene molecule, breaking its double bond, and attaches to only one bonding site, creating an ethylene- initiator RADICAL. The dot represents a free, highly reactive electron.
Propagation
The ethylene radical then attaches to another ethylene monomer, opening another bonding site, then another attaches, and so on. This rapidly increases the length of the chain.
Leading to:
Termination
The reaction terminates when two such chains collide and the two radicals react, forming a longer chain. This is a random process, so the length of polyethylene chains can vary greatly. Note : The peroxide initiator is engulfed by the reaction and is no longer present.
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Identify the following as commercially significant monomersby both their systematic and common names Common name: Vinyl Chloride Systematic name: Chloroethene Molecular formula: CHCl This molecule is produced by the substitution of hydrogen for a chlorine atom in an ethylene molecule. This can then undergo polymerisation to form a very important polymer known as polyvinyl chloride (polychloroethene):
Common name: Styrene
Systematic name: Phenylethylene
Molecular formula: CH Styrene is an ethylene molecule with one of its hydrog en atoms replaced by a benzene ring. Note: A benzene ring is a six- carbon ring with alternating double bonds. These double bonds within benzene are not reactive. This substance can undergo polymerisation to form polystyrene (polyphenylethene).
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Describe the uses of the polymers made from the above monomers in terms of their properties Polymer
Uses
Properties for the use
Low-density polyethylene
Cling wrap Disposable shopping bags Milk bottles
High-density polyethylene
Pipes to carry natural gas Rubbish bins Kitchen utensils
PVC
Garden hoses Pipes and guttering
Crystal Polystyrene
CD cases and cassette tapes Screw driver handles
Flexible, clear, non-toxic, permeable to O& CO but not water allow the food to be kept fresh and prevent it from drying out High chemical resistance thus HDPE can be moulded to hold petrol, oil, detergents, acids and solvents. It is rigid, slightly flexible and hard. Strong and non-toxic With the addition of certain additives, PVC may improve its flexibility and thermal stability. It is very rigid and hard, and unreactive Clear, hard, rigid, easily shaped and is a good insulator. Very durable and strong, hard and inflexible.
Expanded Polystyrene polystyrene foam aka Styrofoam is produced by blowing gas through liquid polystyrene until it froths up into a foam which is then allowed to cool down and solidify
Packaging and disposable cups Sound proofing
It is light (full of air), cheap, thermal insulator. It is a shock absorbent material, light, easily shaped
Factors affecting the properties of polymers 1. The length of the chain (number of monomer units) Plastics composed of longer polymer chains are stronger than those with shorter chains due to the greater amount of dispersion forces between them. 2. The arrangement of the chains- When the molecules are lined up and closely packed, they form crystalline regions resulting in a stronger, less flexible polymer. Amorphous (no distinct shape) regions in which the polymers have a random arrangement produce weaker and softer plastics. 3. The degree of branching- More branching restricts t he orderly packing arrangement and therefore reduces the density and hardness of the polymer but increases its flexibility. 4. The inclusion of additives- certain additives are included to improve or extend the properties of the polymer. Production Of Materials
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Practical:
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Identify data, pl an and perform a first -hand investigation to compare the reactivities of
appropriate alk enes with the corresponding a lk anes in bromine water AIM: To determine the difference between the reactivity of alkanes and alkenes .
MATERIALS: y
Bromine water
y
Pure cyclohexane
y
Pure cyclohexene
y
Molecular model kits
y
Test tubes
y
Measuring cylinders
y
Beakers
DISPOSAL: Place in an organic waste bottle, rinse the test tube and place rinse in an organic waste
bottle RISK ASSESSMENT: Chemical
Risk
Management
Bromine Water
Highly toxic if ingested and slightly corrosive
Use eye and skin protection Use in a fume cupboard and eye skin protection or use in
Cyclohexane
Toxic by all routes of exposure, highly flammable
small amounts in a well
and vapour moderately toxic
ventilated area Use in a fume cupboard and eye skin protection or use in
Cyclohexene
Toxic by all routes of exposure, highly flammable
small amounts in a well
and vapour moderately toxic
ventilated area
METHOD:
1. Place 3mL of Br water into 2 test tubes. 2. To one of the tubes add 3 drops of cyclohexane and shake the tube by flicking it briskly. 3. To the other tube, add 3 drops of cyclohexane and shake the tube by flicking it briskly. 4. Draw to diagram to illustrate observations. 5. Construct a model of cyclohexane and cyclohexene molecule, with a model kit. VARIABLES:
Controlled variables: y
Amount of Br water
y
Amount of Cyclohexane Production Of Materials
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o
y
Amount of Cyclohexene
y
Concentration of Br water
y
Temperature of surroundings
y
Air Pressure
y
Humidity of surroundings
y
Method of mixing chemicals (Flicking 3 times)
Dependent variable: y
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The amount of Br water required
Independent variable: y
The alkane and alkenes
IMPROVING VALIDITY: y
Repetition
y
Using a range of different Alkanes and Alkenes
Note: Cyclohexene and Cyclohexane were used, instead of ethylene or propene because C1 to C4 are gases at room temperature, and would be hard to manage; cyclohexene/ane are liquid at room temperature.
Also cyclohexene/ane was used instead of hexene/ane because cyclic hydrocarbons are more stable than their linear counterparts
IMPROVING RELIABILITY: y
Repetition
y
Improving measurements o
Using a pipette to measure out the amount of bromine water, rather then a measuring cylinder
RESULTS:
%URPLQH:DWHU&\FORKH[DQHZ%:&\FORKH[HQHZ%:
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DISCUSSION: Alkenes are more reactive than alkanes due to the presence of their double bond. Alkenes
go through what is called an addition reaction. It is where in an alkene; a double bond is broken and replaced with two new covalent bonds added across double bond where a foreign element/compound attaches itself. E.g. Ethene + Bromine
1, 2 dibromoethane
As mentioned above this is described as an addition reaction because extra elements are being added to the hydrocarbon and breaking the double bond. Alkanes go through a process called a substitution reaction. The substitution needs the presence of UV light to go through. An example is shown below : +++&O ,,89,, (J+&&+&O+&&++&O ,,,, ++++
When for example Ethane and Bromine undergo a sub stitution reaction the compound HBr is formed. This is due to one of the Br being substituted with an H on the hydrocarbon. Therefore Br 2 is broken into two, one atom goes and joins the hydrocarbon and the H which the Br replaces attaches on to the free Br to make HBr. CONCLUSION: Alkenes are more reactive than alkanes as seen in the results of this experiment due to
the presence of their double bond. Practical : Anal yse information from secondary sources such as
computer simul ations, mol ecul ar model
k its or mul timedia resources to model the pol ymerization process
± In this experiment, molecular modelling kits were used to show how polyethylene is produced through the polymerisation of ethylene.
± The class was divided into groups, and each group was provided with a kit. ± 3 ethylene monomers were created by each group, with bl ack balls representing carbons and smaller, white balls representing hydrogen.
± Then the monomers were polymerised : each group combined their monomers with every other group until a large chain was created a section of polyethylene.
±
JUSTIFY
the method:
The models created a 3D representation of the chemical process, which led to greater understanding of polymerisation.
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The use of ball-and-stick models, depicting the double-bond with flexible rubber rods, greater increased understanding of the process.
± LIMITATIONS of the method:
The model only provided a very limited section of a polyethylene molecule, as there were limited numbers of kits.
The use of catalysts (such as Zeigler-Natta catalysts) was not shown in the process, and thus it was not completely accurate.
