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Preliminary Chemistry Topic 4
ENERGY
What is this topic about?
To keep it as simple as possible, (K.I.S.S.) this topic involves the study of: 1. HIGH ENERGY COMPOUNDS & FUELS 2. CARBON & CARBON COMPOUNDS 3. THE CHEMISTRY OF COMBUSTION 4. RATE OF CHEMICAL REACTIONS ...all in the context of society’s use of fuels.
but first, an introduction... Energy includes heat, light, sound, the kinetic energy of a
To understand the Chemistry of Combustion you need to learn about the element
moving object, and Potential Energy which is stored in various ways.
Carbon
This topic is all about Chemical Potential Energy which is stored in the chemical bonds within many compounds and released during chemical reactions, especially
Photo by Alex M
Combustion and its many and varied compounds.
You will learn more about
Exothermic & Endothermic
From the humble match to a rocket launch, Combustion is the chemical reaction that releases the energy of the
and then extend your chemical knowledge to include
Fuels upon which our entire civilization is dependent for transport, industry, heating & cooking.
Rate of Reaction
Photo: Russian Soyez lift-off, courtesy Ali Cimen, senior reporter, Zaman Daily, Istanbul.
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chemical changes,
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CONCEPT DIAGRAM (“Mind Map”) OF TOPIC Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic, come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.
Origin of Fossil Fuels
Allotropes of Carbon
Natural Gas
The Variety of Possible Carbon Compounds Alkanes & Alkenes
Significance of Photosynthesis
Names Formulas Structures
Carbon & Carbon Compounds
High Energy Compounds & Fuels
Properties of Alkanes & Alkenes
Safety Issues Refining of Petroleum
ENERGY
The Combustion Reaction
Chemistry of Combustion
Rate of Chemical Reactions
Factors that Effect Reaction Rate • Temperature • Concentration • Size of solid particles • Catalysts
Catalysts & Activation Energy
Activation Energy
Slow, Fast & Explosive Combustions. Safety Issues Pollution from Fuel Combustion
Temperature & Kinetic Energy of Particles
Exothermic & Endothermic
Incomplete Combustion
Models of Catalyst Action
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Bond Breaking, Bond Making
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1. HIGH ENERGY COMPOUNDS & FUELS The Origin of the “Fossil Fuels”
Chemical Potential Energy
Coal, Petroleum and Natural Gas are the main fuels which power our industries, our homes and vehicles, and generate our electricity.
is the energy stored within chemical compounds. It is stored in the bonds between the atoms, and every chemical reaction results in the release, or absorption, of energy as bonds are broken and formed.
They are all mixtures of high-energy compounds because they are the fossil remnants of life-forms which lived millions of years ago. They have been chemically altered by the fossilization process, but are basically derived from carbohydrates originally made by photosynthesis.
Some bonds store more energy than others. Many of the complex compounds of carbon are especially important as high-energy compounds.
Photosynthesis
Therefore, whenever coal, petroleum or natural gas are burned as fuels, the energy released is energy which came originally from the Sun and was captured by photosynthesis in some ancient plant cell.
is the process by which plants make food. Photosynthesis is the most important Biological process on Earth, because it makes all the FOOD and OXYGEN for all living things.
ligh
t en er
WATER + CARBON DIOXIDE
from soil
6H2O
from air
+
6CO2
Composition of Natural Gas
green pigment in chloroplasts of plant cells
As an example of just which high-energy compounds are present in a fossil fuel, the following table summarizes the composition of “Natural Gas” which is chemically the simplest of the fossil fuels. (K.I.S.S. Principle!)
gy
chlorophyll
GLUCOSE + OXYGEN to air
high-energy sugar (food)
C6H12O6
+
6O2
Formula
Typical % in Natural Gas
Methane Ethane Propane Butane Other gases
CH4 C2H6 C3H8 C4H10 CO2, SO2, H2
90% 5% 2% 1% 2%
Location of Australian Deposits of Natural Gas Natural Gas, or “Petroleum Gas” is usually associated with “Oil Fields” where petroleum (crude oil) is found. The major Australian deposits are marked on this map:
This essential process captures the energy of the Sun, and stores it in the chemical bonds in the glucose molecule, C6H12O6. Glucose is just one of many compounds known as “carbohydrates”... the sugars and starches. All are high energy compounds made from or derived from glucose, which in turn is made from the low energy compounds carbon dioxide and water.
North West Shelf Oil-ffield
Plants make their own carbohydrates; animals get theirs by eating their food via the food chains.
Mereenie, NT Moonie, QLD Moomba, SA
Glucose molecule C6H12O6
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Compound Name
Bass Strait Oil-ffield
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2. CARBON & CARBON COMPOUNDS Diamond is also a covalent lattice of carbon atoms, but the atoms are arranged in a tetrahedral pattern, forming a huge 3-D crystal lattice.
The Element Carbon Carbon is element number 6 in the Periodic Table.
6
C
Therefore, each atom has 6 electrons.
Diamond has extremely high m.p. & b.p., and is the hardest natural substance known.
Electron Configuration = 2.4
Carbon
The beauty of its sparkling crystals has made diamond treasured (literally) for jewellery throughout history.
12.01
In modern times, its extreme hardness has resulted in the widespread use of “industrial diamonds” for drill bits (e.g. for oil-drilling equipment to bore through solid rock) and high-speed saw blades.
Carbon can exist in several different physical forms called “allotropes”. Each allotrope is the same element, containing exactly the same atoms, but the atoms are arranged differently, giving each form quite different physical properties.
“Bucky Balls” Discovered much more recently is a third allotrope of carbon... the Buckminster-Fullerenes. Named after the architect who invented the “geodesic dome” structure, Bucky Balls and Bucky Tubes come in a variety of shapes and sizes.
Some other elements that have allotropes include sulfur, phosphorus, oxygen and tin.
The best known has the formula C60, in which the carbon atoms are arranged to form a sphere resembling a soccer ball.
Allotropes
C
Allotropes of Carbon In Graphite, the atoms are arranged in hexagonal rings which connect to form flat sheets. The atoms in each sheet are strongly bonded, (m.p. & b.p. are high) but the bonds between the sheets are very weak. They can easily slide past each other, so graphite is “slippery”.
60
Because of its “slipperiness”, graphite is an excellent lubricant, used for example, in door locks. Its most familiar use is the “lead” in a lead pencil.
The Bucky Balls have not yet found a practical use, but they have potential for use as high temperature lubricants, for making super-conducting polymers or even as specialized “capsules” for administering medicines.
Unusually for a non-metal and covalent lattice, graphite is a good conductor of electricity and is used in electric motors in the rotating contacts called “brushes”; in this role it both conducts electricity and helps lubricate the rotating axle.
ALLOTROPES: Same element, same atoms. Different atomic arrangements, different physical properties.
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Carbon Compounds
The Hydrocarbons
All life on Earth is based on carbon compounds, operating within an aqueous (water) environment. We have good reason to believe that, if there is life elsewhere in the Universe, it will also be carbon and water based, although the details of E.T.’s Chemistry, Biology and appearance cannot be predicted.
With so much variety of carbon compounds, where do you start? It turns out that many of our important fuels are composed of mixtures of the very simplest carbon compounds... the Hydrocarbons. These are compounds containing only carbon and hydrogen.
Water is the solvent without equal, and carbon is the only element capable of forming the variety of intricate and complex molecules needed to make a living organism.
