IG Chemsitry Papers
May 3, 2017 | Author: Murad | Category: N/A
Short Description
IGCSE Chemistry Notes. Cambridge Intenational General Certificate for Secondary Education (Chemistry) O-Levels comlete S...
Description
.I.G.C®S.E
h
i
Prof. Dr. Fathy Abdel Galil
-
:
·.
Chemistry IGCSE
(Dr.rathyADC\el uatuJ
Contents Ef'~
Topic L
Topic 2.
. To.pic 3.
Topic 4.
Topic 5.
Topic 6
The particulate nature of matter .........•.....................................•. States of matter·.......... : ............................................................. . . partie . 1e theory ................................................................ . K.metJc Diffusion ............................................... , ............................... . Experimental technique ..•......................................................... Laboratory apparatus ................................................................. . Safety in laboratory .................................................................. .. Methods of purification ............................................................. .. Separation of mixtures ............................................................... ·. Chromatography ~ .................................................................... .. Atoms, elements and compounds .............................................•.. Atomic structure ..................................................................... . .Radioactive isotopes ................................................................ .. . Electron distribution ............................................. :.. ................. . lons and ionic bonds ................................................................. . Molecules and covalent bonds ..................................................... . Macromolecular (giant) structure .................................................. . Stiochiometry ........................................................... .'.......... .. Chemical formula .................................................................. ·.. . Chemical equations· .................................................................. . Chemical calculations .............................................................. .. The mole .............................................................................. .. Empirical formula and molecular formula ........................................ .
1
1
2 6 8 8
10 12 12 14
16 17 19 20 22
25 28 30
30 33
35 37 ·44
46
The periodic table ................................................................. .. Trends in group I [The alkali metals] ............................................. . Transitions elements ................................................................. . Trends in group VII [The halogens] ................. ·.............................. . Nobel gases .......................................................................... .. Test and collection of gases ......................................................... . Oxidation and reduction ............................................................. .
48 51 52 55 56 58
Electricity and chemistry [Electrochemistry] ................................. . Electrolysis ............................................................................ . Applications of electrolysis ........................................................ ..
6D 62 67
Chemistry IGCSE
Topic 7.
Topic 8.
Topic9.
(Dr.FathyAbdel Galil)
Acids, bases and salts .-............................................................. . Acids ............................................................ , ...................... . Bases ................................................................................... . The pH scale .......................................................................... . 1vfaking salts ........................................................................... . Types of oxides ...................................................................... :. Identification of ions ................................................................. .
69
69 71 72 73 77 79
Metals and reactivity series ..................................................... ... Alloys .............. c •. : •.. ; •..••...•••..•.•..•••.•..•.•••.•••••••••..•..•..••.••••.••.•. R.eacfIVI"ty senes . ....................................................................... . Extraction of iron ................................................... ·.................. . Steel making .......................................................................... . Extraction of zinc ..................................................................... . Rusting ................................................................................. .
90 91
Chemical changes Production of energy ................................................................ . Electricity from chemical reactions .............................................. .
95 97
Topic 10. Chemical reactions ····································'····························· Speed of reactions ...................................................... . :........... . Reversible reactions ................................................................. . Chemical equilibrium ............................................................... .
81 81 83 88
89
99 99 106 106
Topic 11. Air and water •••........••..•..••.••.•.•..••..•••••..••••.•.•••..•••.•••••..•••..•••• Purification of water supply ...................................................... .. .. f. ·. ComposttiOn o atr ................................................................. .. Manufacture of ammonia .......................................................... . Fertilisers ................................................ : ............................ . Air pollution .......................................................................... .
108 108 109
Topic 12. Sulphur ............................................................................. . Making sulphuric acid ............................................................. .
116
Topic 13. Carbonates ........................................................ ,..................
118
Topic 14. Organic chemistry ................................ ••.•...•.•••••• .•••..••••. .• ..... ..
120
111 ll2 113
117
- -··
Topic 1:
[Dr. Fathy Abdel Galil]
- 1-
Chemistry IGCSE
. THEPARTICULATENATUREOFMATIER
All matter is made .up of tiny particles, These may be atoms, mole_cules or ions. Using the idea of parti?les, we can explain the way matter behaves.
States of-Matter All matter can exist in three ·states- solid, liquid or gas. Solids have a fixed shape. Liquids have no fixed shape, but their volume is fixed. Gases have no fixed shape or volume. They spread out (diffuse) to fill all the available space. Unlike solids ang liq~ids, gases are easy to compress into · smaller size. The state of a substance can be changed by heating or cooling. 6Jt.eez.i.ttg
c.onde.Jt6.Wg
me..Wng
so LID
f W.r
)-
f"i
y
w
bo-U-Wg
0 0
.JDI
0 0
0
LIQUID
u
0.. GAS
iodina cry--< --'.r"[
( )[ " J: "'[_ J:
J[
0 0 0
0
0
0
0·
..
.'
Arrangement of Particles
close together in a regular arrangement
still close tog ether in an irregular arrangement
particles far apart
Movement of Particles
only vibrate in their places
can move around slowly
free moving rapidly
Kinetic Particle Theory 1. All matter is made up of very small invisible particles (atoms, ions or molecules). 2. The particles move all the time. The higher the temperature, the faster they move. 3. Lighter particles move faster than heavier ones .
.J
4. In a gas, there is relatively large distance between the particles. They are free to move anywhere [i.e. a gas can fill any v.olume].
Using the idea of particles and kinetic theory, we can explain the changes of states of matter and diffusion .
...
;·..
- 3 -
(Dr. Fa thy Abdel Cali!)
Chemistry IGCSE
Changes of State
Mel.:thtg When a sol~d ~s heated, ~ts part~cles ge~more ene,gy and v~brate more. Th~s makes the sol~d
expand.
At the melt~g po~nt
the part~cles v~brate so much that they oreak-aYay from the~r pos~t~ons. The sol~d becomes a liquid.
nrraan·a· o·' " ........ ".,
'offifi8§.
heat
heat at
O:ffil..DOG DQO: · "0'0' .
:a.: 'G ' .........
melting point
·-
........
~
!he partici!"S vibcate.~~T~
a liquid is formed
Mel.:thtg Po.zn.t ~s ~nto
a
the temperature at
wh~ch
the
sol~d
melts,
~.e.
changes
l~quid.
Bo..
·'C'.'k\0
.................................................................................................................................................................... [1 1 '
Safety problem z .....
.'
0
I
-"
l
\
?.:::.::.......... ..l.R:?.!L"f..:..........::o:.......:................~..~'............ :::-~.:::::..............................
..................................................................................................................................................................... [ 1]
- lZ -
Chemistry IGCSE
(Dr. Fathy Abdel Galli)
Methods of Purification
17'--filter paper · fimnel residue.
a) To .5epaJta..te a. 6Mm a. Uqr.Ud
Mild (J.n.6 o.tub.te)
Filtration The solid is left on the filter paper as the residue while the liquid passes through.
Just by pouring the liquid insoluble
off from the solid,
b)
To
ob~
substance
a. .5o.Ud 6Aom a. .5o.f.utLon
t:===::J-..J-salt solution
A solution consists of a solut.e dissolved in a solvent, Sea water is a solution
solution~-_____ evop oratlng :- .. -~_.... ba.s: 1n
/ gauze·
---
of salt (sodium chloride) in
K~~r
water. By heating the solution, water evaporates and solid ·salt ·is left,
! -----
c ..... -
'%?~~~-
:C,c;eifc!c"(
I
I ~
I
__.- ~'~ I
·
tripou·
.f1__/ / ,
u
The solution is evaporated to the crystallization point; i.e, the point at which crystals of solute will form on cooling the solution to room temperature and can then be f"iltered out, and dried.
What method could be used to show the crystallising point had been reached •
'I'
'""' -'r ,) ~,- t I_ ' -::,•.::-,,_i \ - ) v •· · ••••••••'-r•:f·••••••••••·•••••••••••••••••••••••••••••••••••••••••••••••• v
•••••••••••••••••••••••••••••••••••••••••••••••••••••••• '
........... t
••••
- 13 -
Chemistry IGCSE
c)
To
a 6olvent
4epaAaXe
6~om
(Dr. Fathy Abdel Gal!!)
a 4otution
Distillation ------------
_
tharmometor
When the solution ia boiled, the solvent changes to vapour, the vapour passes down· a condenser where it is converted back to liquid and is collected as the distillate.
sea water
d}
To
6 epaAaXe
cold W;ttcr in pun distilled
HEAt
:iJJJo .Uqu).cL6
water
Fractional distillation ----------------------Miscible liquids (i.e. ones that mix together completely) may be separated by fractional
distil~
thermometer
lation provided that their boiling points are different, e.g.