2. Some scientists research the extraction of materials from biomass to reduce our dependence o n
fossil fuels
Discuss the need for alternative sources of the compounds presently obtained from the petr ochemical industry The petrochemical industry is one of the largest in the world, and provides fuels such as petrol, diesel and the raw materials used to make polymers. The source of petrochemicals is the fossil fuel petroleum (crude oil) or natural gas, which are non-renewable resources as they are being consumed at a far greater rate than produced. The demand for petroleum in recent times has increased, and will continue to increase due to the demands of developing countries such as China and India. Currently Australia has petroleum reserves that will last about ten years and natural gas reserves that will last about one hundred years. It will eventually run out, and if energy and material needs are to be met in the future, renewable alternatives need to be found. Renewable alternatives that have been sugg ested include biomass (organic matter produced by living things) : Vegetable oils as a possible substitute for diesel fuel, and ethanol (produced from the fermentation of sugars) as a possible substitute for petrol. There are also concerns that most polymers produced by the petrochemical industry are nonbiodegradable and cause environmental problems. Natural polymers are more likely to be biodegradable, and cause less problems. The main problem at present is the high cost of these alternatives. However, it is likely that as petrochemicals become scarcer and therefore more e xpensive; these alternatives will become more economically viable. Explain what is meant by a condensational polymer A condensational polymer is a high molecular height substance formed when many simple molecules (monomers) chemically joined by eliminating a small molecule usually water. E.g. Natural polymers such as cellulose, starch, protein, DNA, and manufactured polymer fabrics such as silk, polyester and nylon. Describe the reaction involved when a condensational polymer is formed Production Of Materials
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Condensational polymerisation involves a reaction between two different functional groups in which a water molecule (or some other small molecule) is eliminated and the two functional groups become linked together. Condensation polymerisation usually involves a reaction between two different monomers, but can occur where a molecule contains t wo different functional groups. Note: There is no double bond that opens (as in addition); the functional groups of the two monomers react together, forming a new bond and water. E.g. Cellulose Cellulose is a natural polymer formed through the polymerisation of glucose ß-Glucose (CHO) is the monomer for cellulose. It forms a beta 1,4-glycosidic bond which links each glucose molecule in the growing chain.
The reaction sites are the hydroxyl (OH) groups on the first and fourth carbons. Each glucose molecule has 2 reaction sites; that is why they can polymerise. One C-OH bonds to another C-OH, forming a C-O-C +
-
bond (glycosidic bond). The left over H and OH combine, forming water. Describe the structure of cellulose and identify it as an example of a condensation polymer found as a major component of biomass Cellulose is a natural occurring condensational biopolymer (a biopolymer). It is the most abundant polymer on Earth, making up to 50% of the total biomass of the planet (biomass is the mass of all organisms in a given area). There are two structural forms of glucose called alpha and beta glucose. It is the beta glucose that leads to cellulose formation. Ultimately up to 10000 glucose units will form the long, unbranched cellulose chain. Diagram of Beta Glucose
This alternating arrangement maintains a linear structure in the polymer. Cellulose is insoluble in water because its structure Production Of Materials
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exposes few OH groups to the water molecules in the environment.
Identify that cellulose contains the basic carbon-chain structures needed to build petrochemicals and discuss its potential as a raw material The basic carbon-chain structures that are used to make petrochemicals are short-chained alkenes such as ethylene (2C), propene (3C) and butene (4C). Glucose, the basic structure in cellulose, is a 6C molecule. Hence it has to potential to be transformed i nto the above compounds. The Potential of Cellulose as a Raw Material:
Although theoretically, cellulose can provide limitless amounts of renewable raw materials, this is currently too expensive and impractical. This is because in order to derive ethylene, etc., from cellulose, firstly, cellulose must be broken into glucose (using either bacterial digestion or acidic decomposition), then fermented (with yeast) into ethanol and then dehydrated (using H 2SO4) into ethene; this is a lengthy and expensive process. Hence, cellulose has great potential, but is currently not economical. Report: Use avai la bl e evidence to gather and present data from secondary sources and ana l yse progress
in the recent devel opment and use of a named biopo l ymer. This anal ysis shoul d name the specific enzyme used or organism used to synthesise the materia l and an eval uation of the use or potential use of the pol ymer produced rel ated to its properties Name of Biopolymer: Biopol
It is made of pol yhydroxybutyrate (PHB) and pol yhydroxyval erate (PHV). Organism Used:
Alcaligenes eutrophus (a bacterium).
Production:
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In industrial production, A. Eutrophus is grown in an environment favourabl e to its growth to create a very large population of bacteria (such as high nitrates, phosphates and other nutrients). When a sufficiently large population has been produced, the environment is changed to one that is high in gl ucose, high in valeric acid and low in nitrogen. This unnatural environment induces the production of the polymer by the bacterium; the polymer is actually a natural fat storage material, created by the A Eutrophus in adverse conditions. Large amounts of a chlorinated hydrocarbon, such as trichl oromethane are added to the bacteria/polymer mix; this dissolves the polymer. The mixture is then filtered to remove the bacteria. The polymer is extracted from the hydrocarbon solvent as a powder, which is then melted or treated further to create a usable polymer. Properties:
-It is BIODEGRADABLE and BIOCOMPATIBLE -It is non-toxic, insoluble in water, permeable to oxygen, resistant to UV light, acids and bases, high melting point, high tensile strength Uses in Relation to Properties:
-It has many medical applications (e.g. biocompatible stiches that dissolve or are absorbed by the body) -Disposable containers for shampoo, cosmetics, milk bottles, etc., as it only takes 2 years to decompose back into natural components -Disposable razors, cutlery, rubbish bags, plastic plates, etc. Advantages:
-It is biodegradable, unlike polyethylene and other petroleum derived plastics, and so will help to reduce levels of rubbish in land fills -It is compatible with organisms (biocompatible); it is not rejected by the bodys immune system and so can be used safely -It is a renewable resource
Disadvantages:
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-It is currently very expensive, and currently the demand is not high enough for it to be economically viable Future Developments:
-Recently, the gene for producing Biopol polymer strands from the Alcaligenes Eutrophus bacteria was extracted and implanted into E. coli using genetic engineering techniques. E. coli bacteria are much easier to grow than other bacteria, and thus are cheaper -Nutrient sources are starting to be derived from waste materials, such as molasses and other agricultural wastes. This greatly reduces costs. 3. Other resources, such as ethanol, are readily available from renewable resources such as plant s
Describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used Before ethylene became readily available through the catalytic cracking of petroleum fractions, it was mainly produced from ethanol. This involved heating ethanol vapour over a catalyst at 350°C. Ethanol
ethylene + water
CHOH
CH (G) + HO (L)
An acid catalyst (concentrated sulphuric acid) is needed because the acid breaks the C-OH and C-H bonds, allowing the formation of a double bond and water. It also reduces the activ ation energy. Describe the addition of water to ethylene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used The hydration of ethylene is the chemical process whereby a water molecule is added to ethylene, forming ethanol. Ethylene + water
ethanol
CH (G) + HO(L)
CHOH (L)
Note that a diluted acid is required to open the double bond, allowing water to attach to it, forming ethanol.