Names & Formulas for the “Alkanes” Methane
Carbon rarely forms ions. The immense scope of carbon’s Chemistry is based on covalent bonding, and the key is that a carbon atom can readily form bonds with other carbon atoms, as well as (notably) hydrogen, oxygen and nitrogen atoms.
Molecular Model
Molecular Formula CH4 H
Structural Formula
H
C
H
H
C
C
Ethane
Molecular Model
Molecular Formula C2H6 Single Bond A single C-C bond involves sharing one pair of electrons. Each carbon atom has 3 other bond positions available, allowing the formation of chains, rings and networks.
C
Structural Formula H
C
H
C
C
H
H
H
“Condensed Structural Formula” CH3CH3
Propane
Double Bonds (sharing 2 pairs of electrons) and Triple Bonds (sharing 3 pairs) create even more possibilities.
C
H
Molecular Model
Molecular Formula C3H8
C
Structural Formula H H H H
The result is that carbon can form more possible compounds than all the other elements put together.
C
C
C
H
H
H
H “Condensed Structural Formula” CH3CH2CH3
See the pattern beginning to emerge? The Syllabus requires that you learn the first 8 compounds in this series... the “Alkanes” The ALKANES are HYDROCARBONS containing only SINGLE C-C Bonds. The Molecular Formulas form a pattern
CnH2n+2 where n = number of carbon atoms
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How to Name the Alkanes
The Alkenes
To name, just add “-ENE” are another, similar to the appropriate prefix. Homologous Series. The Alkenes contain one C=C double bond.
The Alkanes are just one of many “Homologous Series” of carbon compounds. An Homologous Series means a group of chemicals which form a logical series or pattern, in terms of their molecular structure. As you’ll learn, they have similar properties which also follow patterns.
Since there have to be at least 2 carbon atoms to have a double bond, the series begins with
You must learn the following rules for naming:
Number of Carbon Atoms 1 2 3 4 5 6 7 8
Ethene
Prefix
Molecular Formula C2H4
MethEthPropButPentHexHeptOct-
Structural Formula H
H
C
C
H
H
“Condensed Structural Formula” CH2CH2
Propene
Double C=C Bond
Molecular Formula C3H6
...there are many more, but this is as far as the Syllabus expects you to learn.
Structural Formula H H H
To name any ALKANE, just add “-ANE” to the appropriate prefix above.
C
C
C H H
H
The first 3 were on the previous page. We now continue...
Butene Butane Molecular Formula C4H8
Molecular Formula C4H10
H
H
H
H
C
C
C
C H
H
H
H
H
C
“Condensed Structural Formula” CH3(CH2)2CH3
Double C=C Bond
Molecular Formula C5H12 Structural Formula H H H H
H
C
C
C
C
C H
H
H
H
H
H
“Condensed Structural Formula” CH3(CH2)3CH3
Hexane
C6H14
CH3(CH2)4CH3
Heptane
C7H16
CH3(CH2)5CH3
Octane
C8H18
CH3(CH2)6CH3
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“Condensed Structural Formula” CH2CHCH2CH3
H
C C
C
H H
H
Pentane
H
“Condensed Structural Formula” CH2CHCH3
Structural Formula H H H H
Structural Formula H
Double C=C Bond
Pentene
C5H10
CH2CH(CH2)2CH3
Hexene
C6H12
CH2CH(CH2)3CH3
Heptene
C7H14
CH2CH(CH2)4CH3
Octene
C8H16
CH2CH(CH2)5CH3
The ALKENES are HYDROCARBONS containing one DOUBLE C=C Bond. The Molecular Formulas form a pattern
CnH2n 6
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Properties of the Alkanes and Alkenes
Another Important Physical Property: Volatility
As you know by now, the properties of any chemical substance are determined by the bonding within.
A substance is said to be volatile if, at room temperature, it vaporizes readily. A simple indicator of volatility is the boiling point, but its not the only factor.
Alkanes and alkenes contain only 3 types of bond within the molecules: • Single C-C bonds • Double C=C bonds • C-H bonds
}
Obviously, the alkanes and alkenes with 4 or less carbon atoms are already gases at room temperature, but even those which are liquids are highly volatile.
all non-polar covalent
Consider octane, a component of petrol. Its boiling point is a little above that of water, but it is much more volatile. At room temperature, octane (petrol) in an open container vaporizes rapidly compared to water under the same conditions.
Therefore, you would expect these compounds to: • have relatively low m.p. & b.p.’s • be insoluble in water • be non-conductors of electricity
and you’d be correct !
The explanation is, again, all about bonding. Water molecules tend to cling together because of the dipoledipole attractions of hydrogen bonding. Octane molecules have only the weak “Dispersion Forces” holding them, and many molecules have enough energy to escape into the gas state, even at temperatures well below the boiling point.
To keep it simple, (K.I.S.S. principle) consider just the boiling points: Alkane
b.p. (oC) Methane -162 Ethane -89 Propane -42 Butane -1 Pentane 36 Hexane 69 Heptane 98 Octane 126
State 25oC gas gas gas gas liquid liquid liquid liquid
Alkene
b.p. (oC)
State 25oC
Ethene -104 Propene -48 Butene -6 Pentene 30 Hexene 64 Heptene 94 Octene 121
gas gas gas liquid liquid liquid liquid
The volatility of the alkanes and alkene has important...
Safety Consequences. We haven’t yet looked at the Chemical Properties of alkanes and alkenes, but don’t forget that these are fuel compounds... they contain a lot of energy, are highly inflammable, and now we find out they are highly volatile as well! DANGER, DANGER!
When these values are graphed the pattern becomes obvious, and also the great similarity between the alkanes and alkenes (at least for this property).
0
es an k l A
es en k l A
Propane & Butane are used as bottled “BBQ gas”. They are stored as pressurized liquids in pressure cylinders and used outdoors, or in wellventilated areas only.
-1 100 -2 200
Boiling Point (oC)
100
Natural Gas is mostly methane. It must be stored in high pressure cylinders, outdoors and kept cool.
1
2
3
4
5
6
7
8
No. of Carbon Atoms
Inter-molecular Forces?
Small amounts of “smelly” chemicals are added to gas fuels so that leaks are easily detected by smell.
Inside each molecule are strong, non-polar, covalent bonds. However, the only forces between the molecules are the very weak “Dispersion Forces”, so m.p. & b.p. are generally low.
Petrol, Kerosene & Diesel are highly volatile liquid fuels. They must be stored in sealed drums or tanks, and all sparks or flames (even mobile phones) kept well away. All transfer of fuel from tank to tank (e.g. filling the car) must be done outdoors. SAFETY IS CHEMICAL COMMON SENSE
Dispersion forces become stronger as the size and mass of the molecule increases, which explains the pattern of the graph. Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
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The Refining of Petroleum
Prac Work; Fractional Distillation
Petroleum deposits (including Natural Gas) are the fossilized chemical remnants of ancient sea creatures. Petroleum is a complex mixture containing alkanes, alkenes and many other carbon compounds, trapped in sedimentary rocks.
You may have done a laboratory exercise to separate a mixture by fractional distillation. A common mixture used is water and ethanol, a vastly simpler mixture than petroleum, but the principle is the same. Fractional Distillation thermometer monitors temp. of vapours.
To obtain the fuels we need, the first step is, of course, to locate the petroleum and pump it to the surface. Oil Well Pump, Alberta, Canada
Mixture
Condenser
Heat er
Photo by Diana
Ethanol’s b.p. = 78oC. Water’s b.p. = 100oC.
The next step is to separate the petroleum mixture into more useful “fractions”. This is achieved by...