Cr;Jolest paQ:·
of column·---
mixture of ethanol and water, The mixture in the flask is heated so that it boils. Both
cold water In fractionating ---W~
column filled with tilc:ss rods
go up the fractionating column. Ethanol has a lower boiling point
Frt~ctlons
collected at
(78°C) than water (i.e, is more
lower temperatures.
volatile), The liquid with the lOI.ta.nc.e -L.6 puJte. How
\\c-.~
0"-·fl"'--'
• • • 'l. . . . . . ~ •• ~--u··························::.··t···I
Electron diagram~ ( 1 do·t and cross 1 type diagrams) or just a knowledge of the number of shared electrons in the outer shell can be used to work our formulae.
Complete 'the follow·ing table
Symbol
Va1.enc'y
Carbon
c
4
Nitrogen
N
Oxygen
0
Sulphur.
s
Hydrogen
'H
Chlorine
Cl
Bromine
Br
Ee.emen.t
What are the formulae· of the follwing molecules ·: Ammonia Methane
Hydrogen chlorid.e
........ ......... . ~
Bromine
...................
Chemistry IGCSE
(Dr. Fathy Abdel Galil)
- 31 -
._;_;·
2)
The. FolUnu.C.a.e. o o 1on.i.c Compound& We cannot talk about molecule!! of·ionic compounds. In ionic crystals we have an alternating arrangement of p_ositive and . negative ·ions.
More about ions :
----------------Some elements can lose or gain electrons and change into atoms with positive. or negative charges. These atoms are ··called ions·. Some examples are : Na+ Ca
2+
Cl.,..
..
sodium ion
)(+
potassium ion
calcium ion
A13+
aluminium ion
02-
oxide ion·
. cnlorid~. ion
Spmetimes, groups of atoms can have charges too~ and beh,ave
as ions. The sulphate ion, SO
2-
is made of one sulphur atom 4 and four oxygen atoms, bonded· t.oge'ther; The whole group-of atoms has two nega.tive charges. The nitrate ion,
No 3 - ,
the carbonate ion,
co 3 2- ,
and the
ammonium ion, NH +, are other examples • 4 .Here is a lis't of· most of the ions that you are likely to meet and will need to know :
Valency
I
2
3
Positive ions
Negative ions
H+. u+ Na+ K+ Ag+ NH 4.+ Ca 2+ " .. 2+ Mg. Bal+ Pb2+ Cu-7~. Zn2+ Fea·
hydrogen ion lithium ion sodium ion potassium ion silver lqn ammonium ion •
hydroxide ion • chlo'ride ior, bromide ion iodide ion F- fluoride ion Nol- nitrate ion •
calcium ion magnesium ion ·barium ion iead,,ion copper(II) ion zinc ion irori(II) ion
S2~
FeH Al 3+
iron(III) ion aluminium ion
oHcrBrI-
o2-
so/-
sulphide ion oxide ion· ·sulphate ion
..
col2- carbonate ion •
PO/- phosphate ion
•
(Dr. Fa thy Abdel Ga!il}
- 32 -
Chemlstr)' IGCSE
Predicting the formula of ionic corr Jounds In an ionic cempound, ·th
ions are attracted to each other
-
by their opposite charges. The number of
+ charg.es
is alYays
equal to the number of -charges. Knowing this, the formulae of ionic compounds can be written down. Look at these examples in which the.numbers of positive ions· and negative ions are balanced to make the charges· the same.
·ammonium sulphate
aluminium oxide
magnesium oxide·
this is then written &l .12-l_ Mg ~ Mg~0 2 - - - - - MgO
Remember i) Compounds ending in 1 ide 1 are made of just 2 elements only. ii) Compounds ending in 'ate' have a third element, 'oxygen'.
Question: Write down the formula for each ofthese ionic compounds: 1. sodium chloride 2. aluminium nitrate 3. zinc chloride 4. sodium sulphate 5: copper (II) sulphate 6. zirlc carbonate 7. alllrniniurn carbonate 8. pctassium iodide
9 Cll!ciurn oxide
... ... .. .. ..
10. amrnonium.carbOnate
............
............. ... ... ... ...
11. calcium hydroxide 12. ammonium phosphate
............. ...............
.. .......... ............ ... ... ... ...
1Liron (JII)..oxide.
...............
............ ... -......... .............
16. iron (JII) chloride
............. .:. ...........
17. aluminium hydroxide
.... ·- ··- .....
18.. silver nitrate
.............
~
14~ potassiUI\1 carbonate
15. zinc sulphide
............
(Dr. Fathy Abde! Galli)
Chemistry IGCSE - 33 -
When a che'mical reaction occurs, substances react together. These reacting substances are called tne reactants. They form new chemicals called the products. How to write the equation for a reaction
These are the steps to follow when writing an equation l-
Write the equation in words (word equation).
z~
Now write the equation using symbols. Make sure the· all correct~
formulae are·
3-
th~
Check that the equation is balanced, for each atom in turn;
Make
b~e
you do
no~
change any 6okmulae.
Example l. When hydrogen burns in oxygen or air, it forms water
+
oxygen
H2
+
02
2 H
+
02
hydrogen
2
water
> 2
H H
2
2
o
o
~V!!g111~0 :tha.:t .the c.ommon ga.eb (n.Ultogen, oxygen, hydMgenJ and ~lie MlogeM Me dULtom.i.c. Th.U, mean.b .:tlra.t .they .ex..U..:t a. motec.ul.eb c.ol1.:ta-i.JU.ng wo -
+ ........... +
• • • • .. .. • • • • • • • • • • • • ..
..................... -........................ . water
[Dr. Fathy 'Abdel Cali!)
- 35 -
Chemistry IGCSE
Chemical Calculations
Relative Molecular Mass [Mrl Each element has its own relative atomic mass [Ar ]. This is the average mass of its isotopes compared with the mass of- a standard atom of carbon, l~c. Compounds have a relative molecular mass (sometimes called formula
mass). The relative molecular mass (formula mass) of a compound is founq by adding up the relative atomic masses of the elements in the compound according to the number of each element's atoms.
Look at these examples: 1 Sodium chloride, NaC! 1 X 23 1 atom of Na 1 atom of Cl 1 X 35.5 relative molecular mass
2 Calcium chloride, CaC1 2 I X 40 I atom ofCa 2 X 35.5 2 atoms of Cl relative molecular mass 3 Zinc nitrate, Zn(N0 3)z 1 atom of Zn 1 X 65 2 atoms of N 2 X 14 2 X 3 X 16 6 atoms of 0 relative molecular mass
-
-
-
-
23 35.5 58.5
40 7I 111
+
.65 28 96 189
+
+
Calculate the relative molecular masses of the following compounds : l.
3.
Carbon dio:cide,
co 2
Sodium carbonate, Na C0 2 3
Chemistry IGCSE
(Dr. Fathy Abdel Gali!)
- 36 -
Percentage Composition by Mass
Sometimes it is important to know the exact composition of a compound. For example, all bags of fertilizer must show the percentage of the clements nitrogen, phosphorus and potassium they contain on the outside.
The % of an element by
Ar of the element x no. of its atoms
mass a compound in
Mr of the compound
Look at these examples:
1 The % mass of calcium in calcium carbonate, CaC0 3" Ca 40 100 = 40 + 12 + 48. X IOO = C~ + C + 3XO X
=
40 %
40% of the mass of calcium carbonate is calcium.
2 The % mass of nitrogen in ammonium nitrate, NH.N0 3 2X N 28 = X 100 = ZXN + 4XH + 3XO 28 + 4 + 48 X 100
---==---
35 % of the mass of ammonium niuate is nitrogen.
=
35%
X 100
........ .... ,, .. ~
.
.
(Dr. Fathy Abdel Galil)
- 37-
Chemistry !GCSE
THE MOLE
A mo.e.e. (mo.tM. ma.o.6
r o6
any .6Ub.6.ta.nc.e. )_;,
w
Jr.e-
.f.a.:Uve a.tom.Lc. ma.o.6 [Air.) OJt Jtel.a.tive. mo.tec.u..ta.lr. ma.6.6 [MltJ {.e!U.c. {M l
.too/u, .Uk.e a. me.ta..t, .{..e . .oiU.ny bu;t
doe.o no.t belutve a..o a. me.ta..t. He
H
"_:It,._,_ · .,.j.·. ".>0 \ ~01.
~oy.. :.r
2.
Elements of the same group have similar properties (the same no. of electrons in the o.utershell).
-47-
Chemistry IGCSE
3.