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Practical
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Process information from secondary sources such as mo l ecul ar model k its, digital
technol ogies or computer simu l ations to model the addition of water to ethy l ene and the dehydration of ethanol :
Molecular modelling kits were used to model : The addition of water to ethylene (hydration) and the removal of water from ethanol (dehydration) Ball-and-stick kits were used, where bl ack balls represented carbons, smaller, white balls representing hydrogen, and red balls represented oxygen. Firstly, and ethylene molecule was created, and a water molecule created: +
-
Then, the water molecule was split into a H ion and an OH ion The double-bond of ethylene was opened, and the ions were attached where there were free bonding sites; the resultant molecule was ethanol Secondly, a separate ethanol molecule was created : -
The hydroxide group (OH ) and a hydrogen was removed from the ends They were combines, and water was formed; the two open bonding sites of the ethanol were joined, and ethylene was formed. JUSTIFY
the method:
The models created a 3D representation of the chemical process, which led to greater understanding of dehydration and hydration processes. The use of ball-and-stick models, depicting the double-bond with flexible rubber rods, greater increased understanding of chemical reactions. LIMITATIONS of the method:
Both were severely simplified representations of chemical processes, which had many multiple steps and consisted of a series of aqueous (dilute sulfuric acid) or solid catalysts. Describe and account for many uses of ethanol as a solvent for polar and non-polar substances A POLAR covalent bond is a bond where one of the atoms in it is more electronegative than the other, and so the bond has a slight charge. Electronegativity is the ability of an atom to attract electrons; the more electro-negative an atom, the stronger it will hold onto electrons in a chemical bond .
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The order of electronegativity, from most electronegative to least, for relevant atoms is : f l uorine, oxygen, chl orine, nitrogen, carbon and then hydrogen.
For example, a bond between oxygen and hydrogen is a polar bond because oxygen holds onto negative electrons stronger; thus, in this bond, oxygen is slightly negative. Solubility Rules:
- Polar substances dissolve other polar substances : This is because the slightly negative end is attracted to the slightly positive end of another polar bond, forming a slight intermolecular bond. - Non-polar substances dissolve other non-polar substances : This is due to very weak dispersion forces between molecules. Ethanol is a very useful solvent. A range of substances including polar, non-polar and some ionic compounds dissolve readily in ethanol. The solubility of ethanol in both water (a polar compound) and hexane (a non-polar compound) are due to its m olecular structure. The ethanol molecule consists of two parts, the polar hydroxyl end (OH) and the non-polar alkyl end (CHCH-) end. The ability of ethanol to act as a solvent for polar substances is due to the polar nature of the O-H bond. This end of the ethanol molecule interacts with other polar molecules via dipole-dipole forces or hydrogen bonds. The alkyl chain, although short, is essentially non-polar and this allows ethanol to act as a solvent for some non-polar substances including some hydrocarbons, oils and resins. The non-polar alkyl chain forms dispersion forces with non-polar solutes, similar to the intermolecular forces between solute molecules. This tends to favour the solubility of non-polar solutes in ethanol.
Outline the use of ethanol as a fuel and explain why it can be called a renewable resource As supplies of petroleum dwindle, the use of renewable energy sources has become more attractive. Ethanol is one such renewable fuel that has received much attention. Ethanol can be produced from the starch or sugars present in sugar cane, corn, wheat, maize and other cereal crops. Although no commercially viable method of obtaining ethanol from cellulose is currently available, large-scale production by fermentation of starch and sugars has been carried out for decades. Ethanol is able to undergo combustion, so it can be used as a fuel : Production Of Materials
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CHOH (L) + 3O2(G)
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2CO (G) + 3HO (G)
Despite its short chain, ethanol is a liquid (due to strong polar bonds). Describe conditions under which fermentation of sugars is promoted Fermentation is the biochemical process in which glucose is turned into ethanol and carbon dioxide by the action of enzymes produced by microbes (esp. yeast). Glucose
ethanol + carbon dioxide
Yeast CHO (aq) 2CHOH (aq) + 2CO (G) Raw materials and conditions for fermentation
1. Source of glucose is needed e.g. suitable g rains and/ or fruits are mashed with water 2. Yeast is added 3. Exclude oxygen (anaerobic conditions) to avoid the oxidation of ethanol 4. Temp: body temperature = 37°C The max concentration of ethanol obtained directly from fermentation is 15% v/v. This is because as the concentration of ethanol is greater than 15% v/v, the yeast will be intoxicated. To obtain a higher concentration of ethanol, distillation is required. Research- Process information from secondary sources to summarise the p rocesses invol ved in the
industrial production of ethanol from sugar cane
Ethanol/Petrol Mixtures: Significant quantities of 10% ethanol are sold in some parts of Australia;
however, there has not been much success as the public holds suspicions about the effect of ethanol on their engines. However, in other countries, ethanol/petrol mixtures are very successful. In the United States, many states require a minimum of 10% ethanol in all fuel sold. In Sweden, 85% ethanol mixtures are common. Brazil requires that ALL car engines are able to accept at least 25% ethanol. Thus in certain countries, use of ethanol as a fuel is quite successful. Pure Ethanol Fuels: Pure ethanol is ethanol with AT MOST 1% water. It is a very clean fuel. Engines
must be modified to deal with such high levels of ethanol. It is currently being used in Brazil and Argentina as a complete alternative to gasoline. A quarter of all Brazilian cars run on pure ethanol. It has proven to be a very efficient fuel. Production Of Materials
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Ethanol Fuel-Cells: This is still in an experimental stage; it is the proposition that fuel cells be used to run
cars; success of such a scheme is still not known. Inf o- Present information from secondary sources by writing a bal anced equation for the fermentation of
gl ucose and monitor mass changes
Fermentation of glucose
Summarise the chemistry of the fermentation process Fermentation is composed of a series of chemical reactions whereby complex organic compounds are split into more simple ones. Cane sugar, such as molasses, is rich in sucrose (C 12H22O11), however, it is uneconomic to separate. Hence, water and yeast are added, which react with the sucrose to produce glucose and fructose, both of which have the molecular formula C 6H12O6.
The glucose/fructose can then be converted to carbon dioxide and yeast via the anaerobic respiration of yeast (fermentation). C6H12O6(s) 2 CO2(g) + 2 C2H5OH(l) Practical -Sol ve
probl ems, pl an and perform a first -hand investigation to carry out the fermentation of
gl ucose and monitor mass changes
In this experiment, sucrose solution was fermented to form ethanol and carbon dioxide. The yeast cells first split sucrose into two glucose molecules using the invertase enzyme.
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- A 250 mL side-arm conical flask with a rubber stopper was used. A plastic hose was connected to the side arm, and the end of the hose was placed in another conical flask in a solution of l imewater . No gas was allowed to escape the apparatus: - 100 ml of 0.15 M sucrose solution was placed in the conical flask. ONE gram of active yeast was placed into the sucrose, along with a pinch of sodium biphosphate (Na2HPO4) as a yeast nutrient. This was mixed thoroughly. - The stopper firmly put on, and the flask was WEIGHED with an electronic scale. The apparatus was then set up as shown, with the yeast beaker in a water bath at a constant temperature (37°C). Both flasks were weighed daily for 5 days. RESULTS:
1. The yeast flask turned foamy and smelt clearly of alcohol, while the limewater turned cloudy; this proved that CO2 and ethanol were produced, and that fermentation occurred. 2. The mass of the yeast flask also steadily decreased by about half a gram each day; this is due to the carbon dioxide escape; the limewater flask also gained approximately the same mass. JUSTIFY
the method:
1. A closed system (where no gas was allowed to escape) was used to ensure an accurate experiment. 2. Limewater was employed to prove CO 2 was produced. 3. The water bath ensured that the most optimal fermentation occurred.