The mixture is heated gently until the vapour temperature o is about 80 C. At this temperature the “distillate” collected from the condenser is relatively pure ethanol, perhaps about 90%. Once all the ethanol has evaporated from the mixture, the vapour temperature will rise to around 100oC, indicating that now water is being collected.
Fractional Distillation which separates the mixture according to differences in boiling point. Fractionating Tower in a Refinery Simplified Schematic Diagram
By watching the thermometer, and changing collection beakers at the right time, it is possible to collect 2 separate “fractions” from the original mixture.
Gas fraction L.P.G.
Oil Refinery
Temperature decreases up the tower
Fractionating Towers Vapours condense on collection trays...
at different levels...
Petrol
Diesel & Kerosene Photo: Nick Fletcher
according to boiling point
Vapours rise through the tower Vaporized Crude Oil injected
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Heating & Fuel Oil
In an Oil Refinery the same basic process separates crude petroleum into many fractions... petrol, diesel, kerosene, etc. based upon different boiling points.
Lubricationg Oils
Each fraction is still a mixture. For example, petrol contains dozens of individual compounds, and the exact composition varies from place to place, and from time to time. It depends on which crude oil source is being refined, and on any adjustments made by the chemical engineers at the refinery.
Residue: Grease & Bitumen
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Worksheet 1
p)................................ bonds. Carbon atoms readily bond with each other forming q).........................., rings or networks. They can form single, r)......................... or ............................ bonds.
Fill in the blank spaces Chemical a).................................. Energy is the energy stored in the b)........................................................... within a compound. Biologically, all high-energy compounds are compounds of c)....................................
The simplest carbon compounds, containing only carbon and hydrogen, are collectively called the s)............................................., of which the t)............................ and the u)........................................ are 2 types. Hydrocarbons are named by using a prefix for the number of v)................... .........................., and adding an ending to identify which precise type it is.
In all living things, chemical energy came originally from the d)....................... The plants capture the energy during e)......................................................... and store it initially in the compound f)......................................, a member of the g)................................................. group which includes all the h)................................... and .........................................
The Alkanes have names ending in “w).......................”. They all contain x)............................... C-C bonds and have a general formula y).................................... Alkanes are z)............................................. in water and have relatively aa)....................... melting and boiling points. Their properties relate to their molecules being nonab)......................... The only inter-molecular forces are the very weak ac)............................................ forces. Melting and boiling points show a very regular pattern of rising steadily as molecular size ad).....................................
Photosynthesis is the most important Biological process on Earth because it produces all the i)..................................... and j)..................................... for all living things. Coal, k)................................... and Natural Gas are all “l)................................... fuels” because they are the fossilized remains of ancient living things. When these fuels are burned, the energy released came originally from the m).................... via the process of n)................................... in plants.
The Alkenes have very ae)................................ properties to the alkanes. Alkenes contain one af)........................................ bond, and have general formula ag)....................................
Natural Gas is associated with o)....................................... deposits. Its composition is about 90% p)..............................
Each of these “types” form an ah)........................................ Series, with the same general formula and similar properties.
Worksheet 2 Part A
Fill in the blank spaces.
Another property of alkanes and alkenes is that they are highly volatile. This means that they tend to ai)....................... readily. This has important safety consequences for these highly inflammable, aj)......................-energy compounds.
Carbon is element No.6 and therefore has electron configuration a)...................... Carbon has 3 allotropes; b)......................................., ........................................... and the so-called “buckyballs”. Allotropes are different physical forms of the same element. Allotropes are composed of c).............................. atoms, but they are d)................................ differently, giving each allotrope different e).......................... .....................................
Crude petroleum is a complex ak)............................ of many compounds. At a refinery, it is separated into useful “al).............................” by the process of am)............................ ................................................... The crude mixture is vaporized and as the vapours rise and the temperature an)............................., each fraction ao)................................... at a different level in the tower.
Carbon’s 2 main allotropes have different properties which determine their uses: • In Graphite, the atoms are arranged in f).............................. which can slide easily, so it is used as a g)............................... It is also a good electrical h)..........................................., it is used in the rotating contacts in i)............................................. Its soft, slipperiness and black colour make it suitable for its major use, as the “j).........................” in a pencil.
Part B Practice Questions 1. There are other Homologous Series which have different name endings, but the prefixes follow the same rules. How many carbon atoms in: a) pentanone? b) butanal? c) propanal? d) octanone? e) methanoic acid? f) hexanol?
• In k).................................. the atoms are arranged in a 3-D l)......................................... (shape) pattern, producing a hard crystal which has been prized for m)................................. throughout history. Its extreme hardness makes it useful for n)..............................................................................................
2. What is the molecular formula, structural formula and condensed structural formula for “nonene” and “decane”, given that the prefixes “non” = 9 and “dec” = 10.
The huge variety of carbon compounds is due to the ability of carbon atoms to each form o)................. (number) of Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
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3. THE CHEMISTRY OF COMBUSTION Indications of a Chemical Change
Changes of State During Combustion
• reactant substance(s) will “disappear” or be eaten away. • new substance(s) will appear. • colour changes may occur. • precipitates may occur in a solution. • a gas may be evolved (bubbles, if in a liquid). • there will be significant energy changes, observed as changes of temperature, or in the case of combustion reactions, as visible flames.
Also, notice that octane (a liquid at room temperature) is shown as a gas because the liquid vapourizes before the combustion reaction begins.
By now you should be aware that when any chemical reaction occurs, one or more of the following will be observed:
Notice that in both the previous examples all the reactants and products are shown as being in the gas state. We are used to water being a liquid, but due to the heat produced by combustion, the water always forms as water vapour, H2O(g).
Combustion generally involves reactants and products in the gas state.
Combustion Reactions occur when something burns. Combustion always involves a “fuel” compound combining with oxygen to form oxide compound(s) of whatever elements are present in the fuel.
A Burning Candle You may have done a simple practical exercise to examine carefully a burning candle to study the changes of state occurring.
An example is a reaction you studied in an earlier topic: magnesium + oxygen 2Mg(s) + O2(g) “Fuel”
+
Oxygen
The wax melts, soaks up into the absorbent wick, and vaporizes before it actually burns.
magnesium oxide 2MgO(s) Oxide compound
Some fuels, just like wax, will not burn easily unless a wick is provided to allow easy vaporization so the fuel vapour can ignite. Kerosene and heating oils are like this. Visible flame is the
Combustion reactions are always exothermic, releasing chemical energy. In many cases, there is a visible flame which is a region of gas (in which the reaction is occurring) heated up by the energy release so that it glows.
zone where combustion is occurring
Our most common and important fuels are mixtures of hydrocarbons. Natural Gas and L.P.G. (Liquified Petroleum Gas) contain Methane: methane + oxygen CH4(g) + 2O2(g) “Fuel” + Oxygen
Molten, liquid wax
carbon dioxide + water CO2(g) + 2H2O(g)
Change of Mass in a Combustion If combustion occurs in the open, the gas products (often carbon dioxide and water vapour) escape and disperse into the surroundings.
Petrol is a mixture of many hydrocarbons, but we can use Octane as an example: octane + oxygen C8H18(g) + 25 O2(g) 2
carbon dioxide + water 8CO2(g) + 9H2O(g)
You may have done a simple practical experiment involving weighing a match before, and after burning. You would have found that it lost mass, due to the escape of the reaction products.