(Dr. Fathy Abdel Galil)
The valency (combining power) increases with_the group till eroup -4, then decreases.
4.
Group number
1
2
3
4
5
6
7
0
Valency
1
2
3
4
3
2
1
0
The reactivity of metals increases down a group Rb
5.
> K > Na
The reactivity.of non-metals decreases down a group
-
'·
> Br > I
F > Cl 6.
> Li
Metals react with non-metals to form ionic compounds. Non-metals either join with other non-metals. to make
:C{)I!.!I.te:n;t.
C:am)'JoWttll. or with metals to make .0J11ic. c.ompaund6.
Relation between group number and number of valency electrons.
I
II
u•
Be
0 ,.---
0
N•• M~·
K•
c.'•
Fe1+
He
III
IV
v
VI
VII
B
c
N
o'-
p-
Ne
!Jl+
Si
p
s'- a-
Ar
Br-
Kr
.Cu2+ Znl+
I-
Ag•
Ba 2 +
Pbl+
r
-~
Group I elc:mcnt5. all h:ave l electron to lo$e from their outer shells. They form + ions.
I
Group '\_'II elements h1ve 1 electrons in their outer 'hells.· They need to gain one electron to.fill their outer Shell and form 1 ne&:uive ion.
Group II dement' lll h:rve 2 electrons; in the: outer .dlell. They form 2'*' iOJ'lt..
Group VI dements hne 6 elcciron11 in their outer s:hcl!s..They need' to gi.in 2 dcctron~> to
Group lll dementt have 3 dectroos to lose. They (arm 3 ~ ions:.
fUI their
shell~. They
form 2- ions:.
'
-48-
Chemistry IGCSE
(Dr. Fa thy Abdel Galil)
Group I elements: the 'alkali metals' Density
Melting point
Name
Symbol
Lithium Sodium Potassium Rubidium Caesium
Li Na
1.
K
Rb Cs
rc
fgcm-J
180
0.53
98
Group lions Lithium ion
u+
0.97
Sodium ion
Na+
64
0.86
Potassium ion
K+
39
!.53
29
1.90
Rubidium ion
Rb+
Caesium ion
cs+
They are called th!'. alkali metals because they react with water to form alkaline solutions.
2.
The outermost shell contains one electron.
Where are Group I metals found? The. me..ta.to .thi!Jil!>e..tvu do na.t ac.c.wr. .-potassium oxide
Metals all react easily with cold water to form alkaline solutions.
The heat from the reaction melts the metal as it reacts:
sodium
+
water
-+
sodium hydroxide
+
hydrogen
2Na(s)
+
2H 2 0(!)
-+
2Na®H(aq)
+
Hz(g)
potassium
+
water
-+
potassium hyclraxide
+
hydrogen
2K(s)
+
2H 2 8(!)
--+
2K@H(a:
Sodium Potassium
lithium 2Li(s)
+ +
chlorine Cl 2 (g)
-+ -+
· lithium chloride 2LiCI(s)
sodium 2Na(s)
+ +
chlorine
-+
Cl 2 (g)
-+
sodiuni chloride 2NaCI(s)
potassium 2K(s)
+ +
chlorine Cl 2 (g)
-+
Rubidium
n
Caesium
potassium chloride -+ 2KCI(s)
So you can see these Group· I metals have similar properties. Lithium is said to be the, least reactive of them, because it reacts the most slowly".· Potassium is the most reactive of the three.
Why they. ha.ve. ·~.i.mliaJI. pltopeJt.;U~ ? In the elements of Group I, all the atoms have.! electron in the outer shell. That is why· the elements hehave"in a similar way.
Elements behave in a similar way if" their atoms have the same number of outer-shell electrons.
Group I compounds Metal
Chloride
Bromide
Nitrate
Sulphate
Carbonate
Lithium
LiCI
LiBr
LiN0 3
Li 2 S0 4
Li2 C03
Sodium
NaO
NaBr
NaN0 3
Na 2 S0 4
Na 2 C0 3
Potassium
KC)
KBr
KN0 3
K2 S0 4
K2 C03
GJtoup r ~a.Lt6 {c.he.o:Ud~ I n..UJr.n.;t~ I c.aJtbona.te..o 1 e..tc.. J ivte. w!U.te., .-Bromine is very volatile [has low b;p. 59°C}. This means ihat it easily turns into a gas. >-
Iodine is a dark grey solid, when warmed, it turns into a purple vapour [it sublimes]-
>-
Aqueous solution of iodine is brown. The halogens all exist as diatomic molecules. The tWo atoms in the molecule are joined by a covalent bond. Cl
- Cl
. a chlorine molecule, Cf2
Uses of Chlorine: • Making bleach .. In water purification [kills bacteria] "' Maldn;; PVC (insulation for electric wires)
..
-53-
Chemistry !GCSE
(Dr. Fa thy Abdel Galil) ·:·:
Chemical Properties .
Reaction with .metals:
Halogens react wi~h. most metals to make salts:
+ +
sodium 2Na(s)
Reaction with.indicators:
-+ -+
sodium chloride 2NaCJ(s)
Chlorine gas dissolves in wa\er to make a powerful bleach. Damp indicator paper is quicldy bleached with chloiine., . -Test for Cl
Displacement reactions:
chlorine Cl 2 (g)
2
gas :
Chlorine bleaches damp litmus paper.
Chlorine can displace bromine and iodine from a solution of their salts: chlorine gas-·===\\
v ~I>Otassium Iodide solution ·1oame belng forroed
chlorine
+
Clz(g)
+
potassium -+ iodide 2KI(aq) __,.
[colourless]
Ionic equation:
+
Clz
. chlorine
+
Cl 2 (g)
+
zr
In o:thvr.
WOJtd.l,,
Clz
7
potassium -+ bromide 2KBr(aq). -+
[colourless]·
ionic equation:
->
+
2 Br·
-> 7
iodine
+
I,(aq)
+
potassium chloride 2KCI(aq)
[.brown]
Iz
zcr
+
bromine
+
Br,(aq)
+
potassium chloride 2KCl(aq)
[red]
Brz
+
zcr
a. mOJte Jtea.c:Uve hai.ogen w.Ut dUp.f.a.c.e a. leo Jtea.c:Uve ha..e.ogen
t)Jtom ffi compound;,.
Halogens have similar properties because their· atoms all have 7 electrons in. the outer shell.
-54-
Chemistry I GCSE
(Dr. Fathy Abdel Gs!il)
Nobel Gases :
Helium, Neon, Argon, Krypton, Xenon 1.
The nobel gases in group 0 are very unreactive (inert), they have complete outershell of electrons.
2.
They do not usually form compounds and exist as isolated single atoms (i.e. monoatomic).
neon etomi"::\\\
~((G
N•
hollum atocn• ·- 6oiL dJty,~ng a. gaJ, w.Uh concen:tJur;te.d a.cJ.d )!, co!LILe.c.t f 2
1
~===:;, r;=='d=r=y==~
concentrated sulphuric acid
concentrated sulphuric acid
2. Ammonia gas is dried by passing it over solid calcium oxide [drying agent]
>- [Conce./Lt!ta.te.d .sul.phU)()_c a.c
Reduction is defined as the removal of oxygen. Reduction and oxidation always take place together in a reacticn. So the reaction is called redox reaction.
Question: Which substance is oxidised and which is reduced in the following . reactions, mention the reducing and oxidising agents.
----7 2Fe
Common reducing agents
hydrogen, carbon,
+ 3CC)z
:
carbon monoxide,
reactive metals
-58-
Chemistry IGCSE
(Dr. Fa thy Abdel Ga!i!)
)> Since hydrogen is a common reducing agent, the addition of hydrogen is a reduction reaction, and the opposite, i.e; removal of hydrogen is oxidation. In which change shown has nitrogen been reduced?
B nitrogen, N2
c
~----------~
N02
D
)> Redox in terms of electron transfer [01 L]
[RIG]
Oxidation
Red·uction
Is
Is
Leiss of electrons
Gain of electrons
From the definitions above, it follows that:
Reducing agents give electrons. Oxidising agents accept electrons.
Consider the reaction of magnesium with oxygen : magnesium
+
oxygen·
--+
magnesium oxide
2 Mg
+
02
~
2 MgO
The ma.gne;,.I.wn ).), oudA.ud a.nd .the axyge.n ).), Jte.du.cvi. Vwring .tw Jt.e.a.Ul.ott, .two el.ecbw 111> a.Jt.e. .til.aiU 6e.Med filtom ma.g lteJ>.i.wn .to oxyg e.n. Ma.gne;,.f.wn I.on a.nd oude ion aM 6oJtmed.