LIMITATIONS of method:
- The combined masses of both flasks steadily decreased as well; this was due to inevitable leakages of gas. - The atmosphere in the flasks was not anaerobic (oxygen-free) and this could have hampered the fermentation process. Define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data
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The molar heat of combustion is the heat energy released when one mol e of a substance undergoes complete combustion with oxygen at a pressure of 101 .3 kPa (or 1 atmosphere), with the final products being CO2 and H2O Formula for change in heat :
For the calculation of the molar heat of combustion of ethanol, the following first hand values, were used (from 2001 HSC, Q17):
In this case, the formula for H is applied to the water (the system) : T = 59 19 = 40 K
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m = 250 g = 0.25 kg 3
C = 4.18 x 10 J kg Therefore:
-1
K
-1
(This value is a constant; given in exams) 3
H = -0.25 x 4.18 x 10 x 40 = -41800 joules = -41.8 kilojoules (kJ)
But the change in mass of the burner was 2.3 grams, therefore only 2.3 grams of ethanol was combusted. Moles = mass / molar mass = 2.3 / 46 = 0.05 mol Therefore, -41.8 kJ/0.05 mol = -836 kJ/mol Assess the potential of ethanol as an alternative fuel and discuss the advantages and disadvantages of its use Advantages of Use Renew abl e Resourc e The chemical energy in ethanol is produced in plants/biomass such as sugarcane,
corn and potatoes, via photosynthesis, and is therefore renewable. Plants can be renewed through agriculture as the ethanol becomes deplete: 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O
Therefore, unlike fossil fuels it will not run out. Greenhouse gas neutral The combustion of ethanol is considered Carbon Dioxide neutral if derived
from biomass. Unlike ethanol derived from petroleum, it does not take carbon dioxide locked deep underground, but from the atmosphere as part of photosynthesis by the plants. Therefore, use of ethanol as fuel would reduce carbon dioxide emissions compared with combusting fossil fuels. Lower Emissions Combusting ethanol produces lower levels of toxic hydrocarbons, carbon monoxide,
benzene, toluene and xylenes. Up to 26% less C O is released when compared with petrol. Improved human health Half burned hydrocarbons produced when combusting petrol in engines, e.g.
chrysene and Buckminster fullerene are carcinogenic, and have an adverse effect on human health. Ethanol produces less of such hydrocarbons than petrol. The reduction in CO emissions by up to 26% will reduce the pollution-related malaise common in urban areas where there a lots of motor vehicles. It is a simple, non-toxic and economically viable replacement as an octane-enhancer for MTBE, which is a toxic compound known to contaminate ground water and t herefore dangerous to human health. Disadvantages of Use
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Expense Ethanol is expensive to produce. The energy required to distil ethanol is half the energy
produced when ethanol is combusted. The energy and cost of fermentation and distillation are almost twice that of mining and refining petroleum. Despite rising oil prices and depletion of fossil fuels, and development of technology that reduces the cost of ethanol production, it still remains too impractical a prospect in terms of cost and energy to be taken up by fuel companies Large land area needed Large areas of arable land need to be cleared in order to grow the biomass
necessary for the production of ethanol. Although this appears less destructive than an oil field, such mass production of crop will result in a mono-culture that would be disastrous for the biodiversity in the region. The need for land could also result in the clearing of irreplaceable natural ecosystems, such as the clearing of tropical rainforests in Queensland. Potential for engine corrosion Ethanol absorbs water. Blends of ethanol that are a round 25% (E-25) or
higher absorb enough water to cause corrosion and damage of the fuel lines in engines. Benefits of emission reductions and effect on greenhouse gases are small Presently, much of the
energy used to power the growing, harvesting and processing of crop to produce ethanol is derived from fossil fuels, which makes the net reduction of carbon dioxide gas emissions much smaller. Increased levels nitrogen oxides in exhaust gases While combustion of ethanol reduces the amount of
carbon dioxide, monoxide and other toxic gases, it rel eases more nitrogen oxides than petrol, leading to an increase in smog and acid rain. Research-Process information from secondary sources to summarise the use of ethano l as an al ternative
car fuel , eval uating the success of current usage
It is a renewable resource and so would reduce the use of non-renewable fossil fuel (provided less fossil fuel was used to make the ethanol than was saved by using the ethanol in cars). It could reduce greenhouse gas emissions (if the amount of CO2 not released from oil because of the use of ethanol in cars was greater than the CO2 released from the fossil fuels used to make the ethanol). The disadvantages are: Large areas of agricultural land would need to be devoted to growing suitable crops with consequent environmental problems such as soil erosion, deforestation, fertiliser runoff and salinity. Disposal of the large amounts of smelly waste fermentation liquors after removal of ethanol would also present major environmental problems. The current situation in Australia:
1. Ethanol costs more than petrol to produce so the federal government has set up subsidies and excise concessions to encourage the production of ethanol (from crops) to be added to petrol (presumably to reduce oil consumption). 2. Car manufacturers accept that up to 10% ethanol in petrol has no detrimental effect on vehicles but have opposed higher concentrations. Production Of Materials
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3. Significant quantities of petrol with 10% ethanol in it are available in some parts of Australia, but there is considerable public suspicion of this blended fuel, largely because of incidents involving some petrol suppliers who put excessive amounts of ethanol in their petrol (to gain from the government subsidies), and because of car manufacturers who have widely claimed that amounts above 10% could damage car engines and may void warranties. 4. There are no reliable studies to show whether ethanol as made in Australia from wheat or molasses produces less greenhouse gas in total than does the petrol it replaces. Identify the IUPAC nomenclature for straight-chained alkanols from C1-C8 Alkanols are a group of alkanes where one or more hydrogens have been replaced by the hydroxy l (OH) functional group When naming alkanols, there are specific rules : The number of carbons determines the prefix of the name :
#Cs
1
2
3
4
5
6
7
8
Prefix
methane-
ethane-
propane-
butane-
pentane-
hexane-
hepane-
octane-
If there is only ONE hydroxyl group, the e is dropped from the prefix and the suffix -ol is added. The carbon the hydroxyl is on must also be stated; this is written before the prefix with a dash. The carbons, depending on how long the chain is, are numbered from 1 to 8. E.G. This alkanol has 5 carbons, but only one hydroxyl, so its prefix is pentan-, and its suffix is -ol. Also, the hydroxyl is on the 2
nd
carbon (the
number is taken either from the left O R the right; the SMALLER number must be taken). HENCE this alkanol is 2-pentanol. INCORRECT naming would be 4-pentanol.
If there is more than one hydroxyl group, the suffixes are (1-4) :
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No. of OHs
1
2
3
4
Suffix
-ol
-diol
-triol
-tetraol
For more than one carbon, the e at the end of the prefix is NOT dropped. The positions of the OH groups must be stated. If there are 2 hydroxyls on the same carbon, then the number is written twice, with a comma in between: E.G. This alkanol has 6 carbons, and 3 hydroxyl groups so its prefix is st
hexane- and its suffix is -triol. Also, one hydroxyl is on the 1 carbon, rd
while the other 2 are on the 3 carbon. HENCE, the IUPAC name for this alkanol is 1,3,3-hexanetriol. INCORRECT naming would be 4,4,6-hexanetriol Practical -
Identify data sources, choose resources and perform a first -hand investigation to determine
and compare heats of combustion of at l east three l iquid alk anol s per gram and per mol e
The 3 alkanols used were : methanol, ethanol, and 1-propanol Each alkanol was placed in a spirit burner; the original mass recorded, and then was used to heat 200 mL of water (at 25°C) in a tin can. A thermometer was used to stir the water as well as m easure the temperature Once the temperature rose by 10 degrees (Kelvin or Celsius, it doesnt matter; they both use the same scale) the spirit burner was capped and imm ediately reweighed. RESULTS:
- The H (change in heat) was calculated for each alkanol by using the formula H = -mCT. This was then calculated per gram, and then per mole, to give the heat of combustion per gram, as well as the molar heat of combustion. - Methanol has the lowest value, followed by ethanol, and then 1-propanol. NOTE: This is not because of the extra bonds in longer hydrocarbon chains, but rather more bonds need to be created in the products (H2O and CO2); recall that: - creating bonds releases energy. - breaking bonds absorbs energy.