Note: When necessary, it is usual to balance the equation with a fractional mole quantity of oxygen so that the equation shows 1 mole of fuel. (Simply add total oxygen atoms on the right side. If an odd number, write this number over 2, as the coefficient for O2(g)) Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
Wick absorbs liquid
Solid wax
Oxide compounds of the elements in the fuel
zone of vaporization
If carried out in a sealed container, there is no mass change, since the same atoms are still present, but simply rearranged to form the reaction products. 10
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Endothermic & Exothermic Reactions
Break Bonds, Make Bonds
These concepts have been introduced in previous topics.
All chemical reactions involve rearranging the atoms of the reactant molecules. This must involve 2 stages:
Now you must learn more detail about the shape of the energy profile diagrams introduced previously.
• Breaking the bonds in the reactant molecules, so the atoms can be rearranged, and • Making new bonds to create the product molecules.
Exothermic Reactions release energy because the products contain less Chemical Potential Energy than the reactants. Because the chemicals lose energy, the energy change (ΔH) is negative.
For example, in the combustion of methane: CH4(g)
+ 2O2(g)
CO2(g)
+ 2H2O(g)
Here is the Energy Profile Diagram as you’ve seen it before:
the reactants are methane and oxygen molecules:
Reactants Energy Level Energy Content
Bonds must be broken... Energy needed
Energy released by chemicals during reaction
negative
Products have LESS energy
Energy must be supplied to break these bonds.
Now we must add the idea of Activation Energy:
Once the atoms are freed from the reactant molecules, they can begin combining into new molecules to form the products.
Reactants Energy Content
The making of these new bonds always releases energy.
New bonds are made. Energy released
Ea
Ea = Activation Energy
ΔH Products
The Activation Energy is like a “hill” or energy barrier which must be overcome before the chemicals can turn into products. The Ea is the energy needed to break some bonds in reactant molecules so the atoms can begin to rearrange themselves. Remember, that this is connected to the value of the “Ignition Temperature” of a fuel.
Activation Energy
The reaction cannot begin until some energy is supplied to break some bonds in reactant molecules. This energy requirement is called the “Activation Energy”. Once a combustion reaction gets started, the energy released by the exothermic reaction provides the activation energy for other molecules, so the reaction continues.
Ea
Energy Content
The temperature at which a fuel-air mixture will ignite is called the Ignition Temperature. Its value is related to the Activation Energy required to get the combustion started.
ΔH
Energy Content
Ea
It is this activation energy requirement that explains why fuels, like petrol, do not just spontaneously burst into flame when exposed to oxygen in the air. There has to be a spark or flame to ignite them.
ΔH
These 2 fuels release the same amount of energy (ΔH is the same). The one on the left has a lower Ea and therefore a lower Ignition Temperature than that on the right.
Fuel Ignition Temperature (oC) Petrol 500 approx Diesel 250 Methane (Natural Gas) 540 Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
= ΔH
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Incomplete Combustion
Endothermic Reactions are those that require energy to proceed. The products have more Chemical Potential Energy than the reactants. Since the chemicals have gained energy, ΔH is considered positive.
Earlier, you learned about the combustion of typical fuel compounds like octane, a component of petrol: octane + oxygen C8H18(g) + 25 O2(g) 2
These reactions also have an Activation Energy requirement.
This reaction describes complete combustion.
Ea = Activation Energy Energy Content
Ea
carbon dioxide + water 8CO2(g) + 9H2O(g)
In many real situations however, there is not enough oxygen available, or the fuel and oxygen do not get mixed thoroughly enough for all the molecules to “find” each other in the time available. In these cases, the combustion may be incomplete: less octane + oxygen carbon monoxide + water C8H18(g) + 17 O2(g) 8CO(g) + 9H2O(g) 2
Products
ΔH positive Reactants
If there is even less oxygen, or really poor mixing of molecules, the reaction can be even more incomplete: less octane + oxygen carbon + water 8C(s) + 9H2O(g) C8H18(g) + 9 O2(g) 2
Practical Work You may have carried out experiments to observe and describe various exothermic and endothermic reactions. In this topic, you have seen combustions, while back in topic 2 you saw active metals react with water or acids. In topic 3 you measured the heat released during dissolving
In this case, the solid carbon formed is “soot”, often observed from diesel exhausts, burning candles, and bunsen burners set to a yellow flame.
These were all Exothermic. Way back in topic 1, you studied and observed electrolysis causing water to decompose when electrical energy was added. In topic 2 you decomposed carbonates by heat and studied the Endothermic extraction of metals from ores.
In real situations, the total result of burning a fuel can be some combination of all the 3 situations above... some molecules burn completely forming CO2, and some burn incompletely to CO or even soot.
Incomplete Combustion of carbon-based fuels results in the formation of Carbon Monoxide and/or “soot” Endothermic Reactions
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Pollution from Combustion of Fuels
Photo: Ken Kiser
When pure hydrocarbon fuel compounds burn completely the products are carbon dioxide and water vapour. Neither of these can be considered as “pollution” since they are part of the natural chemical cycles on Earth. Carbon Dioxide However, the release of billions of tonnes of CO2 from fossil fuel combustion is believed to be leading to a “Greenhouse Effect”, and causing global warming. This may be leading to (is leading to?) major environmental changes, such as alterations to climates and weather patterns.
Carbon Monoxide is produced in all petrol engines due to incomplete combustion of the fuel. It does not cause any large-scale environmental damage, but is very toxic to people.
The Greenhouse Effect can be minimized by reducing the usage of fossil fuels, and switching to fuels which do not increase CO2 levels, such as ethanol from plant sources. Toxic pollution results from either incomplete combustion, or from impurities in the fuels.
Oxides of Nitrogen Oxygen and nitrogen in the air do not normally react with each other. However, at the high temperatures inside an engine, they react:
Photo: Daniel West
nitrogen + oxygen N2(g) + O2(g)
nitric oxide 2 NO(g)
Nitric oxide (or nitrogen monoxide) reacts further to form toxic nitrogen dioxide, NO2(g), and is involved in the formation of “petrochemical smog” which causes a toxic and serious pollution problem in cities. To avoid the problems caused by both carbon monoxide and the oxides of nitrogen, modern cars are fitted with “catalytic converters” on their exhaust systems. These promote the reaction: 2NO(g) + 2CO(g)
Sulfur Dioxide Fossil fuels, especially coal, contain small amounts of sulfur compounds. When these are burnt with the fuel: sulfur + oxygen S + O2
Modern car engines also have advanced fuel-air control systems to ensure the most efficient, complete combustion possible. These measures have been very successful in reducing air pollution problems in some cities.
sulfur dioxide SO2(g)
The final solution to the problems associated with burning fossil fuels may be solved for us when petroleum supplies run out. Petroleum is a non-renewable resource, and many estimates suggest that world supplies will only last about 30 years, at current usage rates.
This gas is a serious pollutant because it reacts with water in the environment to form a strong acid. This can lead to “Acid Rain” which can destroy natural environments such as forest and lake ecosystems.
In 2005, the world economies were shocked by a sudden “spike” in the price of petroleum. It is highly probable that such events will continue to happen (for both political and economic reasons). This is certain to encourage research and development of “alternative”, sustainable fuels. It’s already happening...
