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Chemical Equilibria and Le
Chatelier’s Principle
Question Paper 4
Level
Subject
Exam Board
Module
Topic
Sub-Topic
A Level
Chemistry
AQA
3.1 Physical Chemistry
3.1.6 Chemical Equilibria + Le Chatelier +Kc
3.1.6.1 Chemical Equilibria and Le Chatelier’s
Principle
Question Paper 4
Booklet
Time Allowed:
51
minutes
Score:
/51
Percentage:
/100
Grade Boundaries:
A*
A
B
C
D
E
U
>85%
75%
70%
60%
55%
50%
<50%
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Q1.At high temperatures and in the presence of a catalyst, sulfur trioxide decomposes
according to the following equation.
2SO3(g)
(a)
2SO2(g)
+
∆Hϴ = +196 kJ mol–1
O2(g)
In an experiment, 8.0mol of sulfur trioxide were placed in a container of volume 12.0
dm3 and allowed to come to equilibrium.
At temperature T1 there were 1.4 mol of oxygen in the equilibrium mixture.
(i)
Calculate the amount, in moles, of sulfur trioxide and of sulfur dioxide in the
equilibrium mixture.
Amount of sulfur trioxide .......................................................................
Amount of sulfur dioxide ........................................................................
(2)
(ii)
Write an expression for the equilibrium constant, Kc, for this equilibrium.
...............................................................................................................
...............................................................................................................
(1)
(iii)
Deduce the units of Kc for this equilibrium.
...............................................................................................................
...............................................................................................................
(1)
(iv)
Calculate a value of Kc for this equilibrium at temperature T1
(If you were unable to complete the calculations in part (a)(i) you should
assume that the amount of sulfur trioxide in the equilibrium mixture was 5.8
mol and the amount of sulfur dioxide was 2.1 mol. These are not the correct
values.)
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
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...............................................................................................................
...............................................................................................................
(Extra space) ........................................................................................
...............................................................................................................
(3)
(b)
The experiment was repeated at the same temperature using the same amount of
sulfur trioxide but in a larger container.
State the effect, if any, of this change on:
(i)
the amount, in moles, of oxygen in the new equilibrium mixture
...............................................................................................................
(1)
(ii)
the value of Kc
...............................................................................................................
(1)
(c)
The experiment was repeated in the original container but at temperature T2
The value of Kc was smaller than the value at temperature T1
State which is the higher temperature, T1 or T2
Explain your answer.
Higher temperature ........................................................................................
Explanation ....................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(Extra space) .................................................................................................
........................................................................................................................
(3)
(Total 12 marks)
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Q2.
A sample of nitrogen dioxide gas (NO2) was prepared by the reaction of copper with
concentrated nitric acid.
(a)
(i)
Balance the equation for the reaction of copper with concentrated nitric acid.
Cu + ........ HNO3 → Cu(NO3)2 + ........ NO2 + ........ H2O
(1)
(ii)
Give the oxidation state of nitrogen in each of the following compounds.
HNO3 ........................................................
NO2 ..........................................................
(2)
(iii)
Deduce the half-equation for the conversion of HNO3 into NO2 in this reaction.
.............................................................................................................
(1)
(b)
The following equilibrium is established between colourless dinitrogen tetraoxide
gas (N2O4) and dark brown nitrogen dioxide gas.
N2O4(g)
(i)
2NO2(g)
ΔH = 58 kJ mol–1
Give two features of a reaction at equilibrium.
Feature 1 ............................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
Feature 2 ............................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(2)
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(ii)
Use Le Chatelier’s principle to explain why the mixture of gases becomes
darker in colour when the mixture is heated at constant pressure.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(2)
(iii)
Use Le Chatelier’s principle to explain why the amount of NO2 decreases
when the pressure is increased at constant temperature.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(2)
(Total 10 marks)
Q3.
Sulfuric acid is made from SO3 which can be manufactured in a series of stages from
iron(II) disulfide (FeS2), found in the mineral iron pyrites.
(a)
In the first stage, FeS2 is roasted in air to form iron(III) oxide and sulfur dioxide.