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JUSTIFY
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the method:
- A tin can was used as it is a better thermal conductor than a glass beaker - Methanol, ethanol and 1-propanol were used as they are the shortest alkanols and thus are the most likely to undergo complete combustion LIMITATIONS of method:
- Molar heat of combustion refers ONLY to complete combustion; the yellow flames and soot formed indicated that the combustion was incomplete. Thus the experimental data gathered is inaccurate. - Also, much heat was lost to the air, as there was not 100% efficiency of heat transfer from flame to tin can. Heat was also radiated from the can to the air; insulation would reduce this. 4. Oxidation reduction reactions are increasingly important as a source of energy
Explain the displacement of metals from the solution in terms of transfer of electrons A displacement reaction is a reduction-oxidation reaction in which a more reactive product changes a less reactive products ions into solid or gaseous atoms . In other words, the less reactive products ions are displaced out of a solution and neutralised into atoms. E.g. the reaction between zinc and copper sulphate is an example of an oxidation-reduction reaction. Chemical equation- Zn(s) + CuSO (aq) ZnSo (aq) + Cu (s) +
+
Ionic equation Zn + Cu² + SO² Zn² + SO² + Cu Net ionic equation- Zn + Cu²
+
+
Zn²
+ Cu
These equations symbolise the transfer of electrons, where one substance donates electrons to another. Half equations These half equations is the net ionic equation split into two halves. E.g. From above Zn Zn²
+ 2e Oxidation
Cu² + 2e Cu
Reduction
Oxidation represents the loss of electrons and reduction represents the gain of electrons. Production Of Materials
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Oxidation
Reduction
Said to be oxidised
Said to be reduced
Is the reducing agent
Is the oxidising agent
Is the reductant
Is the oxidant
Loses electrons
Gains electrons
Increase in oxidation state/number
Decrease in oxidation number/state
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Note: OIL-RIG, where Oxidation Is Loss, Reduction Is Gain There are many types of redox reactions: Displacement reactions; e.g. magnesium and silver nitrate :
Mg (s) + 2AgNO3 (aq) +
Mg(NO3)2 (aq) + 2Ag (s)
-
2+
Mg + 2Ag + 2NO3 +
2+
Mg + 2Ag
-
Mg + 2Ag (2NO3 is the spectator ion) 2+
Mg
-
Mg + 2NO3 + 2Ag
¯
Mg + 2e (Oxidation) +
¯
2Ag + 2e
2Ag (Reduction)
Acid/Metal reactions; e.g. sulfuric acid and zinc :
H2SO4 (aq) + Zn (s) +
ZnSO4 (aq)
2-
2+
2H + SO4 + Zn +
2+
2H + Zn
Zn
Zn
Zn +
Zn
¯
2H + 2e
2+
+ H2 (g) 2-
+ SO4 + H2 2-
+ H2 (SO4 is the spectator ion)
¯
+ 2e (Oxidation) H2 (Reduction)
Metal/Non-metal reactions; e.g. reacting sodium and chlorine gas :
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2Na (s) + Cl2 (g)
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2NaCl (s) +
2Na + Cl 2
-
2Na + 2Cl (No spectator ion in this case) +
2Na
¯
2Na + 2e (Oxidation) ¯
-
Cl2 + 2e
2Cl (Reduction)
Alkali metal/water reactions; e.g. reacting potassium and water :
2K (s) + 2H2O (l)
2KOH (aq) + H2 (g) +
2K + 2H2O +
2K
-
2K + 2OH + H2 ¯
2K + 2e (Oxidation)
2H2O + 2e
¯
-
2OH + H2 (Reduction)
This is slightly different as there are TWO elements in the reduction half. Metal combustion reactions; e.g. burning magnesium in oxygen :
2Mg (s) + O2 (g)
2MgO (s) 2+
2Mg + O2
2+
2Mg
2-
2Mg + 2O ¯
2Mg + 2e (Oxidation) ¯
O2 + 2e
2-
2O (Reduction)
Identify the relationship between displacement of metal ions in solution by other metals to the relative activity of metals Metals were ranked in order of reactivity by comparing their reactivities with oxygen, water and acids. They can also be arranged into an activity series by their ability to displace one another from solution. If a strip of copper is placed in a solution of zinc sulphate, no reaction occurs. This is because Zn² ions have a lesser tendency to gain electrons (reduced) than Cupper ions. As a result, Cu atoms will not giv e up electrons to Zn² ions. However, if a piece of copper is placed in a solution of silver ions, the copper metal displaces silver ions from the solution. Therefore Ag² ions have a greater tendency to gain electrons than Cu² ions.
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These experiments allow us to arrange these three me tal ions in increasing order of ease to be reduced : Zn²
< Cu² < Ag.