Emission of SO2 is minimized by • removing sulfur impurities during the refining of liquid and gas fuels. • “scrubbing” the exhaust emissions from coal-burning power stations and sulfide-ore metal smelters. The SO2 can be collected and used to make sulfuric acid for industry. Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
N2(g) + CO2(g)
Now there’s a career idea for today’s Chemistry students! 13
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Large scale burning of fossil fuels has added so much extra x)................................................. to the atmosphere that this has resulted in y)................................. warming due to the z)........................................... Effect. Fuels containing sulfur will produce aa)........................................... when burned, and this can cause “ab)........................ Rain”. Petrol engines produce ac)..................................... gas from incomplete combustion, and ad)................................................ gas from the high temperature reaction of ae)................................. and ........................................ NO(g) is involved in the production of “af)..............................” in cities.
Worksheet 3 Part A Fill in the Blank Spaces The indications that a chemical change has occurred are: • reactants a).......................................... • b)..................................... appear. • c)................................. changes may occur. • in a solution, a d)................................ may appear. • a e)............................ may be evolved. • there may be significant f)............................. changes. Combustion reactions always involve 2 reactants: the “g)...............................” and ............................... The product(s) of the reaction are always the h)........................... compound(s) of the elements in the fuel. For all the common hydrocarbon fuels, the products of complete combustion are i)............................................. and .............................. In general, hydrocarbon fuels need to be in the j)............................ state before they will ignite. Many fuels, such as k).................................... need a wick to help them vaporize.
Part B Practice Questions 1. Equations for Combustion Write a balanced equation for the complete combustion of a) calcium metal b) hydrogen gas c) ethane d) ethene e) pentane f) pentene g) propane
For a reaction to get started, energy must be supplied to l).............................. some of the bonds in m)........................... molecules. This energy requirement is called the “n)............................................... Energy”. The atoms can then form new bonds to form the o)...................................... of the reaction. Making new bonds always p)......................... energy which can provide the activation energy for further bond breaking so the reaction becomes q).............................. ...................................
2. Equations for Incomplete Combustion Write a balanced equation for the incomplete burning of a) hexane, if all the carbon atoms form carbon monoxide. b) hexane, if the carbon atoms form CO2 & CO in equal proportions. c) hexane, if CO and “soot” are formed in equal proportions. d) butane, if CO2 & CO are formed equally.
On an “Energy Profile Diagram”, activation energy is like an energy “r).....................................” which must be overcome before the reaction can proceed. For fuels, the activation energy is related to the value for “s)........................ ......................................”, which is the temperature at which a fuel-air mixture will ignite.
3. Energy Profile Diagrams Label each diagram to show ΔH and Ea. Sketch diagrams to represent:
Incomplete Combustion occurs when there is insufficient t)............................................ available, or when the fuel and air are not u)............................................. thoroughly. In hydrocarbon fuels, incomplete combustion leads to the production of v)............................................. gas, or even particles of solid w).....................................
a) an exothermic reaction with large ΔH and small Ea. b) an exothermic reaction with small ΔH and large Ea. c) an endothermic reaction. d) Which diagram (a, b or c) could describe a fuel with a high ignition temperature? Explain your answer.
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4. RATE OF CHEMICAL REACTIONS Explosive Reactions occur when fuel molecules are well mixed with air, and there is no limit on the flow of fuel to the combustion area. For example, if a gas leak fills a room with a fuel gas mixed with air, any spark or flame can start a reaction that accelerates so fast that the heat release and gas pressure creates a shock wave... it explodes!
Slow, Fast and Explosive Combustions Combustion reactions can occur at different rates, depending on conditions. In a camp fire, or slow combustion stove, large pieces of solid fuel (e.g. wood) burn slowly and steadily. There’s plenty of fuel, so why is the reaction slow? Although there is a large piece of fuel available, it can only burn at the surface where oxygen can get to it. The reaction rate is limited by the surface area available. If the wood had been chopped into small “chips”, there would be a greater surface area and it would burn faster, assuming plenty of air could get to it. (Solid fuel heating stoves are often slowed right down by limiting the amount of air getting in.)
Fast Combustions In a gas stove or bunsen burner, the fuel is mixed with air before ignition. The fuel and oxygen molecules are intimately mixed together, then ignited as they reach the burner nozzle. The reaction is fast, and limited only by the flow of fuel-air mixture to the burner.
Its not just gas molecules that can mix with air to form explosive mixtures. Fine dust particles suspended in air can explode too, if the particles are at all capable of combustion. Explosions have occurred with wheat dust, coal dust, cotton fibres and even custard powder, in factories where dust or fibre was allowed to mix with air.
The point is that the rate of a combustion reaction depends on the fuel and oxygen molecules colliding with each other CH4
The safety consequences are obvious. Any work environments where dust can be produced need to ensure dust supression, good ventilation, air filters, or whatever is necessary to prevent explosive mixtures forming in the air.
O2 collision
Reaction
The RATE of a Chemical Reaction depends on the reactant molecules COLLIDING
energy released
Any situation which increases the Rate of Collisions will increase the REACTION RATE CO2 Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
H2O 15
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Temperature & Kinetic Energy of Particles
The Effect of Concentration
According to the “Kinetic Theory of Matter”, all matter is composed of particles (molecules, atoms or ions) which are constantly moving.
If a chemical reaction is occurring in solution, the concentration of the reactant(s) can affect the chances of a collision, and control the reaction rate.
• In solids, the particles can only vibrate in one place. • In liquids, they are close together, but move around. • In gases, they are far apart and flying in all directions.
Concentration Low, collisions less likely, Rate of Reaction Slow
As temperature increases, the movement increases. Increased heat energy causes an increase in the kinetic energy of the particles. TEMPERATURE is a measure of the average KINETIC ENERGY of the particles
At higher concentration, the chances of reactants colliding is increased, so reaction rate is higher.
Therefore, at higher temperatures the particles move faster and are more likely to collide with enough “activation energy” for the reaction to proceed. The result is that:
Product
The RATE of Chemical Reactions Increases with TEMPERATURE Product
Practical Work: Temperature & Rate of Reaction
In reactions involving gases, higher gas pressure increases the concentration of the particles, by forcing them closer together. So gas pressure also increases reaction rates.
You may have done experiments in the laboratory to observe and measure this relationship.
The RATE of Chemical Reactions Increases with CONCENTRATION or with GAS PRESSURE
Many reactions are suitable for study; a simple one is: Mg(s) + 2HCl(aq)
H2(g) + MgCl2(aq)
More Practical Work: Effect of Concentration
Three identical test tubes of the same acid solution can be set up at different temperatures.
Using the same reaction as on the left, you could set up three test tubes of acid at different concentrations by using the same acid, but diluting the solution in each tube:
Different temperatures are achieved by immersing each tube in a beaker containing
Tube 1 Tube 2 Tube 3 Vol. of acid (mL) 10 10 10 10 0 Vol. of water (mL) 20 30 20 10 Note that the amount of acid is the same in each tube, but the concentrations are different.
• ice and water • tap water • hot water Identical pieces of magnesium are dropped into each tube. The rate of the reaction can be observed by the rate of bubbling (of H2 gas). To measure the rate, the time taken for the magnesium to “disappear” could be measured. (For graphing purposes, the reciprocal (1/time taken) can be used as a value for “rate of reaction”.) You would find that higher temperature
Product
Identical pieces of magnesium could then be dropped into each tube, and the rate of reaction observed or measured as before. You would find that Tube 3 would react the fastest.
faster rate of reaction
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How Catalysts Work
Effect of Surface Area on Reaction Rate
There is no single, simple model to explain how a catalyst speeds up reactions by reducing the Activation Energy. There may be dozens of ways that various catalysts work; here are just two.
