Chemistry 12 (HL)
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
Equilibrium Review – Past Paper Questions
PAPER 1 QUESTIONS …. no calculators!
1.
2SO2(g) + O2 (g)  2SO3(g)
ο
∆H = –200 kJ
According to the above information, what temperature and pressure conditions produce the
greatest amount of SO3?
Temperature
Pressure
A.
low
low
B.
low
high
C.
high
high
D.
high
low
2.
Which statement(s) is/are true for a mixture of ice and water at equilibrium?
I.
The rates of melting and freezing are equal.
II.
The amounts of ice and water are equal.
III.
The same position of equilibrium can be reached by cooling water and heating ice.
A.
I only
B. I and III only
C. II only
D. III only
3.
What will happen to the position of equilibrium and the value of the equilibrium constant when the
temperature is increased in the following reaction?
Br2(g) + Cl2(g)  2BrCl(g)
A.
B.
C.
D.
Position of equilibrium
Shifts towards the reactants
Shifts towards the reactants
Shifts towards the products
Shifts towards the products
∆H = +14 kJ
Value of equilibrium constant
Decreases
Increases
Decreases
Increases
4.
The rate of a reversible reaction is altered by the addition of a heterogeneous catalyst.
Which statement correctly describes the role of the catalyst?
A.
It alters the enthalpy change of the reaction.
B.
It decreases the activation energy of the forward reaction.
C.
It increases the activation energy of the reverse reaction.
D.
It increases the rate of the forward reaction but decreases the rate of the reverse reaction.
5.
A liquid and its vapour are at equilibrium inside a sealed container. Which change would alter the
equilibrium vapour pressure of the liquid in the container?
A.
Adding more liquid
B.
Adding more vapour
C.
Decreasing the volume of the container
D.
Decreasing the temperature
6.
Which statement concerning a chemical reaction at equilibrium is not correct?
A.
The concentrations of reactants and products remain constant.
B.
Equilibrium can be approached from both directions.
C.
The rate of the forward reaction equals the rate of the reverse reaction.
D.
All reaction stops.
p. 1
Chemistry 12 (HL)
7.
For the reaction
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
H2(g) + I2(g)  2HI(g)
at a certain temperature, the equilibrium concentrations are (in mol dm
[H2] = 0.30, [I2] = 0.30, [HI] = 3.0
What is the value of K?
A.
5.0
B.
8.
10
C.
15
–3
)
D.
100
Which statements are correct for a reaction at equilibrium?
I.
The forward and reverse reactions both continue.
II.
The rates of the forward and reverse reactions are equal.
III.
The concentrations of reactants and products are equal.
A.
I and II only
B. I and III only
C. II and III only
D. I, II and III
9.
A sealed container at room temperature is half full of water. The temperature of the container is
increased and left for equilibrium to re-establish. Which statement is correct when the equilibrium
is re-established at the higher temperature?
A.
The rate of vaporization is greater than the rate of condensation.
B.
The amount of water vapour is greater than the amount of liquid water.
C.
The amount of water vapour is greater than it is at the lower temperature.
D.
The rate of condensation is greater than the rate of vaporization.
10.
The equation for a reversible reaction used in industry to convert methane to hydrogen is shown
below.
CH4(g) + H2O(g)  CO(g) + 3H2(g)
∆H˚ = +210 kJ
Which statement is always correct about this reaction when equilibrium has been reached?
A.
The concentrations of methane and carbon monoxide are equal.
B.
The rate of the forward reaction is greater than the rate of the reverse reaction.
C.
The amount of hydrogen is three times the amount of methane.
D.
The value of ∆Hº for the reverse reaction is –210 kJ.
11.
The expression for the equilibrium constant for a reaction is
Kc =
[B][C]
[A]2
-3
At a certain temperature the values of [A], [B] and [C] are all 0.2 mol dm . What happens to the
-3
value of Kc when all three values are doubled to 0.4 mol dm ?
A.
It is halved.
B.
It does not change.
C.
It doubles.
D.
It increases by a factor of four.
12.
Which changes cause an increase in the equilibrium yield of SO3(g) in this reaction?
2SO2(g) + O2(g)  2SO3(g)
∆Hº = –196 kJ
I.
increasing the pressure
II.
decreasing the temperature
III.
adding oxygen
A.
13.
I and II only
B.
I and III only
C.
II and III only
D.
I, II and III
A liquid and its vapour are at equilibrium inside a sealed container. Which change will alter the
equilibrium vapour pressure of the liquid in the container?
A.
Adding more liquid
B.
Adding more vapour
C.
Decreasing the volume of the container
D.
Decreasing the temperature
p. 2
Chemistry 12 (HL)
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
14.