0 +
Mg
2+ Mg.
2 e
oz-
+ 2 e
Ma.gne;,.f.wn, w/U.c.h ).), oufued, .to~>e;, el.ectJto111>; oxygen, w/U.ch ).), Jte.duc.ed, ga.i..1u el.ectltoJ1-6.
-59-
Chemistry IGCSE
[Dr. Fathy Abdel Galil]
Oxidation States [Oxidation Numbers] The oxidation state of iron in iron(II) oxide is 2 and in iron(III) oxide is 3. The oxidation state of manganese in manganese(IV) oxide is 4 and in · ·potassium manganate(Vll) is -7. The oxidation state of chromium in chromium(III) oxide is 3 and in potassium dichromate(VI) is 6.
Change in oxidation state )>
Oxidation involves an increase in oxidation state, e.g.
)>
Reduction involves a decrease in oxidation state, e.g.
Oxidation and Reduction [summary] with respect to: O!fgen hydrogen electrons oxidation state
oxidation addition of oxygen removal of hydrogen I loss of electrons [OILJ . increase in oxidation state·
reduction removal of oXygen addition of hydrogen gain of electrons [RIG] . decrease in oxidation state -
Tests for oxidising and reducing agents a) Oxidising agent (oxidant) + potassium iodide solution The colour changes from colourless to brown [h]. b) i. Reducing agent (reductant) +.·acidified potassium magnate (yii) solution : The colour changes from purple to colourless. ii. Reducing agent (reductant) + acidified potassium dichromate (VI) solution : The colour changes from orange to green.
- 60-
Chemistry IGCSE
Topic 6.
(Dr. Fa thy Abdel Galil)
ELECTRICITY AND CHEMISTRY [ELECTROCHEMISTRY]
Conductors and Non-Conductor:; Solid substances that allow electricity to flow through them nre
called conductors, All metalo are good conductors of electricity. why., Substances that do not allow electricity to flow thr·ough them are called non-conductors or insulators. All non-metal aubatancea such as glass, plastic, wood, and-rubber
ar~
insulators. There is, how-
ever, one very important exception, Carbon, in the form of graphHe, is non-metal, but a very good conductor of ele.ctricity. why ? Semi-Conductors,
They are substances which contain no "free electrons" and no "ions" bu·t allow very small cunerit ~O··'pass through them, i.e. they conduct weakly e.g.
c,Wc.on. ·
. Electrolytes and Non-Electrolytes :
. Liquids that conduct an ele.ctric cun:ent are called electrolytes. Those that do not conduct are called non-electrolytes. Electrolytes are substances that contain ions which are fi:ee to move about, so they are ionic compounds that are molten or are dissolved in water, Covalent compounds such as ethanol, pure water.or sugar solution are non-electrolytes. The following figure shows the apparatus that could be used to see if a liquid is an electrolyte .
.-----1
F
a battery A lamp. If it glows, electricity must be flowing through the liquid.
electrode called cathode 8
electrodes. Generally made of graphite or :::::::H-+--1 unreactive metal like platinum
-
Can you suggest why -ve ions are called anions and +ve ions are called cations? ........................................... .
.. ! .
. ••••••••••••
0
0
0
••••••
0
••
0
•••••••
0
0
•
0
••••
0
•••• ' •••••
0
•
0
0
....
Chemical changes take place at the electrodes. At the cathode, +ve ions gain electrons and become -atoms or molecules. At the anode,. -ve :!:ons lose electrons and become atoms or molecules.
The electrons flow in the external circuit from the anode into the cathode. a battery
r-~--t'I t------, electron flow
electron flow
e
cathode ·.
:_·anode
(£)
·negative ions lose electrons at tho"· anode
electrolyte
.
.,
/
-62-
Chemistry IGCSE
(Dr.FathyAbdel Gali!)
Electrolysis of Molten Substances using Carbon Electrodes The compounds are decomposed into their elements. The rules for the . electrolysis of molten compounds are : Molten Compound •::
At the anode + Non-metal
At the cathode Metal
> Electrolysis of molten lead bromide [PbBr2] +
-
a battery ;.·
carbon anode +
carbon cathode -
bubbles ~F brown gas {bromine)
dlr.ap.5 a 6 moLten lead ·(gJte.y} .
~··.
-
molten lead bromide
t
heat
>:Explain. why the lead bromide needs to be melted for electrolysis to .
:take place ........................... ·.................... .
> The. electrode reactions are summarized as follows : Electrolyte Lead bromide, PbBr2
At the cathode Pbt+ + 2e- ~ Pb
At the anode + 2Br- ~ Brz + 2e·
> Complete the following table: molten electrolyte. lead bromide sodium chloride . magnesium fluoride aluminium oXIde
Product at cathode lead .. . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . ..
· Product at anode bromine
t-
................... ..................
...................
-63-
Chemistry IGCSE
[Dr. Fathy Abdel Galil] ..-._
Electrolysis of Concentrated Uydrochloric Acid: · At the anode +
At the cathode -
Electrolyte
.
2W (aq) +
Hydrochloric acid, HCI (aq)
2 e- -7 H2 (g) 2Cr (aq) -7 Ch(g) + 2e··.·.
Electrolysis of Aqueous Solutions: · The products of electrolysis of aqueous solutions are difficult to predict than molten compounds. This is because as well as there is a cation and a anion from the salt, there is a cation (H") and a anion (OH) from the water.
Electrolysis of Concentrated Aqueous Sodim Chloride :[Brine} : At cathode. ~
Atanode +
,___
__.---,
LNa+ - - - - - _ --- \ __ d-;
FromNaCL
-- oH_., /
-~
1r '
~-,--
·..-.+ ' , ~
-- --
---
I
,
--" Only F takes part in electrolysis [H being lower in the reactivity ·series than Naj
I2 F
(aq) + 2 e· -7 H 2 (g)
.
f
-- --J Only Cr takes part in electrolysis because it is present in much greater concentration than OH
jz Cr{aq) -7 Cl2 (g)'+ 2 e·
/
This leaves a solution containing sodium and hydroxide ions (a solution of sodiwn hydroxide, NaOH). The solution is alkaline (i.e. turns red .litmus paper blue). Compare the products of e./ectrolysis of: (i) Molten sodium chloride (ii) Concentrated aqueous sodi!fm chloride )>
-64-
Chemistry IGCSE
[Dr. Fathy Abdel Galil]
Manufacture of Chlorine and Sodium Hydroxide : The method is based on the electrolysis of concentrated aqueous sodium chloride (brine)•. brine
chlorine
lui~_____;
...
..... ...
.·.·. ·•· ~
pos!Uve eleclrodo
aqueous sodium hydroxide
neg alive
elec[rode
Predicting the Products o[Electrolysis o[Aqueous Solutions :
)>
)>
At the cathode If the metal is higher than hydrogen in the reactivity series, then hydrogen gas is produced If the metal is lower than hydrogen in the reactivity series, then the metal is deposited (formeq}.
The reactivity series
Na Ca Mg AI Zn
above hydrogen, so hydrogen gas is evolved at the cathode
Fe Pb Cu
Ag least reactive
Au
below hydrogen so metal deposited at the cathode
)>
)>
At the anode + For concentrated solutions of chlorides,· chlorine gas is produced For sulphates and nitrates, oxygen gas is produced
-65-
Chemistry IGCSE
[Dr. Fathy Abdel Galil]
Electrolysis of Aqueous Copper(ll)sulphate : Using carbon electrodes (i.e. inert)
-{ Cu +) ....
--so - -- ----'
• - ..:- 1
....
--· -· -· ·
2
FromCuS04
Atanode +
~
Atcathode
... -
2- I
4
\.-- /
1-
....
,rr\-·-·
FromHzO
I
/ 10Ir' 'r
-----
Onlythe copper ions take part in the electrolysis.
Only the OH ions take part in the electrolysis.
Cu 2+ (aq) + 2e· -7 Cu (s) Copper deposited on the cathode (as brown deposit)
Oxygen gas is given off
The solution left is dilute sulphuric acid (H2SO_J. )>
The solution changes from blue to colourless I I
2) -
cathode
anode
+
I
0
.. b~r.own 0
de.po.oU
6 c.o ppe.IC.
'
0
c
'
bubbles of oxygen
6Oc_ c.oppe.IC. (II) .;ul..pha..te. M.fu:ti..o n
Summary: Electrolysis of :;:tqueous copper-(II) sulphate:
atcathode copper
The solution left is dilute sulphur~ acid (H2 S04).
atanode + oxygen
[Dr. Fathy Abdel Galil]
-66-
Chemistry IGCSE
'•;
Types of electrodes : . Inert electrodes: Carbon (graphite) or platinum,. They do not take part in the electrolysis.. Active electrodes: e.g. copper, silver, zinc, nickel. They take part in the electrolysis and the active metal anode dissolves, (i.e. passes into solution as ions. In other words, the mass of the active· metal anode decreases).