You may have done a simple experiment as suggested by the diagram: Same quantity of solid calcium carbonate on each spoon
Lumps
Powder
Model 1: Surface Adsorption Some catalysts work by holding the reactant(s) in such a way that collision and reaction become more likely. In some cases too, the bonds within a reactant molecule are placed “under strain” so that less energy is required to break bonds to start the reaction process.
Both lumps and powder react with acid in exactly the same way, but you would observe that the powder reacts faster.
Reactant molecules
Product molecule
Same quantity of same strength acid
The reaction occurring is CaCO3(s) + 2HCl(aq)
Collision & Reaction
Reactant molecule(s) adsorb to surface
CO2(g) + H2O(l) + CaCl2(aq)
You would have found that the reaction occurs faster with the powdered solid than with larger lumps. This is because the smaller solid pieces in the powder add up to a larger total surface area for the acid to react with. Collisions between reactants occur more often, so the reaction proceeds faster.
Bond placed under strain by adsorption to catalyst
CATALYST
Many Industrial processes rely on catalysts of this type: • In the manufacture of Ammonia
Catalysts
N2(g) + 3H2(g)
A “Catalyst” is a chemical substance which increases the rate of a reaction, without being consumed or permanently changed by the reaction.
Fe catalyst
2NH3(g)
the reaction rate is speeded up using an iron catalyst, finely divided to provide greater surface area.
Catalysts are widely used in Industrial Chemistry (examples at right) but perhaps the most notable example of catalysts is in Biology. All living cells carry out thousands of chemical reactions which would occur far too slowly for life to function if not for catalysts. In every cell, every reaction is made possible by protein catalysts called “Enzymes”.
• In the “Catalytic Cracking” of petroleum compounds at an oil refinery (to be studied in a later topic) larger hydrocarbon molecules are broken up to increase the yield of the valuable petrol fraction. The reaction is speeded up by a catalyst of Zeolite; a natural clay mineral with a large surface area, and an affinity for hydrocarbon molecules.
Catalysts Lower Activation Energy The effect of catalysts is to reduce the Activation Energy requirement for the reaction.
Model 2: Alternative Chemical Pathways Some catalysts work by providing an alternative chemical pathway which has a lower Activation Energy. In these cases, the catalyst might be a reactant in a series of reactions which then “regenerate” the catalyst at the end.
Without Catalyst
Ea
For example, the reaction
Energy Content
Reactants
ΔH
2SO2(g) + O2(g) With Catalyst. Activation Energy reduced
is very slow, but if a small amount of NO2(g) is added to the mixture, 2 faster reactions occur in sequence:
Products
Reaction 1: SO2(g) + NO2(g)
Note that ΔH is NOT affected
Reaction 2:
If the Activation Energy requirement is lower, then at any given temperature there will be more reactant molecules having the energy to react... so the reaction goes faster. Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
2SO3(g)
2NO(g) + O2(g)
SO3(g) + NO(g) 2NO2(g)
The NO2 is regenerated at the end, and has not been consumed... it is a catalyst for the reaction, and each NO2 molecule can be recycled over and over. 17
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Practical Work: Catalysts
Another Example of an experiment on catalysts uses the following reaction:
You may have carried out a practical experiment to see the effect of a catalyst on a reaction rate.
hydrogen + iodide + hydrogen peroxide ions ions H2O2(aq) + 2I-(aq) + 2H+(aq)
A popular experiment is to use the familar reaction Mg(s) + 2HCl(aq)
H2(g) + MgCl2(aq)
iodine
+ water
I2(aq) + 2H2O(l)
The reaction is quite slow, but can be followed visually over several minutes because while the reactants are colourless, the iodine product is a yellow-brown colour.
with, and without a catalyst. The catalyst used is a small piece of copper metal, which a strip of magnesium ribbon is wrapped around. (The metals must be in contact.)
Solution darkens as Product I2 forms
Colourless Reactants
In contact with the copper, it will be found that the magnesium reacts much faster than an identical experiment without the copper. At the end of the reaction, the copper is totally unchanged... it speeds the reaction up, but is not consumed or changed itself; it is a catalyst.
If a few drops of a dilute solution of sodium molybdate (NaMoO4) is added to another identical mixture, the reaction proceeds to completion much faster... a few seconds compared to many minutes.
The method by which the copper catalyses the reaction does not fit either of the “models” described on the previous page.
The molybdate ion (MoO4 , a polyatomic ion) acts as a catalyst and remains in the solution unchanged at the end.
-
It works by providing an alternative sequence of loweractivation energy reactions, which regenerate the molybdate ion at the end.
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Worksheet 4 Fill in the Blank Spaces If solid substances are involved, then reaction rate can be increased by increasing the u).................................................... of the solid, such as by v).......................................................... .............
The rate of a combustion reaction depends on the reactant molecules a)................................. with each other. In a slow combustion, such as the burning of b)............................... .............................., the reaction only occurs at the surface where c)..................................... can reach the fuel. In a gas stove or bunsen burner the fuel and air are mixed before d)......................................... The rate of reaction is limited only by the flow of e).................... & ................................ to the burner nozzle.
A “catalyst” is a substance which w)..................................... the rate of a reaction, without x)............................................... or ............................................ by the reaction. In living things, all chemical reactions are controlled by catalytic proteins called y)..................................... Catalysts effect the energy profile of a reaction; they have no effect on the value of z)......................, but reduce the aa).............................. .................................. This means that, at any given temperature, there are more reactant molecules with enough ab)............................................ to commence the reaction, so the ac)................................................. increases.
If a fuel-air mixture is allowed to build up, a spark or flame can result in an f).......................................... Any combustible gas or even g)............................ can become an explosive mixture if allowed to mix with air. This has important h)........................................... implications for workplaces which produce dust or fibres, such as i)................................., .................................., etc. (examples)
Some catalysts work by ad)....................................... reactant molecules to their surface in such a way that collisions become more ae)..................................... Chemical bonds may also be placed under af)...................................... which makes it more likely that they can be ag)................................... to begin the reaction. This type of catalyst is common in industry, such as the use of ah).................................. catalyst in the production of ammonia, and the use of the clay mineral ai)......................................... in the “aj).......................... Cracking” in petroleum refining.
According to the “j)................................. Theory of Matter”, all substances contain particles which are constantly k)................................... Adding heat energy causes the particles to l)................ ......................................... Temperature is a measure of the average m).................................................. of the particles. An increase in temperature causes all chemical reactions to n)............................................................. This is because the particles will move o)......................................, and the chances of p)........................................... increases. Also, the reactant molecules are more likely to have the necessary q).................................. energy at higher temperatures.
Other catalysts work by providing an ak)................................ ...................................... pathway from reactants to products. In these cases, the catalyst actually takes part in the reactions, but is al)....................................... unchanged at the end.
Higher r)......................................... also increases reaction rates, by making it more likely that s)...................................... will occur. In gases, the t)............................................. of the gas is equivalent to the concentration.
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WHEN COMPLETED, WORKSHEETS BECOME SECTION SUMMARIES
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CONCEPT DIAGRAM (“Mind Map”) OF TOPIC Some students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. Practise on this blank version.
ENERGY
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8. If the compound hydrogen sulfide (H2S) underwent combustion, you would expect the products to be: A. carbon dioxide and water B. water and oxygen C. sulfur dioxide and water D. hydrogen and sulfur dioxide
Practice Questions
These are not intended to be "HSC style" questions, but to challenge your basic knowledge and understanding of the topic, and remind you of what you NEED to know at the K.I.S.S. Principle level. When you have confidently mastered this level, it is strongly recommended you work on questions from past exam papers.