Which reaction occurs with the largest increase in entropy?
A.
Pb(NO3)2(s) + 2KI(s) → PbI2(s) + 2KNO3(s)
B.
CaCO3(s) → CaO(s) + CO2(g)
C.
3H2(g) + N2(g) → 2NH3(g)
D.
H2(g) + I2(g) → 2HI(g)
15.
Which reaction has the largest positive value of ΔSӨ?
A.
CO2(g) + 3H2(g) → CH3OH(g) + H2O(g)
B.
2Al(s) + 3S(s) → Al2S3(s)
C.
CH4(g) + H2O(g) → 3H2(g) + CO(g)
D.
2S(s) + 3O2(g) → 2SO3(g)
16.
Which reaction causes a decrease in the entropy of the system?
A.
CaCO3(s) → CaO(s) + CO2(g)
B.
2H2(g) + O2(g) → 2H2O(l)
C.
2C(s) + O2(g) → 2CO(g)
D.
2SO3(g) → 2SO2(g) + O2(g)
17.
What are the signs of ∆Hο and ∆Sο for a reaction that is non-spontaneous at low
temperature but spontaneous at high temperature?
ΔHο
ΔSο
A.
–
–
B.
+
–
C.
–
+
D.
+
+
18.
The ΔHӨ and ΔSӨ values for a reaction are both negative. What will happen to the
spontaneity of this reaction as the temperature is increased?
A.
The reaction will become more spontaneous as the temperature is increased.
B.
The reaction will become less spontaneous as the temperature is increased.
C.
The reaction will remain spontaneous at all temperatures.
D.
The reaction will remain non-spontaneous at any temperature.
19.
For a certain reaction at 298 K the values of both ∆Hο and ∆Sο are negative. Which
statement about the sign of ∆Gο for this reaction must be correct?
A.
It is negative at all temperatures.
B.
It is positive at all temperatures.
C.
It is negative at high temperatures and positive at low temperatures.
D.
It cannot be determined without knowing the temperature.
MULTIPLE CHOICE ANSWERS
1 B
8 A
15 C
2 B
9 C
16 B
3 D
10 D
17 D
4 B
11 B
18 B
5 D
12 D
19 D
6 D
13 D
7 D
14 B
p. 3
Chemistry 12 (HL)
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
PAPER 2 QUESTIONS
20.
The table below gives information about the percentage yield of ammonia obtained in the Haber
process under different conditions.
Pressure/
atmosphere
10
100
200
300
400
600
Temperature/°C
200
50.7
81.7
89.1
89.9
94.6
95.4
300
14.7
52.5
66.7
71.1
79.7
84.2
400
3.9
25.2
38.8
47.1
55.4
65.2
500
1.2
10.6
18.3
24.4
31.9
42.3
(a)
From the table, identify which combination of temperature and pressure gives the highest
yield of ammonia. (1)
(b)
The equation for the main reaction in the Haber process is
N2(g) + 3H2(g)  2NH3(g)
∆H is negative
Use this information to state and explain the effect on the yield of ammonia of increasing
(i)
pressure: (2)
(ii) temperature: (2)
(c)
In practice, typical conditions used in the Haber process are a temperature of 500 °C and a
pressure of 200 atmospheres. Explain why these conditions are used rather than those that
give the highest yield. (2)
(d)
Write the equilibrium constant expression, Kc, for the production of ammonia. (1)
(Total 8 marks)
21.
(a)
The following equilibrium is established at 1700°C.
CO2(g) + H2(g)  H2O(g) + CO(g)
If only carbon dioxide gas and hydrogen gas are present initially, sketch on a graph a
line representing rate against time for (i) the forward reaction and (ii) the reverse reaction
until shortly after equilibrium is established. Explain the shape of each line.
(7)
(b)
Kc for the equilibrium reaction is determined at two different temperatures. At 850°C,
Kc = 1.1 whereas at 1700°C, Kc = 4.9.
On the basis of these Kc values explain whether the reaction is exothermic or endothermic.
(3)
(Total 10 marks)
p. 4
Chemistry 12 (HL)
22.
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
The equation for one reversible reaction involving oxides of nitrogen is shown below:
N2O4(g)  2NO2(g)
∆Hº = +58 kJ
Experimental data for this reaction can be represented on the following graph:
23.