Or we can arrange them in order of the tendency to be oxidised : Zn > Cu > Ag The metal activity series
Note: reactive metals will tend to be oxidised, the unreactive metals ions will tend to be reduced. Account for the changes in the oxidation state of species in terms of their loss or gain of electrons Rules of oxidation number
1. O.N of all elements in free forms (uncombined forms) is zero E.g. Ca, H, O, F, Na, Fe 2. O.N of fluorine in all compounds/polyatomic ions is -1 E.g. NaF¹ 3. O.N of oxygen in most compounds/ polyatomic ions is -2 E.g. KCO² 4. O.N of hydrogen is +1 in most compounds (except when it reacts with reactive metals from group 1 or 2) E.g. HCl 5. O.N of the alkaline metals (group 1) is +1 O.N of the alkaline Earth metals (group 2) is +2 6. The total sum of the O.N in a compound is zero 7. The total sum of the O.N in a polyatomic ion is the charge it carries 8. The O.N of a monatomic ion is the charge of that ion The oxidation state provides a useful way of determining whether an oxidation-reduction reaction has taken place. An increase in oxidation number represents oxidation whilst a decrease in oxi dation number represents a reduction. Production Of Materials
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Relating this back to the metal Zn
Zn 2+
¯
Cu + 2e
2+
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displacement reactions:
¯
+ 2e (Oxidation) Cu (Reduction)
It can be seen that the oxidation state of zinc changes from (0 +2); this increase signifies an oxidation, while the oxidation state of copper changes from (+2 0); this decrease shows it is a reduction. Thus, relating this back to transfer of electrons: - An increase in oxidation number means that electrons have been lost, and the oxidation number is increasing (moving towards the positive numbers) due to the loss of negative electrons. - A decrease in oxidation number means that electrons have been gained, and the oxidation number is decreasing (moving towards the negative numbers) due to a gain of negative electrons. Describe and explain galvanic cells in terms of oxidation/reduction reactions A galvanic cell is a device or apparatus that converts the chemical energy of a spontaneous redox reaction into electrical energy. Electricity is simply a flow of electrons. Thus redox reactions are electrontransfer reactions; if this electron flow can be exploited, electricity could be produced. There are two half cells; oxidation takes place in one and reduction takes place in the other. A conducting wire and salt bridge connects the two half-cells and completes the circuit; as electrons have to flow from the oxidation cell to the reduction cell, a flow of electrons is produced in the wire, and hence electricity is produced. Outline the construction of galvanic cells and trace the direction of electron fl ow A galvanic cell consists of two half-cells. Each half-cell consists of an electrode, which is a conductive metal or graphite strip in contact with an electrolytic solution. These solutions are joined by a salt bridge containing an electrolytic solution such as KNO. The salt bridge may consist of a filter paper saturated with KNO solution or a U-tube containing KNO solution in an agar jelly. The salt bridge completes the circuit and allows ions to move between each half cell. Galvanic cell setup : There are two cells, each containing a solution of the metal-sulfate; one cell contains zinc sul fate, the other cell copper sul fate. In the zinc sulfate, there is a solid zinc electrode, connected by a wire to a solid copper electrode, which is in the copper sulfate solution. A salt bridge, soaked in potassium nitrate
solution, connects the two cells. The chemical reactions :
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In the zinc-sulfate cell, oxidation is occurring, as SOLID zinc is oxidised to zinc IONS, which then flow into the zinc sulfate solution. The electrons that are released flow into the wire : Zn
Zn
2+
¯
+ 2e (Oxidation)
In the copper-sulfate cell, reduction is occurring, as copper IONS are reduced to SOLID copper, when then build up on the copper electrode. Electrons are received through the wire, which then reduce the 2+
¯
ions: Cu + 2e
Cu (Reduction)
NOTE: The oxidation and reduction cells can be on the left O R the right, it does not matter, although
oxidation is conventionally on the right. As the zinc is slowly oxidised, and more zinc ions build-up, the zinc sulfate solution builds up in POSITIVE 2+
2-
charge (more Zn than SO4 ). Similarly, as the copper ions are reduced, the copper sulfate solution builds in NEGATIVE charge (more 2-
2+
SO4 than Cu ). However, this will affect the flow of electrons; electrically neutral solutions are needed for optimal electricity production. Hence the role of the salt bridge : - The salt bridge completes the circuit, but also has another function. - The salt bridge maintains electrical neutrality; this means that it keeps the charges in both the halfcells at zero, by allowing the flow of ions. - The salt bridge is soaked in potassium nitrate solution: Thus, as the positive charge builds up in the left cell, NEGATIVE nitrate ions migrate towards the cell to neutralise the charge; as the negative charge builds up in the right cell, the POSITIVE potassium ions move towards the cell to neu tralise it as well.
This type of galvanic cell is called a Daniell cell.
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Define the terms anode, cathode, electrode and electrolyte to describe galvanic cells Electrode- anything through which electric current passes; in the context of galvanic cells, they are the metal conductors placed in the electrolytes (solution) Types of electrodes 1. Metal-metal electrodes Cu(s)l Cu²ll Agl Ag(s) 2. Metal-Inert electrodes Cu(s) l Cu² ll Fe²(aq) l Pt 3. Inert-Inert electrodes Pt l Cl(g), Cl ll Fe²(aq), Fe³(ag) l Pt Electrolyte- a substance which dissociates in solution to form ions, and i s therefore electrically conductive Anode- the negative electrode in which electrons come up from the cell and oxidation occurs (loses electrons) Cathode- the positive electrode in which electrons enter the cell and reduction occurs (gains electrons)
Practical -
Perform a first -hand investigation to identify the conditions under which a gal vanic cell is
produced
A galvanic cell was produced in the lab in the same set-up as above; that is, in the form of a DANIELL CELL Production Of Materials
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A 2 cm strip of zinc and copper were cut from metal strips. Using wire leads and crocodile clips, the zinc strip was connected to the NEGATIVE terminal of a voltmeter, and the copper strip connected to the POSITIVE terminal The zinc was then placed in 50 mL of 1M solution of zinc sulfate, and the copper in 50 mL of 1M solution of copper sulfate. A strip of filter paper was soaked in potassium nitrate; the two cells were then connected using this salt bridge RESULTS: The voltmeter showed a reading of 0.4 volts. When more electrolyte was added, the voltage
stayed at 0.4 volts; thus the voltage is determined ONLY by the metals used, and has nothing to do with the amount of copper or zinc. JUSTIFY
the method:
- Copper and zinc were used as they are readily available, non-toxic metals - 1M solution was used as the ratio of moles of the salts was 1 :1 - A potassium nitrate salt bridge was used as potassium and nitrate ions do not react with zinc, copper of sulfate ions. Practical -
Perform a first -hand investigation and gather first -hand information to measure the difference
in potential of different combinations of meta l s in an el ectrol yte sol ution Report - Gather and present information on the structure and chemistry of a dry ce ll or l ead -acid cell and
eval uate it in comparison to one of the fo ll owing: -button cell , fuel cell , vanadium redox ce ll , l ithium cell , l iquid junction photovol taic device ( E. g. the Gratzel cell) in terms of chemistry, cost and practical ity,
impact on society and environmental impact
Dry cells and lead-acid batteries are extremely useful and cost effective sources of portable power. The DRY CELL (or Leclanché Cell) :
STRUCTURE: The dry cell is made of a carbon rod surrounded by a mixture of manganese (IV) oxide and carbon (in the form of graphite); this is the cathode. This is then surrounded by a paste of ammonium chloride which acts as the electrolyte. All of this is contained in a zinc shell, which is the anode. CHEMISTRY:
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The complete chemical equation is shown as: Zn (s)
+ 2MnO2 (s) + 2NH4Cl (aq)
ZnCl2 (aq)
+ Mn2O3 (s) + 2NH3 (aq) + H2O (l)
The ionic equation is: Zn + 2Mn
4+
2-
+
+ 2O + 2NH4 + 2Cl
-
2+
Zn
+
-
3+
2-
+ 2Cl + 2Mn + 3O + 2NH3 + H2O +
But there is also another reaction: 2NH4
2NH3 + 2H -
This occurs on the left hand side; hence 2NH 3 and 2Cl are spectator ions: Thus the net ionic equation is: Zn + 2Mn
4+
2-
+ 2O + 2H
+
Zn
2+
3+
2-
+ 2Mn + 3O + H2O
Splitting this into redox half-cells : OXIDATION: Zn REDUCTION: Zn
Zn 4+
2+
¯
+ 2e
2-
+
¯
2Mn + 2O + 2H + 2e
3+
2-
2Mn + 3O + H2O
is oxidised: (0 +2) and Mn is reduced : (+4 +3).
The SIMPLIFIED galvanic cell representation is : 2+
4+
3+
[ Zn | Zn || Mn | Mn ] COST and PRACTICALITY: The dry cell is relatively cheap It is very practical to manufacture as there is little wastage of materials; all the materials (including the casing) take part in the reaction It is light and portable, and can be used in small appliances +
It has a short shelf-life however, as the acidic NH 4 slowly corrodes the zinc. IMPACT on SOCIETY : It was the first commercially produced battery, and so it had a HUGE impact on society; it made portable devices such as torches, radios and clocks possible It is very widely used, as it is a cheap, portable source of steady electricity
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ENVIRONMENTAL IMPACT: Very minimal environmental impact The manganese(III) product is readily oxidised back to manganese(IV), which is stable and insoluble; it is harmless. Ammonium salts and carbon are also harmless; smal l quantities of zinc pose no environmental risk. The Silver-Oxide BUTTON CELL:
STRUCTURE: The silver-oxide button-cell is made of layers of chemicals within a steel case. There is powdered silver(I) oxide at the bottom; this is the cathode. At the top is powdered zinc; this is the anode. They are separated by a paste of alkaline potassium hydroxide which acts as the electrolyte and a catalyst. The steel case does not take part in the reaction.