Electrolysis of Aqueous Copper(II)sulphate : . Using copper electrodes (i.e. active) .
I,
ll
~ copp er caih ode -
copper an·ode +
1...
-7 Cu2+
-7
-7 Cu 2;-
vaqueo us copper(II)sulpltate """~........ (blue)
Atanode +
Atcathode -
Copper passes into solution as ions (i.e. the anode dissolves) [anode: decreases in mass] [ Cu (s)
~
2
Cu + (aq) + k
J
Copper deposited [cathode: increases in mass]
I Cu2+ (aq) + 2e-
~
The aqueous copper(ii) sulphate remains unchanged.
Cu (s)
I
[Dr. Fathy Abdel Galil]
-67-
Chemistry IGCSE
Application of Electrolysis: 1} Refining of Copper: Copper can be refined by electrolysis using a block of impure copper as the anode and thin sheet of the pure copper as the cathode. The electrolyte is aqueous copper(II)sulphate. Impurities sink to the bottom of the container as anode sludge. The overall result is the transfer of copper atoms from the impure copper to the cathode. );>
·EB
e
an. ode.
cathode. pU/tl!. CO(I(Il!./t
cop.fll!./t( I I ).out.phai.e. .oot.uil.on.
Zinc can be refined by electrolysis. The method is similar to that used to refine copper. Complete the following statements. about refining of zinc. The cathode is made from ................................. . The anode is made from .................................. . The electrolyte is aqueous ................................. .
2) Electroplating: Many metal objects are electroplated for: a) Protecting them from corrosion and rust b) Making them look attractive
anode: _ _:::,-
cathode: (object to be plated)
(metal used to plate the• ::>bject)
·='='""='!-·electrolyte:-
aqueous solr.{tion contairzing.ions of the plating metal
if the object is to
be plated with copper silver .nickel
· the anode will be made fro in copper silver nickel
the electrolyte will be aqueous copper(II)sulphate aqueous silver_nitrate aqueous nickel sulphate .
I
Chemistry IGCSE
-68-
[I?r. Fathy Abdel Galil]
3) Extraction of metals Reactive metals above Zn in the reactivity series such as Na, Mg, Ca, AI can be. extracted only by electrolysis.
Extraction of Aluminium Ore: Bauxite. The purified bauxite (aluminium oxide, Al 20 3) is electrolysed in molten cryolite (Na3A1F6). Aluminium oxide has too high melting point to be used on its own, so it dissolves in molten cryolite at 900°C (i.e. cryolite is used to lower the m.p. ofAl1 0J)
....
Carbon anodes (";), --~,;,.,o."'l"""___ 1±/
Molten electrolyte -HI-(aluminium oxide
fil!.-l~·carbon
lining .ofccll(cathodc)
dissolved in cryolite) ·
At cathode-
The aluminium sinks to the bottom.
8.
..
AI
At anode +
Oxygen gas is produced.
.> As the hot oxygen escc·.pes, it burns away the graphite anode· ( C + 0 2 7 C0 2), ard so they have to be replaced from time to time. .> Making aluminium is an expensive process because large amounts of electricity are needed.
(Dr.FathyAbdel Galil)
-69-
Chemistry IGCSE
Topic 7.
. ACIDS, BASES AND SALTS
Acids: An acid is a substance giving hydrogen ions, II'" (aq), in aqueous solutions. Aqueous solutions of acids tUrn the colour of litmus red. It is the hydrogen ions that cause the change of colour. Remember that the hydrogen ion is simply a proton. . In other w.ords, an acid is a proton donor. .. ·common acids:
strong
Acid
Formula
Hydrochloric acid
HCl
Nitric acid
HN0
Sul.phur:Lc acid
{ weak -
'
T
--+
Thing to remember about hydrochloric acid: Hydrochloric acid is a solution of hydrogen chloride gas in water. water HCl (aq) HCl (g) [covalent] [ionic]
•:':·
-71-
Chem_istry JGCSE
(Dr. Fathy Abdel Galil)
Bases A base is a substance that can accept hydrogen ions, i.e·. a proton acceptor.
Ox..i..du, and hydJr.ox..i..deh of metals are bases. Properties of bases :
1.
·Bases react ,;ith acids to form a salt and ,;ater only (see properties of acids), This reaction is called neutralization reaction.
2.
If a base is soluble in ,;ater, the solution is called an alkatl.
An aiH;ali is a base that is soluble in water
-
'
Common alkalis : ·Alka-li
,crong { weak
Formula
Ions present in the alkali
Sodium hydroxide
NaOH
Na+
'"'"'''= "''""""'
KOH
K+
Calcium hydroxide
Ca(OH)
NH (aq) or 3 NH 0H
f-- Aqueous ammonia
(often called ammonium hydroxide)
3.
2
OH
-
20H OH
Ca 2 + NH+
OH
4
4
Solutions of alkali turn the colour of lit"mus £~~~It is the hydJr.ox..i..de. ~OM that cause the chang.e of colour.
Weak and strong acids and bases Acids strong Completely ionized in solution, !.e. the solution wi!! contain a high ·concentration of hydrogen ions, e.g. hydrochloric acid sulphuric acid nitric acid
we k
Partially ionized In solution,
l ...e. some of the .. molecules .remain un-Ionized In the solution e.g. ethanoic acid citric acid [in citrus fruits]
Similar-ly;_ if an alkali completely ionizes on dissolving in water, a ..;.tJwng
alkatl
is produced;
e.g. sodium hydroxide, potassium .. hydroxide, calcium hydroxide. If an alkali does not. complet.ely ionize in· water, a is formed;
e.g·.
·ammonium hydroxide (aqueous ammonia).
we.a.k.
~
(Dr. Falhy Abdel Galil)
-72~
Chemlstiy IGCSE
nre pH scale The strength of an acid or an alkali Is shown using a scale of numbers called the pH scale. On this scale :
an acidic solution has a pH number less than 7 an alkalic solution has a pH number greater than 7 a neutral solution has a pH number of exactly 7
a
The pH can be measured by using universal indicator. Unlversallnd!cator Is a mixture of dyes. Uke litmus, it can be used as a solution or as universal Indicator paper; it goes a different colour at different pH value as shown In this diagram: yellow violet blue orange red ...__ / / / /
~r~;n
'
'
-
'
unlversal!ndlcator colours
1
pH numbers
2...___
2
.....
3
5
4
...___
/
e.g. hydro-
o.g. otha-
chlol1c acid
nolc acid
6
7
8
11\
g 10 11 12 13 14
-"""';! method and apparatus would. be use
+
d.Uu:te. n.U!Uc. Cl.c).d - - - p.{.pe.tie.
, which is an alloy of copper and zinc is stronger and harder than the pure metals. The following is a diagram showing the structure of an a.lloy ·.·
0
•
me,.ta.£ a.to m6 o;thvr. metal dom6
Uses of Metals related to their Properties
Aluminium high electrieal
conducllimy
yens;cy (ov/
overhead C.tee1. co11.e.
window frames
cans Food containers
aluminium
The L>.teel. c.o11.e L>.t11.eng.then6 ;the c.a.b.f..e.
g£EE0_.: I.t L6 u.oed e.g. : 1. In· el.ec:tiUc w.Ute6 bec.a.u.oe c.oppeJr. L6 du.c.t.Ue, a: good .c.andu.c.tolt o6 el.ec;tJUcA.:ty, on .tow Jtea.di..vU!f a:nd haL> a. IU.gh m.p. 2. In c.oo!U.ng u;tert.6ili beCJJ.LL6e c.oppvr. L6 a. good c.ondu.c.tolt o6 hea-t. 3.
Making brass which is an alloy of copper and zinc
- 83 -
Chemistry I GCSE
(Dr. Fathy Abdel Galli)
Reactivity series
K
.l
::;
extracted by electrolysis
a..tta.c.k Lll:lteJL --r hy d!to :Ude. +
Hz Jtea.c.t.
Mg
C-+
A.t
Zn
.....-.._,. ->a..U + Hz
a.tta.ck ;,.te.a.m ---'7 o:Ude +
..