9. The correctly balanced combustion equation is: CO2(g) + H2O(g) A. CH4(g) + 3O2(g) 2 B. CH4(g) + 3O2(g) CO2(g) + 2H2O(g)
Multiple Choice
1. The chemical raw materials for photosynthesis are A. water & oxygen B. glucose & oxygen C. carbon dioxide & glucose D. water & carbon dioxide 2. When a fuel such as coal is burned, large amounts of energy are released. This energy was previously in the form of A. low energy compounds such as CO2 and water. B. chemical energy stored in carbohydrates. C. geothermal energy from mineral formation. D. chlorophyll used in photosynthesis in ancient plants.
+ 2O2(g)
CO2(g)
D. CH4(g)
+ 3O2(g)
2CO2(g)
+ 2H2O(g) + 2H2O(g)
11. The diagram shows an energy profile for a chemical reaction. It would be true to say that the reaction is: A. exothermic, and Ea = +20kJ/mol. B. endothermic, and Ea = +40kJ/mol. C. exothermic, and Ea = +40kJ/mol. D. endothermic, and ΔH = -10kJ/mol.
4. The molecular model shown is A. pentene B. butene C. propane D. butane 5. The compound with molecula formula C15H32 is likely to: A. be a member of the alkene homologous series. B. contain a triple carbon-carbon bond. C. have a name ending in “-ANE”. D. have polar molecules.
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10. During a chemical reaction, some chemical bonds are broken and others are formed. The correct statement is: A. energy is released by both making & breaking of bonds. B. energy is released when bonds are made, and absorbed when bonds are broken. C. energy is released when bonds are broken, and absorbed when bonds are made. D. energy is absorbed by both making & breaking bonds.
3. Allotropes of an element: A. contain identical atoms in different arrangements. B. are different atoms arranged in the same way. C. have the same physical properties as each other. D. have the same appearance and uses.
12. A major problem associated with large scale use of fossil fuels is: A. the toxic effects of CO2 gas. B. SO2 gas release which causes petrochemical “smog”. C. carbon monoxide destruction of the ozone layer. D. global climate change due to CO2 gas release.
6. Which of the following is NOT a property of the alkanes? A. soluble in water B. relatively low melting & boiling points C. non-conductors of electricity D. hydrophobic
13. The risk of an explosive combustion is increased by: A. increasing the particle size of solid fuels. B. decreasing the collision rate of reactant molecules. C. storing liquid fuel in larger tanks or drums. D. allowing build-up of dust or fibres in the air.
7. In petroleum refining, Fractional Distillation separates the mixture according to differences in: A. melting point B. density C. solubility D. boiling point
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C. CH4(g)
Energy Content (kJ/mol) 10 20 30
Part A
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19. (continued) The boiling points (oC)for some alkenes: ethene -104 propene -48 hexene 64 heptene 94 a) Plot the data as points on the grid provided. b) Use the graph to estimate b.p.’s for butene and pentene. c) Account for the fact that the alkenes have low b.p.’s relative to some other similar sized molecules.
14. The rate of a chemical reaction could NOT be increased by A. increasing the temperature. B. reducing the pressure of a reactant gas. C. adding a catalyst. D. increasing the concentration of a reactant solution. 15. Catalysts can: A. decrease ΔH of a reaction by providing alternative chemical pathways. B. speed reactions up by increasing Ea. C. lower Ea by increasing reactant collisions. D. cause different products to form, via different pathways.
20. (5 marks) a) What is meant by the property of “volatility”? b) Octane, a compound found in petrol, has a boiling point comparable to that of water, but is much more volatile. Explain why. c) Outline any safety concerns and precautions arising from the volatility of octane and associated compounds in petrol.
Longer Response Questions Mark values shown are suggestions only, and are to give you an idea of how detailed an answer is appropriate. 16. (6 marks) Using a named fossil fuel as an example: a) give the name, and formula for a significant compound within the fuel. b) justify the use of the term “fossil fuel”. c) explain how the fuel comes to have energy stored within it, including mention of the origin of that energy.
21. (5 marks) Give an outline of the key process in the refining of petroleum, including a list of some of the main products of the process. 22. (4 marks) In an experiment, a student burnt 2 identical pieces of coal; one in an open container, the other in a sealed container which had an adequate supply of air within it.
17. (6 marks) For each of the 2 main allotropes of carbon: a) list the significant physical properties of the substance. b) explain how a named use of the allotrope is linked to its physical properties.
The containers (including pieces of coal) were weighed before and after burning. One was found to lose mass, the other did not change.
18. (6 marks) a) Write the molecular formula for hexane. b) Draw the structural formula for pentene. c) Write the condensed structural formula for heptane. d) Name the following compounds: i) CH2CH(CH2)5CH3 H H H ii) C C C H
23. (9 marks) Write a balanced equation to describe: a) the complete combustion of butane gas. b) the complete combustion of ethene gas. c) the incomplete combustion of hexane, assuming that 2 different oxides of carbon are formed in equal molar quantities.
H
H
iii) C2H6
Identify which container lost mass and account for the different results between the 2 containers.
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24. (4 marks) During the combustion of petrol and diesel fuels both release approximately the same amount of energy (per unit of mass). However, petrol has a significantly higher ignition temperature. Sketch an energy profile diagram to compare these two fuels. No values are required, but the diagrams should be labelled to emphasize any important comparisons.
-1 100 -2 200
Boiling Point (oC)
100
19. (5 marks) Use this grid to help answer the question following.
1
2
3
4
5
6
7
8
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27. (5 marks) In the upper atmosphere is a region called the “ozone layer” where the gas ozone (O3) protects the Earth by absorbing dangerous U.V. radiation from the Sun. Certain human-made chemicals can release single chlorine atoms which react with ozone as follows:
25. (5 marks) a) Name a toxic pollutant associated with the use of a named fossil fuel. b) Write a balanced symbol equation to describe the production of this pollutant during the use of the fuel. c) Describe the steps that can be, or have been, taken to minimize the release of this pollutant into the environment.
Cl(g) + O3(g)
26. (5 marks) During your study of this topic you have carried out a “hands-on” practical exercise to observe the effect of certain factors on the rate of a chemical reaction.
ClO(g) + O2(g)
Then the ClO(g) reacts with the occasional single oxygen atoms that occur at this altitude: ClO(g) + O(g)
Describe the experiment you did to investigate the effect of either temperature or concentration on reaction rate.
Cl(g)
+ O2(g)
Overall, the result is the destruction of the ozone allowing more U.V. radiation to penetrate to the surface of the Earth.
Include a) a brief outline of the method used. b) description of exactly what observation/measurement was used to get a result. c) what general conclusion was reached.
a) In what sense can the chlorine atom (Cl(g)) be considered as a “catalyst” in the overall reaction? b) The diagram shows the energy profile for the overall reaction which can occur without the presence of chlorine. i) Describe this reaction as “endo-” or “exo-” thermic, and state the sign of the value for “ΔH”.
Energy Content
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Products
Reactants
ii) Alter the diagram to show the expected change when chlorine atoms are present.