CHEMISTRY:
The overall chemical equation is: Zn (s)
+ Ag2O (s)
ZnO (s)
+ 2Ag (s)
However, this ignores the role of the potassium hydroxide; a more complete chemical equation (with aqueous KOH as catalyst) is : Zn (s)
+ Ag2O (s) + 2KOH (aq) + H2O (l)
ZnO (s)
+ 2Ag (s) + 2KOH (aq) + H2O (l)
The ionic equation is: Zn +
+
2-
+
-
2Ag + O + 2K + 2OH + H2O
2+
Zn
2-
+
-
+ O + 2Ag + 2K + 2OH + H2O
-
NOTE: The water and OH need to be shown for the half-equations. +
The net ionic equation (without K ions) is: Production Of Materials
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Zn +
+
2-
-
2+
2Ag + O + 2OH + H2O
Zn
2-
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-
+ O + 2Ag + 2OH + H2O
Splitting this into redox half-cells: OXIDATION: Zn + 2OH OR: Zn + 2OH REDUCTION:
-
Zn
ZnO +
2-
¯
2+
2-
+ O + H2O + 2e
+ H2O + 2e
2Ag + O + H2O + 2e
OR: Ag2O + H2O + 2e Zn
-
¯
¯
¯
2Ag + 2OH
2Ag + 2OH
-
-
is oxidised: (0 +2) and Ag is reduced: (+1 0).
THUS, KOH takes part in the equation as a catalyst, by splitting into its ions, but at the end of the reaction, the ions reform. The SIMPLIFIED galvanic cell representation is : 2+
+
[ Zn | Zn || Ag | Ag ] COST and PRACTICALITY:
Silver oxide button cells are very expensive, due to the high cost of silver However, even small cells are able to provide large amounts of electricity with a very constant voltage for a long period of time; so the benefits balance out the costs of the silver It is non-rechargeable The steel case does not take part in the reaction, and there is very little chance of leakage. IMPACT on SOCIETY:
The very small size of the cell, and high constant voltage allows it to have many applications, such as in wristwatches (where they last for many years), calculators and digital cameras. ENVIRONMENTAL IMPACT:
Environmental impact is minimal The zinc, zinc oxide, si l ver , and si l ver oxide are all stable, insoluble and non-toxic compounds. Potassium hydroxide is strongly alkaline; however, it is in quite a dilute form as an electrolyte, and small
amounts are not harmful.
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EVALUATING the dry-cell in relation to the button cell :
- In terms of chemistry, the dry-cell and the button cell have very similar reactions; however, the button cell is able to produce larger and more constant voltage supply in relation to size; this is because silver has a higher reduction potential than manganese(IV). Also the alkaline environment of the button cell encourages the reaction to occur at a constant rate, for long periods. - In terms of cost and practicality, the dry cell is more practical in most situations as it is cheaper. However, some situations require smaller cells that are able to last much longer than the short life of the dry cell, such as in watches. Also, the button cell is much less likely to leak, and thus is a more reliable battery. - In terms of impact on society, the dry cell has had a far greater impact than the button cell. This is due to the historical significance of the Leclanché cell, the first battery ever produced. The dry cell made portable electrical devices possible; however, the button cell has allowed the size of electrical devices to reduce greatly, as some devices these days are even smaller than dry cells. - Both cells have very minimal environmental impact, and hence are advisable to be used over other more polluting cells, such as mercury cells. Sol ve probl ems and ana l yse information to ca l cul ate the potential
E°
requirement of named
el ectrochemical processes using tab l es of standard potentia l s and hal f -equations
The potential difference (or voltage or E.M.F (electromotive force)) of a galvanic cell (or electrochemical cell) is the sum of oxidation and reduction potential. The voltage of a Galvanic cell can be measured but the individual oxidation or reduction potential cannot be measured. The cell potential can be determined by finding the difference in reduction potentials of the couples involved in the two half-cell equations. The following example shows how the cell potential E º of the Zn | Zn2+ || Ag+ | Ag cell is calculated. Consider Ag (aq) + e Ag(s) E º = +0.80 V Zn²
(aq) + 2e Zn(s) E º = 0.76 V
Select the half-cell reaction with the most positive E º, and write the half-equation and E º. This halfreaction is the reduction reaction. Ag (aq) + e Ag(s)
E º = +0.80
V
Reverse the direction of the other half-cell reaction and change the sig n of the E º. This half-reaction is
the oxidation reaction.
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Zn(s) Zn² (aq) + 2e E º = +0.76 V
Calculate the cell voltage by adding the half-cell potentials. E º (cell)
= 0.80 + 0.76 = 1.56 V
The cell potential E º (e.m.f.) under standard conditions is 1.56 V. To determine the overall cell reaction, multiply the half-cell equations by factors that balance the electrons in the two equations, and add the resulting equations. 2Ag (aq) + 2e 2Ag(s) Zn(s) Zn² (aq) + 2e Zn(s) + 2Ag(aq) Zn2(aq)
+ 2Ag(s)
Note that although the half-equations may have to be multiplied by factors to balance the number of electrons, the E º values are not multiplied. As the cell operates, the Ag+ concentration decreases and the Zn2+ concentration increases. Gradually the reaction tends to equilibrium and the cell E.M.F. falls, finally reaching zero. A higher E.M.F. than 1.56 V could be obtained by using conditions that favour the forward reaction. This could be achieved by having a Ag+ concentration greater than 1 mol L¹, or a Zn² concentration less than 1 mol L¹, or both. This would increase the tendency for the forward reaction to take place and therefore the E.M.F. would be greater. 5. Nuclear chemistry provides a range of materials Reminder
Isotopes are atoms of the same element with different numbers of NEUT RONS; but they have the same number of P ROTONS In nucl ear chemistry (chemistry dealing with nuclear reactions), isotopes are shown in the following form:
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E.G. chlorine-35 is written as Cl and rubidium-85 as
P a g e | 42 85
Rb
There are three types of radiation : (alpha), (beta) and (gamma) radiation: Alpha Decay: Alpha radiation is made of helium nuclei (2 protons and 2 neutrons) that are ejected
from unstable large nuclei (too heavy). For example, the decay of uranium-238:
Beta Decay: Beta radiation is made up of electrons ejected from an unstable nucleus (too many
neutrons); BUT nuclei do not contain electrons. Hence, the underlying reaction is the decomposition of a neutron:
When a neutron decomposes, it forms an electron, wh ich is immediately ejected as beta radiation, and a proton (hydrogen nucleus), which is captured by the nucleus. Thus, beta decay results in an increase in atomic number (IMPORTANT). For example, the beta decay of cobalt-60 results in an increase of atomic number, creating nicklel-60:
There is no such thing as gamma decay; gamma radiation, high energy electromagnetic waves, are emitted in addition to some beta or alpha decays
Name
Identity
Charge
Penetrating power
Ionising power
Magnetic field
Alpha(He)
Helium nucleus
positive
low
high
deflected
Beta (-e)
electron
negative
intermediate
intermediate
deflected
Gamma (Y)
EM radiation
neutral
High
Low
Not deflected
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Distinguish between stable and radioactive isotopes and describe the conditions under which a nucleus is unstable Many naturally occurring nuclei are unstable and will spontaneously emit radiation. All nuclei with an atomic number greater than 83 are unstable and radioactive, and most of the lighter elements have some isotopes that are unstable to some degree. The stability of the nucleus depends on many factors. 1. The neutron-proton ratio. Elements with an atomic number less than 20 tend to have approximately equal numbers of protons and neutrons, and the neutron-proton ratio is close to one. As the atomic number increases, the neutron-proton ratio wil l tend to increase thus the ratio will be greater than one. Note- n of neutrons > n protons after Z=20.