Hz
Fe
o:Udu a.Jte. Jte.duc ed w.Uh c.Mbo rt
w.Uh
d.Uu..te. a.cJ.do
Pb (H)
Cu
Ag Au Action of· Water on Metals
a)
Potassium, Sodium, Calcium
--------------------------
Reactive Metal + Water ----7 Metal Hydroxide + Hydrogen
Potassium and sodium react violently
+----+-Hydrogen
· with cold water, ( see page-49 J Calc·ium reacts raadily but not violently
Water
with cold water to form hydrogen and calcium hydroxide solution, the alkali -U.me wct.te.Jt.
Calcium reacts steadily wi!h walet.
calcium + water ----7 calcium hydroxide + hydrogen
Ca (s) +2H20 Q) --'-+
Ca(O~
(aq)
+ H2 (g)
St~te two observations you could make when a small piece_ ofcalciu~ Is added to cold' water. Observation 1 ·-···-····-·····--··················--···--.--........... - ................- ................................................ .. Observation 2 ·-·-·-·--------................................................................................................................... [2]
- 84-
Chemistry IGCSE
(Dr. Fathy Abdel Cali!)
Magnesium
Reacts very slowly with cold water to form hydrogen and magnesium hydroxide.·
rI
When heated in steam, magnesium turns to a white powder (magnesium. oxide)
Heal
Magnesium
+ _Steam
Magnesium oxide
+
Hydrogen
Mg(s)
+
• • • • • • • • • • . . • ( s)
+
• • • • • • (g)
--> H O(g) ·-> 2
Hydrogen burn1ng
React only with steam to form hydrogen and the oxide.
Reaction with Dilute Acids Metals above hydrogen in the series react with dilute acids and hydrogen is produced. This is a displacement reaction in which the metal displaced the hydrogen of_ the acid.
ri nc
+
Zn(s) + zinc
+
+ hydrogen
hydrochloric add ....-...;:. rinc chlor-ide
+
- - 7 .......... ..
sulphuric acid
--->- rlnc sulphate
+
hydrogen
Zn(s) +
)>
)>
)>
It is too dangerous to react the very reactive metals potassium and sodium with acids and would caitse aplosion. Aluminium seems to be unreactive due to the protective layer of Al203 . Remember that Cu, Ag and Au do not react with ·,vater, steam and dilute acids.
-(Dr. Fathy-Abdel ·calif)
- 85-
Cherr:lstry IGCSE
Making hydrogen in laboratory:
Hydrogen is _prepared in laboratory by reacting zinc or magnesium with -dilute hydrochloric acid or sulphuric acid. acid
hydrogen
water
zinc
Reaction of Metal Oxides with Carbon Carbon, on heating, will reduce the oxides of zinc and of metals below it in the reactivity series.
iron (Ill) oxide _+ ··Fe 0 2 3
+
carbon
.; c
---~
iron
~··Fe_
+ carbon dioxide + .. co
2
Competition for Oxygen : [Reduction of metal oxides by a more reactive_ metal]
A metal oxide can be reduced tr:_ ·.:he metal by heating it ·with another metal which is above it ir, Why a polystyrene (plastic cup) is used instead of a beaker? .................................................... )>
Predict what the temperature of the reaction mixture in the experiment would be after 1 hour. Explain your answer. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .
2. All displacement reactions are exothermic:
7
MgS04 (aq) + H 2 (g)
7
ZnS04 (aq)
+ H 2 (g)
By measuring the rise in temperature we can find mit the order of reactivity of metals.
Chemistry IGCSE
(Dr. Fa thy Abdel Galil)
- 95-
Examples (or endothermic reactions: 2 NO(g]
1.
This means that energy is
2.
needed for the reaction to take place
Thermal decomposition.. Melting of ice and evaporation of water.
3.
PRODUCTION OF ENERGY ,·,>1
Burning Fuels Fuels release heat energy when they burn in air or oxygen.
~
burning reactions are exothermic. Natural gas
i~
largely methane,. When it burns,
methane + oxygen CH
4
+
z 02.
~carbon
------1-
diox1de + water
J ) )
co 2
he.a..t .{;, given 'ocU
Ethanol (used in cars in Brazil). When i t burns,
ethanol
+ oxygen --7 carbon dioxide + water +30
2
--72C0 . 2
+3H 0 2
) he.a..t .{;, J given aU-t )
What is needed for a fire ?
Hydrogen as a Fuel Hydrogen is a possible energy source of the future. Hydrogen can be used as a fuel for cars. It causes no pollution because the only product of combustion is water.
Nuclear Energy Uranium- 235 as a source of energy [see page !9].
-.----~----
~-
,_
···- ..
..
Chemistry IGCSE
-96-
(Dr. Fathy Abdel Galil)
Comparing the amount of energy produced by the burning of different liquid fuels: The apparatus below could be used. The same mass of each fuel is burnt in turn. The initial and final temperature of the same v-;iume of water is recorded
metal
container
f--
thermometer
clam p :.·
water
i) wick
draught shield
j
0:,
t
:~:~· ......"r.
r----
-----
spi rit bu mer
l iquid
.:.-
fuel
Questions: Suggest how you could work out which fuel produced more energy .
. ..
. . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What are the sources of errors in the above experiment?
............................................ ; ................ _- ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - ... ' .
I.
-97-
Chemistry IGCSE
(Dr. Fathy Abdel Galil)
-.-.·
Electricity from Chemical Reactions : Cell :
a device to produce electrical energy from a chemical reaction
This is the reverse of what happens during electrolysis, where chemical reactions are brought about by the use of electrical energy. Electrolysis (endothermic)
Electrical energy
----------__:~
t
Cell (exothermic)
Chemical reaction
Simple cells Two metals
that are far apart from one another in the reactivity
series are placed in an electrolyte. The more reactive metal becomes the negative pole from which electrons flow.
The amount of electricity produced (voltage) depends on the position of the metals in the reactivity series, The rule is : The further apart the metals are in the reactivity series the more electricity is produced.
voltmeter
Mg AI
copper - - - l l l
Zn Fe Pb H Cu Ag
Au dilute acid
:>
The voltage of an -ironicoppercell is about 0.8 volis. Predict whether the voltage of a zinc/copper cell would be less; the s·ame ar bigger. Explain your an?Wer. ·
.,-.,-.-.-- "''"'"~'··-.-.··.----,-----:-..-.-;-
··--·
-- -- ....
Chemistry !GCSE
(Or. Fathy Abdel Cali!)
- 98-
The simple cell can be used to compare the reactivity of two metals l.
The more reactive metal is the one which loses electrons and i~e.
passes as ions in the solution,
2.
becomes thinner;
The direction of flow of e- is from the more reactive metal to the less reactive one.
Question : :::::========= How could the simple cell shown below be used to find out if zinc is more reactive than cadmium ?
·-
voltmeter
zinc
cadmium electrode
electrod.:;
-=1--- dilute sulphuric acid
............................................................................... •
•
•
..
•
0
•
0
.......................................
0
0
•
0
....................
0
..............
.
The Zinc Carbon Dry Cells: [Batteries] Batteries are convenient source of energy because they are portable and small, and are used in toys, radios, torches and many other things.
-99-
Chemistry IGCSE
(Dr. Fathy Abdel Cali!) ·.·.·
Topic 10.
CHEMICAL REACTIONS
-Rate of Reaction The rate l.
of a chemical reaction can be increased by :
l/,_a_king the size of reacting par·ticles smaller, i.e. increasing the surface area of the reactants.
2.
Increasing the. concentration.
j·:
Iricreaaing the temperature..
4.
Adding a suitable catalyst.
5.
Light (applied to a few re.actions).
Plotting Graphs of Reaction Rates FoUow.i.ng
IUJ.;te
o6 1r.eac.tion by mea..&UJU.ng ga..& vo.twne :
Take for example the reaction between magnesium and excess dilute hydrochloric acid :
magnesium + hydrochloric acid--+ magnesium chloride + Mg(s)
+
2 HCI(aq)
Measurements made are
---+
MgCI (aq) 2
+
gas volume and time.
__,.. the plunger can move out
gas s.yringe
stop clock
The volume of gas in the ayrin•;e is noted at intervals, for exal'lple at the end of each half-minute, How would you know when the reaction is complete ?
...........................................................................
- 100-
Chemistry I GCSE
(Dr. Fathy Abdel Galil)
Here are some typical results: Time/minutes
0
l
1
ll
2
2!
3
3!
4
4!
5
5!
6
6!
Volume of hydrogen/em'
0
8
14
20
25
29
33
36
38
39
40
40
4D
40
These results can be plotted on a graph, as shown
'.
Shape a 6 .the GJtir.ph I.
The 6a..o.tvr. .the II.e.ac..Uon,
curve llal, raacllon over
.the. .teepvr. .the. cWLve.
curve· loss sleep, reaction slower
, 2.