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Answer Section
Worksheet 3 (continued)
Worksheet 1
q) self-sustaining r) hill/barrier s) ignition temperature t) oxygen u) mixed v) carbon monoxide w) soot/carbon x) carbon dioxide y) global z) Greenhouse aa) sulfur dioxide ab) Acid ac) carbon monoxide ad) nitrogen monoxide /nitric oxide ae) nitrogen & oxygen af) (petrochemical) smog
a) Potential c) carbon e) photosynthesis g) carbohydrate i) food k) petroleum m) Sun o) petroleum
b) chemical bonds d) Sun f) glucose h) sugars & starches j) oxygen l) fossil n) photosynthesis p) methane
Part B 1. a) 2Ca(s) + O2(g)
Worksheet 2 Part A a) 2.4 c) identical e) physical properties g) lubricant i) electric motors k) diamond m) jewellery o) 4 q) chains s) hydrocarbons u) alkenes w) -ANE y) CnH2n+2 aa) low ac) dispersion ae) similar ag) CnH2n ai) vaporize ak) mixture am) Fractional Distillation ao) condenses
b) 2H2(g) + O2(g)
b) graphite, diamond d) arranged f) flat sheets h) conductor j) lead l) tetrahedral n) drill bits/saw blades p) covalent r) double or triple t) alkanes v) carbon atoms x) single z) insoluble ab) polar ad) increases af) double ah) Homologous aj) high al) fractions an) drops
2H2O(g)
c) C2H6(g) + 7 O2(g) 2 d) C2H4(g) + 3O2(g)
2CO2(g) 2CO2(g)
+ 2H2O(g)
e) C5H12(g) + 8O2(g)
5CO2(g)
+ 6H2O(g)
f) C5H10(g) + 15 O2(g) 2 g) C3H8(g) + 5O2(g)
+ 3H2O(g)
5CO2(g) 3CO2(g)
+ 5H2O(g) + 4H2O(g)
2. a) C6H14(g) + 13 O2(g) 2 b) C6H14(g) + 8O2(g)
3CO2(g) + 3CO(g) + 7H2O(g)
c) b) C6H14(g) + 5O2(g)
3CO(g) + 3 C(s) + 7H2O(g)
d) C4H10(g) + 11 O2(g) 2
2CO2(g) + 2CO(g) + 5H2O(g)
6CO(g)
+ 7H2O(g)
3. a)
b)
small
Ea
d) 8
2. Decane: C10H22
e) 1
large
Ea
f) 6
CH3(CH2)8CH3
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C H
H
H
H
H
H
H
H
H
H
H
Energy Content
c) 3
Energy Content
Part B 1. a) 5 b) 4
H
2CaO(s)
large
ΔH
Ea
Energy Content
Part A b) new substances d) precipitate f) energy h) oxide j) gas l) break n) Activation p) releases
Preliminary Chemistry Topic 4 Copyright © 2005-2007 keep it simple science
ΔH
c)
Worksheet 3
a) disappear c) colour e) gas g) fuel & oxygen i) carbon dioxide & water k) kerosene/ wax m) reactant o) products
small
Products
ΔH positive Reactants
d) Diagram (b). To be a fuel, it must be exothermic. To have a high ignition temperature Ea must be large. 24
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Baulkham Hills High School SL#802445
Worksheet 4
19. a) & b) on graph
d) i) octene
ii) propene
4
5
6
7
8
0
22. The coal in the open container lost mass because the gas products of combustion (e.g. CO2 & H2O) escaped into the surroundings. In the sealed container there is no mass change because nothing escapes.
H
iii) ethane
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21. Key process is “Fractional Distillation” in which the mixture is separated according to boiling points. Heated crude oil is injected into the “fractionating tower” and the vapour mixture rises through the tower. Temperatures fall with height and each “fraction” condenses at different levels according to b.p. Important fractions include: lubricating oils, fuel oil, kerosene, diesel, petrol and petroleum gas.
17. Graphite: a) high m.p., electrical conductor, “slippery”. b) its slipperiness makes it useful as a dry lubricant in (for example) door locks. Diamond: a) high m.p., clear sparkling crystals, extremely hard. b) its hardness makes it useful for high performance drill bits and saw blades. H H H H H 18. C C C C C H a) C6H14 b) H H
2
20. a) A substance is “volatile” if it readily evaporates at room temperature, or at temperatures well below its boiling point. b) Octane, like all alkanes, has only very weak dispersion forces between its molecules. (because its intra-molecular bonding is non-polar) So, even at temps well below b.p., many molecules have enough energy to escape into the gas state. c) Since it is volatile and combustible, and the vapour can form explosive mixtures with air, safety is a major concern. Precautions include: • storage must be in sealed containers. • fuel transfer must occur outdoors, with no sparks, flames or electrical devices nearby.
Part B Longer Response In some cases there may be more than one correct answer possible. The following “model” answers are correct, but not necessarily perfect. 16. Natural Gas (or L.P.G.) a) is largely composed of methane, CH4 b) It is a “fossil” fuel because it is a chemical derivative from ancient living things. The life forms were fossilized in sediments and their cell chemicals altered by heat & pressure in the rocks, forming petroleum. c) The energy in it came originally from the Sun. It was captured by ancient plants during photosynthesis, and stored in the carbohydrates in living cells. Fossilization has modified the chemicals, but the energy is still there.
H
1
c) All the bonds within the molecules are non-polar so there are no dipole-dipole attractions between molecules. The only forces are very weak “dispersion forces”. The molecules are easily separated from each other by heat energy, so m.p. & b.p. are relatively low.
Multiple Choice 5. C 9. C 13. D 6. A 10. B 14. B 7. D 11. A 15. C 8. C 12. D
c) CH3(CH2)5CH3
butene ≅ 0oC
No. of Carbon Atoms
Practice Questions Part A 1. D 2. C 3. A 4. B
pentene ≅ 30-40oC
-2 200
Boiling Point (oC)
100
a) colliding b) a large piece of wood c) oxygen d) ignition e) fuel & oxygen f) explosion g) dust h) safety i) wheat, cotton, coal j) Kinetic k) moving l) move faster m) kinetic energy n) proceed faster o) faster p) collision q) activation r) concentration s) collisions t) pressure u) surface area v) breaking it into smaller/finer particles w) increases x) being consumed or changed y) enzymes z) ΔH aa) activation energy ab) energy ac) reaction rate ad) adsorbing ae) likely/frequent af) strain ag) broken ah) iron ai) zeolite aj) Catalytic ak) alternative chemical al) regenerated
-1 100
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23. a) C4H10(g) + 13 O2(g) 2 b) C2H4(g) + 3O2(g)
2CO2(g) + 2H2O(g)
c) C6H14(g) + 8O2(g)
3CO2(g) + 3CO(g) + 7H2O(g)
4CO2(g) + 5H2O(g)
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24.
Diesel
26. Effect of temperature. a) Method outline: • set up 3 identical test tubes of acid; one in an ice bath, one at room temp., one in hot water bath. • Identical pieces of magnesium were dropped into each, in turn. b) The time taken for the Mg to disappear was measured. The faster the reaction, the shorter this time was. c) Reaction rate increases at higher temperatures.
Petrol Petrol’s Ea is larger; = higher Ig.Temp.
Ea
Energy Content
Ea
ΔH
ΔH’s same
ΔH
27. a) It is a catalyst in that it is involved in a series of reactions, but it is “regenerated” at the end, and can be used over and over. It is not consumed or permanently changed by the reactions. b) i) Endothermic, ΔH = +ve ii)
25. a) Sulfur dioxide b) Fossil fuels contain small amounts of sulfur compounds. When burnt: S + O2(g) SO2(g) c) Petroleum fuels can be refined to remove sulfur compounds before combustion. Coal burning is the main source. The exhaust gases are “scrubbed” to collect all the SO2. This is used to make sulfuric acid for industrial use... a useful by-product.
Energy Content
With Cl atoms; Ea reduced
Products
Reactants
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