2. All atoms with an atomic number/ protons greater than 83 are very heavy and are unstable. Bismuth is the heaviest stable nuclide. Nuclides with
Z
(atomic number) > 83 tend to emit an
particle, thus reducing the numbers of protons and neutrons by two. Describe how transuranic elements are produced The transuranic (transuranium) elements are those following uranium in the periodic table. B ecause uranium is the heaviest naturally occurring element on Earth, transuranic elements must be produced synthetically. They are produced in a nuclear reactor or a particle accelerator by bombarding the target nuclei with neutrons or with nuclei of the atoms of another element. Transuranic elements are produced in two ways Neutron Bombardment (in nuclear reactors) : In nuclear reactions, the fission chain-reaction (of
uranium-235 or other elements) produces large amounts of neutrons. When atoms are placed inside the reactor, they are bombarded by these neutrons. Occasionally the atom A BSORBS one of these neutrons; Production Of Materials
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however, it is unstable, and undergoes beta decay (see above). Hence the proton number increases, and a transuranic element can be created. EG: Uranium-238 is not fissile (it cannot undergo the nuclear chain-reaction); when it is placed in the reactor, the following reaction occurs:
Thus, neptunium-239, the first transuranic element, is formed. This method is used to produce the first few of the transuranic elements. Fusion Reactions (in particle accelerators) : The production of larger transuranic elements is achieved by
colliding heavy nuclei with high-speed positive particles (such as helium or carbon nuclei). The positive particles need to be at very high speeds to overcome the positive repulsive force of the heavy nuclei and fuse with them. Particle accelerators are used to bring these particles to the high speeds required. EG: Uranium-238 is fused with a carbon nucleus:
As a result, californium-246, a large transuranic element, is formed. Inf ormati on-Process information from secondary sources to describe recent discoveries of e l ements
Four new elements have been discovered in the 21
st
century. They are listed below. Note that their
strange names are just temporary until the IUPAC decides on permanent names : Ununhexium: Also known as eka-polonium, element 116 was synthesised in December, 2000, by the Joint Institute for Nuclear Research (Dubna, Russia). It was produced through the fusion of curium-248 and calcium-48. The atom decayed 48 milliseconds later. Ununpentium: Also known as eka-bismuth, element 115 was synthesised in February, 2004, by the scientists from the Joint Institute for Nuclear
Research
(Dubna,
Russia)
and the Lawrence Livermore
National Laboratory (America). It was produced through the fusion of americium-243 and calcium-48. The atom then underwent ALPHA decay, forming element 113, a new element Ununtrium: Also known as eka-thallium, element 113 was also synthesised in February, 2004, through the alpha decay of ununpentium. Ununoctium: Also known as eka-radon, element 118 is the most recently produced, and the heaviest element known to man. It was produced by the fusion of californium-249 atoms and calcium-48 Production Of Materials
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Describe how commercial radioisotopes are produced Bombardment with charged particles
A radioisotope can be synthesised by bombarding the targ et nucleus with charged particles, usually the nuclei of other elements such as helium, boron, carbon, oxygen and neon. To overcome the electrostatic repulsion between these positively charged particles and the target nucleus, they must first be accelerated to very high velocities. This can be achieved in a particle accelerator, also known as a positive ion accelerator. By using alternating electric and magnetic fields, these accelerators produce the velocities required for the particles to penetrate the targ et nucleus. Examples of radioisotopes produced from the bombardment of a target nucleus with charged particles include the formation of technetium99, nitrogen-13 and phosphorus-30. Technetium does not occur naturally on Earth but can be synthesised by the bombardment of molybdenum-98 by hydrogen-2 nuclei. Mo + ²H Tc + ¹n The radioisotope nitrogen-13 is produced when carbon-12 is bombarded with protons. ¹²C + ¹H ¹³N Bombarding aluminium-27 with alpha particles produces phosphorus-30 ²Al + He ³ P + ¹n Bombardment with neutrons
Many of the artificial radioisotopes used in industry, m edicine and research are produced by neutroninduced transmutation. Since neutrons are uncharged, they do not experience the strong electrostatic repulsive forces associated with bombardment by positively charged particles. Thus neutrons are more readily absorbed by the target nucleus without the ne ed for them to be accelerated. The source of neutrons for these reactions is a nuclear fissi on reactor. Cobalt-60 is an important medical radioisotope which is produced in three steps. This begins with the bombardment of iron-58 with neutrons to form iron iron-59, followed by the emission of a particle to form cobalt-59. This then absorbs a neutron to form cobalt-60. Fe +¹n Fe Fe Co + e Co + ¹n Co
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Identify instruments and processes that can be used to detect radiation Photographic film
It is used for a routine check-up on the level of radiation at work places. The darkening of photographic film indicated radiation. The degree of darkening increased with the length of exposure and the intensity of the radiation. The darkening of a photographic film is due to the formation of silver crystals as the silver salts on the film absorbs radiation and decomposes. Geiger Muller/ ionisation counter
A Geiger counter is an instrument for detecting and measuring radiation. It consists of a metal tube filled with argon gas. The Geiger counter is based on the fact that radiation causes matter to be ionised, forming positive ions and free electrons. The gaseous cations and electrons produced by the i onisation radiation are attracted to electrodes that conduct a current to a recording device. In a Geiger counter, the metal tube is connected to the negative terminal of a DC power supply and a central wire electrode is connected to the positive terminal. A small window at one end allows radiation to pass into the tube. The radiation causes some of the argon atoms to be ionised. The argon ions and free electrons produced allow a current to flow between the electrodes. Each time the tube is exposed to radiation these bursts of current are converted into audible clicks on a built in speaker or converted into a digital readout to alert the user.
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Identify one use of a named isotope Medicine- Technetium_99m (Tc-99m) Industry Sodium_24 (Na) Describe the way in which the above named industrial and medical radioisotopes are used and explain their use in terms of their chemical properties Technetium_99m
It is a emitter. It is used in a wide range of medical appliances, such as the detection of clogged arteries, observing organs and pinpointing brain tumors. Technetium is reasonably reactive and can react with a wide range of biologically active chemicals. Through this, it is able to accumulate at a target organ which uses the biologically active chemicals, allowing pictures to be taken for investigation. E.g. Tc_99m is being combined with tin, which is then injected into blood where it attaches to red blood cells, accumulated where there are clots. 99m Tc +Tc Technetium_99m is used because it has a short half-life of 6 hours which means that the patients exposure to radiation is minimised thus reducing major effects of radiation. Note- Half life is the time taken for half the radioactive isotopes in a given sample to decay. Sodium_24
It is used to detect leaks in underground water or oil pipes. Na is added to the liquid in the underground pipelines. Gamma ray detectors (Geiger counters or scintillation counters) are used to detect the level of radiation i n the surrounding soil. The unexpected high level of radiation indicates a possible leakage. Advantage Na is used because it has a half-life of about 15 hrs. This half-life allows sufficient time for an investigation to be covered out but not to cause any serious pollution to the water bodies
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