When .the II.e.a.mon ;_;, ovvr., .the cwr.ve goe. u.f.a..t.
curve steepesl, teacllon fastest
The reaction is which a gas given off, e.g. an acid and Ciirbonate, the rate of reaction can be followed up by measuring the decrease in mass of reactants .with time. i.e. the balance reading decrease as the reaction takes place.
/
fast
.
add
, ..
ca.f.uwn
cMbona..te. slow
time (s)
Chemistry IGCSE
Investigating the Rate
(Dr. Fathy Abdel Galil)
- !Ol -
of a Reaction
Effect of particle size This effect can be examined by reacting equal masses of calcium carbonate with different particle sizes (e.g. chalk powder and marble chips) with equal volumes of the same hydrochloric acid.
The volume
~f.
th; _produced
co 2
can be measured using
a_~as
syringe
and is plotted against time.
A
=
U
2 NO
+
3,
.. 2 N.H 3
The effect of chanee in gres·sure r for 'aseous reactions l
:
a:::z:::z:::=m:a=:a::JZJ;:;::.:u::;::::c=a:~at••====c.-:Sit.c-=e=-==•=*'!'.*"':::.:;:a:::;J:..t;:ia;::c;;r:xi2~=~~::t=ti=:Q::=az::.;=x:=o:::.:a:J::=c:
Increasing the pressure shifts the equilibrium in the direction in which there is decJI.e.4Ue in vohwre, i.e. th.e direction which produces fewer gas moles.
;;:==·z
+
NH (g)
3
2mol )> .Predict how the position of equilibrimii might ·change i f there were
an increase·in·the pressure on the f61lowin:g e-quilibria
+
CO (g) )>
__ 02
+ 2 H2 (g)
Why an increase in pressure does not affeCt the position of fhe following
equilibrium : 2NO(g) ·~-~~·-··············································· .. ························································~········
;::'.
·.-··.
Ch~mistry
(Dr. Fathy Abdel Galil) ·
-108-
IGCSE
Topic
11.
AIR AND WATER
PuM..QJ_c.atio n a 6 wa.:teJL Mppty : The water from rivers is. first. stored in reservoirs where the process of purification starts.
The water is still, so the larger
particles of dirt can settle out.
At the su:toface, oxygen· and sun- ·
light break down other impurities and kill some bacteria. The water from the storage rese:z::.voirs. is treated in two stages 1-
Filtration : ===:;::======== It is filtered through heds o·f fine sands to remove suspended (insoluble)
·-
solids. 2-
Chlorination : :ell:====-=-====== Then it is treated with chlorine to kill bacteria.
PUJte wa.:te.Jt c.a.n be ab.t.abted Mom .U, cut
U.6eo
.6e1t
wa.:teJL by liU.:tUJ..a:tWn, bu:t .:tJW,
expe.JU.tion w!U.c.h you c.a.nno.t ue olt .~>me.tl. Th.iJ, .iJ, :the po.LbonoU1> :9a..6 c.a.Jtbon monoude [CO]. When pebt.ol .{..6 b!Litnt bi :the c.aJt wgine, ;the exllO.ll..il.t ga.e-6 c.on:tain .~>orne c.aJcbon ·monoude, orne !Litbwr.n:t hydltoc.a.Jtbonll· a.nd .iome .6oo.t -i.n a.drUtion ;to .:the ha.Jtmle-6.6 pJtod!.tU!> c.a.Jtbon d.toude and Wll.teJL.
2-
As a product of respiration : During breathing, some of the oxygen in the air is used up, and
C0 2 , water and heat energy ·are produced.
-111-
Chemistry I GCSE
(Dr. Fathy Abdel Galil)
Manufacture of ammonia by the Haber process : . Ammonia is made by
~yn.theo.U,
i.e. nitrogen and hydrogen·are
combined together.
1.
Nitrogen is obtained from the air.
2.
Hydrogen in now obta·ined from methane (natural gas). Hethane is heated with steam over a nickel catalysc
CO(g)
+
3 H (g) 2
> The following is the equation of formation of ammonia from Its elements, nitrogen and hydrogen:
+ i.
1NH3 (g)
The reaction is exothermic.
ii. The reaction is reversible.
)>
Conditions: temperature: 450°C pressure: 200 atmosphere catalyst: .iron
Ammonia is liquified by cooling. The unreacted gases are recycled.
:> 1. 11.
Uses of ammonia Making fertilizers Making nitric aCid
. Chemistry IGCSE
- 112-
(Dr. Fa thy Abdel Galil) =:
Ammonia in the Laboratory l.
Ammonia is a colourless; choking gas, which is. formed whenever· an ammonium salt is warmed with an alkali. For example• :
NH,Cl(s) . + NaOH(aq) ____,.. NH 3(g) ammonium sodium ammonia chloride hydro:x:ide
+ NaCl(aq) + H 20(1)
~p
sodium chloride
water
red litmus paper
:unmonium _ _____:~,.y chloride
t
warm
2.
Ammonia is very soluble in water and· aque·ous ammonia is .formed,. which is a weak alkali.
NH (g)· + 3 >-
+
~---these
H 0(!) ---+NH 08(aq) 2 4
ions make the solutionalkaline
Since ammonia is an alkali, it will neutralise adds to make ammonium salts. Ammonia +
nitric acid
---+
ammonium nitrate
Fertilisers
1. Fertilisers are substances, which added to soil to promote plant growth. 2. .The major plant nutrients include nitrogen (N), phosphorus (P) and potassium (K). 3. Nitrogen is the most important of the three elements because plants use it to make oroteins. Nitrogenous fertilizers: Ammonium nitrate Ammonium sulphate Ammonium phosphate Urea
NB4N03 (NB4)2S04 (NB4)3P04 CO(NH2)2
4. Many fertilisers contain phosphorous and potassium compounds as well as nitrogen compounds. These mixed fertilizers are called 'NPK' fertilisers.
16.8.24
NPK FERTILIZER
- 113-
Chemistry .!GCSE
(Dr. Fa thy Abdel Galil)
AIR. POLLUTION
Apart from the gases normally _found in air, other gases such as sulphur dioxide, oxides of nitrogen, and carbon monoxide can be present. These gases can cause air pollution and are called po~.
Common pollutants in the air 1.
Carbon monoxide, CO : Carbon monoxide is produced by the .·incomplete combustion of carbon con~aining
fuels. Much of carbon monoxide comes from the in-
complete combustion of petrol in car engines.
CaJLbon monoxide .U po.UonolLb ga.;, :thtLt pll.even;U ha.emogtobbt .ul.phwr. :tJ;;;l_ux.J..de £6 rUbi>ol.ved .:l.ll. Wod:eJL, L>u1.plu.vc.tc. a.c:id .U pJwduced•. Ba:t .thU .U. no.t dime in p}(O.c.tic.e .
:toa ex.o.the)!Jn)_e a.nd ·
on a. .ta.Jtge .6.c.al.e beaa.u.6e .:the Jt:e.a:c.tioir M boili :the
d ··
0-u1.phwr..i,e a.ci.d. H s o (1) . + 2 2 7
ol.eum
H 0(0 2
wa.teJL.
-~>
2 H so (!} 2 4 .6 ul.phwr..Le a.c.f..d
Since ·all three of these ·r·eaC:tiona are. exothermic, the whole process
prod~ces heat en.e.rgy, The fact that enetgy is ·produced rather than . ' ;
'
needed means· that sulphuric acid can be produced· cheaply.
i- in-making :t~rtilisers (!'.g. ammonium sulphate) ii- ·iri making detergents iii- in making_paint;s i.:iii- ·iii. car batteries. Conc~e.d .6u1.p/w.JUe a.Ud .U paJ~y da.ngelloll.
I
H-C-Cl
I
[,{
H
mc:thanc
·chloromethane
Similarly,
+ ethane
Cl
£:t -c.an ortl..if pl(.odu.c.e. a .6o.tu.tion c.o~g I 0% e.tfutno.f... k6:teJt :tha:t, :the. ife.M:t d.i.v, 6Jtom atc.aho.f. po.i.,;ort.{.ng. · o:th~e.
The apparatus below can be used 1:0 ferment glucose •
. Air lock allows carbon dioxide to escape, butpre~ents air getting in, i.e. allows fermentation under · anaerobic conditions jar
yeast
+
glucose.