(i)
Balance the following equation for this reaction.
..........FeS2 + ..........O2 → ..........Fe2O3 + ..........SO2
(1)
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(ii)
Deduce the oxidation state of sulfur in each of the following compounds.
SO2 .....................................................................................................
FeS2.....................................................................................................
(2)
(b)
In the second stage of the manufacture of sulfuric acid, sulfur dioxide reacts with
oxygen. The equation for the equilibrium that is established is shown below.
SO2(g) +
O2(g)
SO3(g)
ΔH = –98 kJ mol–1
State and explain the effect of an increase in temperature on the equilibrium yield of
SO3
Effect of increase in temperature on yield ..................................................
Explanation …..............................................................................................
......................................................................................................................
......................................................................................................................
(3)
(c)
In the extraction of iron, carbon monoxide reacts with iron(III) oxide. Write an
equation for this reaction and state the role of the carbon monoxide.
Equation ......................................................................................................
Role of the carbon monoxide .......................................................................
(2)
(Total 8 marks)
Q4.
The following dynamic equilibrium was established at temperature T in a closed
container.
P(g) + 2Q(g)
ΔHο = –50 kJ mol–1
2R(g)
The value of Kc for the reaction was 68.0 mol–1 dm3 when the equilibrium mixture contained
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3.82 mol of P and 5.24 mol of R.
(a)
Give the meaning of the term dynamic equilibrium.
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(Extra space) ................................................................................................
......................................................................................................................
(2)
(b)
Write an expression for Kc for this reaction.
......................................................................................................................
......................................................................................................................
(1)
(c)
The volume of the container was 10.0 dm3.
Calculate the concentration, in mol dm–3, of Q in the equilibrium mixture.
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(Extra space) ...............................................................................................
......................................................................................................................
(4)
(d)
State the effect, if any, on the equilibrium amount of P of increasing the
temperature.
All other factors are unchanged.
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......................................................................................................................
(1)
(e)
State the effect, if any, on the equilibrium amount of P of using a container of larger
volume. All other factors are unchanged.
......................................................................................................................
(1)
(f)
State the effect, if any, on the value of Kc of increasing the temperature.
All other factors are unchanged.
......................................................................................................................
(1)
(g)
State the effect, if any, on the value of Kc of using a container of larger volume.
All other factors are unchanged.
......................................................................................................................
(1)
(h)
Deduce the value of the equilibrium constant, at temperature T, for the reaction
2R(g)
P(g) + 2Q(g)
......................................................................................................................
......................................................................................................................
(1)
(Total 12 marks)
Q5.
Oxygen and ozone (O3) both occur as gases in the upper atmosphere.
Chlorine atoms catalyse the decomposition of ozone and contribute to the formation of a
hole in the ozone layer.
These chlorine atoms are formed from chlorofluorocarbons (CFCs) such as CF3Cl
(a)
(i)
Give the IUPAC name of CF3Cl
.............................................................................................................
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(1)
(ii)
Complete the following equation that shows the formation of a chlorine atom
from a molecule of CF3Cl
(1)
(iii)
State what the • represents in Cl•
.............................................................................................................
(1)
(b)
Write two equations that show how chlorine atoms catalyse the decomposition of
ozone into oxygen.
Equation 1 ....................................................................................................
Equation 2 ....................................................................................................
(2)
(c)
An equilibrium is established between oxygen and ozone molecules as shown
below.
3O2(g)
(i)
2O3(g)
ΔH = +284 kJ mol–1
State Le Chatelier’s principle.
.............................................................................................................
.............................................................................................................
.............................................................................................................
(1)
(ii)
Use Le Chatelier’s principle to explain how an increase in temperature causes
an increase in the equilibrium yield of ozone.
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.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(2)
(d)
Chemists supported the legislation to ban the use of CFCs. Modern refrigerators
use pentane rather than CFCs as refrigerants.
With reference to its formula, state why pentane is a more environmentally
acceptable refrigerant.
......................................................................................................................
......................................................................................................................
(1)
(Total 9 marks)
Page 10