--1~::;:::::::;::::::~
+ .water
>
What technique is used to concentrate the aqueous ethanol? ... _.. __
Dur~ng s~nce
a fermentation process,
the reaction is
are also observed.
exotherm~c.
r~se·
in temperature is observed
Bubbles of a colourless. gas (C0 ) 2
(Dr.FathyAbdel Galil)
- 134-
Chemistry IGCSE
A laboratory experiment to demonstrate the fermentation: sugar solution andyeast -~
limewater
Wine (contains about 10% ethanol) is made by fermenting grape juice. Beer is made by fermenting malt. Spirits (whisky, brandy, ..} contain about 30% alcohol. Biotechnology Microorganisms have been used in some methods of food-making fcit .many centuries. Today these methods are often referred to as examples of 'biotechnology'. One of the oldest kriown biotechnologies isferm(mtation. Properties of Ethanol 1. Colourless liq11id. b.p. 78°Cjrteutral to litmus.
2. Ethanol burns in air with a blue flame to form C02 and H 2 0 and heat · is given out.
3. Ethanol undergoes dehydra:tion and is converted into ethene when heated with. concentrated sulphuric acid.
Uses of Ethanol
1. As a: solvent
2. As a fuel
3. In alcoholic drinks
Uses of methanol (toxic)
1. As a solvent
2. As a: fuel
- 135-
Chemistry IGCSE
Ethanoic acid
>
(Dr. Fa thy Abdel Galil)
H Q' I II H-C-C-0-H I
-
H Ethano-ic acid is usually called acetic acid . (from the Latin word for vinegar -
'acetum'). Vinegar is a 5% solui:;Lon of --.eth~noic acid.
Formation of. Ethanoic Acid from Ethanol a)
£L~xi~~!::!:~:;-~:!,_t:!!_~tmosEheric ~?.'X~:;__:
Vinegar is made by exposing ethanol to the air in the presence of' bacteria. Ethanol is o~ed to ethanoic acid.
~ ~
H
H-C-C-0-H I I H H Wta.noi.
I
+
0
0 II
H-C-C-0-H
2
I
+
H
e.tha.no.i.c. o.c..i..d
This is another example of 'biotechnolo'gy'.
~~g~1=~g=,;, Suggest a reason·why wine goes 'sour' when .left open in air·?
..................................................... •.............. ............................................ . .. . .. .. . .. .. . .. . ........................ ,. ....................... · ........................... . ~
_
b)
Ethanol can b~ ox..Ui..U,ed.much more quickly-iri the laboratory by _heating it with an.o~.i.ng
agent
such as potah~ium
d.i.c.hAomo.te{VT}
(adllified with sulphuric acid). On heating the colour changes from
o.lta.n.ge
to. gltee:n.. In other words, when aqueous potassium
dichromate (VI) is
Jteduc.ed,
the colour changes from ••••••••••..•
to ................... .
Generally: alcohol [alkanol]'
oxidation carboxylic ac~d
Oxidation of methanol gives _........... . Oxidation of propanol gives ............ .
Chemist1·y IGCSE
- 136-
(Dr. Fa thy Abdel Galil)
.. :
Et}w.no.i..c. a.c.i..d .i..b a. wea.k. a.c.i..d. Ethanoic acid (and all carboA-ylic acids) is a weak acid, i.e. partially ionized in aqueous solution.
~~~~;!;-£liH~=,; The pH value of ethanoic acid is ..•..••••••••••• ; •••••••••••••••••. What is the colour produced on adding drop of universal indicator to ethanoic acid ?
.................... • ..................................................... .
Ethano.i..c. a.c.i..d .i..b juJ.:t Uke_ MCJ o.theJt a.c.i..d. It reacts with * Metals to form a salt + hydrogen
* *
Bases and alkalis to form a salt Carbonates to form a salt
+ water
+ water + carbon dioxide
In each case the salt is:c.alled ethanoate.
Examples_:
1>- ethanoic add
+ sodium hydroxide --* sodium ethanoate + water
1>- ethanokacid
+ sodium carbonate --* sodium ethanoate + water + carbon dioxide
However, the reaction of ethanoic acid is slower than of other acids such as hydrochloric acid or sulphuric acid. Why? ...... ········· ............................................................................................ );>
Differentiation between ethanol and ethanoic acid
test );>litmus paper );> sodium carbo-nate >potassium dichromate );>lighted splint
ethanol
ethanoic acid
no colour change no reaction . orange -7 green catches fire
turns red effervescence -7 C02 no colour change does not catch fire
Chemistry IGCSE
[Dr. Fathy Abdel Gaiil)
- 137-
Esters: Organic acids like ethanoic.acid react with alcohols to form compounds called eh£~ in a reaction co~~only called an
organic acid + [carboxylic acid]
'
alcohol
ehte4i6ieation.
ester
+
water
e.tha.no.t is heated with e.tha.no.Lc. a.c..Ld in the presence of .Ou.tphuh.Lc. a.c..Ld as a catalyst, the ester ethy.t etha.noa.£e is formed.
When
ethanoic acid +
ethyl ethanoate + water
ethanol
·•·.
~
1;0
~ ~
.. -.
H
+: H;O-C-C-H \~ .. "· ....... · I I. ;oH:: H H
H-C-C
I
H
·· ....
~
I
0 ,--the ester linkage
1/
H-C+C ·I \
H\
'
l
H I
H
H
H
I 'o-+c-c-H + H2o \..........J I I
Similarly: ethanoic acid + methanol
~
methyl ethanoate + ·water
Ethylethanoate has a characteristic pleasant smell. Many fruits' smells are caused by esters. Artificial flavouring are usually mixtures of esters. The reverse reaction of esterification is called hydroly~is.
j ester + water
-·-:t
carboxylic acid + alcohol
Fats and Vegetable Oils Fats and oils are important constituents"of our· food. They are e..6te.Jr..6 ,. i.e contain the ester linkage [::-CQQ;..]
Making Soap (Saponification)
Soa.p
is produced by heating vegetable oils or animal fats with
sodium hydroxide solution (i.e.
.
alka.tine
Fats I oils +sodium hydroxide
~
-.-
hy~d.tybib).
soap + glycerol
Chemistry IGCSE
(Dr. Fathy Abdel Galli)
- !38-
Carbohydrates
1-
A family of energy foods including starch and sugars.
2-
These compounds consist of carbon, hydrogen and oxygen,_ the hydroge~
and oxygen atoms are present in the sariie ratio as in
wa.ter.
3-
Sugars are carbohydrates which are soluble in water and taste ·sweet.
The chemical name for ordinary sugar is .6UCJl0.6e..
LQcto.6e. is the sugar found in milk, and fructose occurs, with
other sugars, in fruit juices. and honey.
CaJtboh!JdN:t;te.6 a.o po.tymeM : [ complex carbohydrates
1
Starch molecule consists of a long chain of g;Lucose molecules .• Glucose HO--Q-OH (a monomer) can be j6ined by in a l.ong chain to form starch (a polyl!ler) by condensad.on ]>o.lymerisation.
glucose rolecules .
li
(monomers)
condensation polymerisation
hydroly•h
-o -c=J- o -c:::J- o· --c=J- o- + H2o starch (polymer)
Ac.JJJ. hyd.Jw.tif.6.U
o6 c.a.JWohydJw.;tu,,
e.g. starch :
.Starch ·can be hydrolysed into gl.ucose molecul.es by boiling with dilute hydrochloric acid starch
hydrochloric acid boiling
I> glucose (simple sugars)
Chemistry lGCSE
- 139-
(Dr. Fathy Abdel Galil)
Proteins Proteins are natural macromolecules. Amino acids are the monomers from which proteins are made. Formation of a p;lrt of protein chain : When proteins are formed, an amino group from one amino add reacts Vlith the carboxyl group from another (VIith elimination of water). This-process can continue repeatedly tc produce long chain (i.e. macromolecule).
H
0
I
II
Ii.
0
.. .:-····· ... \ II H-N-i=t-c In laboratory,- proteins can be hydrolysed into their amino acids by heating with dilute · acids (e.g; HCL or HiS04). !>-- The products of the hydrolysis of proteins and carbohydrates can be separated and identified by chromatography. The products. are colourless so need spraying a locating agent to be seen on the chromatogram. )> )>
These constituents of food can all be hydrolysed by boiling ·with acid or alkali. · linkage
· consliluent of food protein
product of hydrolysis
-
fat complex carbohydrate
~
Complete the table.
[5]
Chemistry IGCSE
(Dr. Fathy Abdel Galli)
- 140-
CONDENSATION POLYMERISATION Occurs when a small molecule such as >'1
-7
56
55
Fr fronc•um
87
·ss-71
72
226
227
Ra
Ac
Rad•um
73
76
75
74
78
77
79
80
81
209
83
82
"
c
86
85
actm•um
88
89
t
Lanthanoid series
t90-1 03 Actinoid series
Key0
140
141
144
Ce
Pr
Nd
Cerium
PraseO
View more...
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