(i)
Write an expression for the equilibrium constant, Kc, for the reaction. Explain the
significance of the horizontal parts of the lines on the graph. State what can be deduced
about the magnitude of Kc for the reaction, giving a reason. (4)
(ii)
Use Le Chatelier’s principle to predict and explain the effect of increasing the temperature
on the position of equilibrium. (2)
(iii)
Use Le Chatelier’s principle to predict and explain the effect of increasing the pressure on
the position of equilibrium. (2)
(iv)
State and explain the effects of a catalyst on the forward and reverse reactions, on the
position of equilibrium and on the value of Kc. (6)
(Total 14 marks)
(a)
The equation for the decomposition of hydrogen iodide is
2HI(g)  H2(g) + I2(g)
∆Hº = +52 kJ
Predict and explain the effect on the position of equilibrium of
(i)
increasing the pressure, at constant temperature. (2)
(ii)
increasing the temperature, at constant pressure. (2)
(iii)
adding a catalyst, at constant temperature and pressure. (2)
(b)
Deduce the expression for Kc for the forward reaction. (1)
(c)
The equilibrium formed during this reaction was investigated in two experiments carried out
at different temperatures. The results are shown in the table below.
Experiment
number
1
2
(i)
Initial concentration / mol
–3
dm
[H2]
[I2]
[HI]
0.06
0.00
0.00
0.00
0.04
0.04
Equilibrium concentration /
–3
mol dm
[H2]
[I2]
[HI]
0.01
0.04
For each experiment, deduce the concentrations of the other species present at
equilibrium. Calculate the values of Kc for the forward reaction for each experiment.
(6)
(ii)
Use the two calculated values of Kc to deduce which of the two experiments was
carried out at the higher temperature, and explain your choice. (If you were not able
to calculate the values of Kc in (c)(i), assume that the values are 0.1 for experiment 1
and 0.2 for experiment 2, although these are not the correct values.) (2)
p. 5
Chemistry 12 (HL)
24.
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
The equation for another reaction used in industry is
CO(g) + H2O(g)  H2(g) + CO2(g)
(i)
(ii)
25.
Under certain conditions of temperature and pressure, 2.0 mol of carbon monoxide and 3.2
mol of steam were left to reach equilibrium. At equilibrium, 1.6 mol of both hydrogen and
carbon dioxide were present.
Calculate the amounts of carbon monoxide and steam at equilibrium and the value of Kc.
(3)
Under the same conditions of temperature and pressure, 2.0 mol of carbon monoxide and
2.0 mol of steam were left to reach equilibrium.
Calculate the amounts of each reactant and product at equilibrium.
(If you were unable to calculate a value for Kc in (i) use the value 9.0, although this is not
the correct value.) (2)
(Total 5 marks)
Hex-1-ene gas, C6H12, burns in oxygen to produce carbon dioxide and water vapour.
(a)
Write an equation to represent this reaction. (1)
(b)
Use the data below to calculate the values of ΔHº and ΔSº for the combustion of
hex-1-ene.
Substance
Standard enthalpy
Ө
of formation, ΔHf /
−1
kJ mol
Ө
−1
Entropy, S / J K
−1
mol
(i)
(ii)
26.
∆Hº= -42 kJ
O2(g)
C6H12(g)
CO2(g)
H2O(g)
0.0
–43
–394
–242
205
385
214
189
Value of ΔHº (2)
Value of ΔSº (2)
(c)
Calculate the standard free energy change for the combustion of hex-1-ene. (2)
(d)
State and explain whether or not the combustion of hex-1-ene is spontaneous at 25°C. (1)
(Total 8 marks)
ο
ο
ο
Explain in terms of ΔG , why a reaction for which both ΔH and ΔS values are positive can
sometimes be spontaneous and sometimes not.
(Total 4 marks)
2+
27. Compounds of heavy metals are one type of toxic substance found in water. Lead(II) ions, Pb , can be
removed by bubbling hydrogen sulfide, H2S, through polluted water. The solubility product of lead
−28
2
-6
sulfide is 1.25 × 10 mol dm at 25°C.
(a) Calculate the concentration of Pb2+ ions in a saturated solution of lead sulfide. [2]
(b) Explain how the addition of hydrogen sulfide decreases the concentration of Pb
saturated solution. [2]
2+
ions in a
p. 6
Chemistry 12 (HL)
Unit 3 / IB Topics 7, 17, E12.1, 15.3, 15.4
28. Waste water can contain metal ions such as chromium. Chromium ions can cause
damage to the liver and kidneys. Chromium ions can be removed from water by chemical
precipitation using hydroxide ions.
3+,
(a) Assuming chromium is present as Cr state an equation for its reaction with hydroxide ions,
including state symbols. [2]
(b) State an expression for the solubility product constant, Ksp, for chromium(III) hydroxide. [1]
−33
4
−12
(c) The solubility product of chromium(III) hydroxide is 1.00×10 mol dm at 298 K. Calculate
–3
3+
the concentration, in mol dm , of Cr in the solution, when chromium(III) hydroxide is
precipitated. [2]
THERE WILL ALSO BE MORE TEST QUESTIONS BASED ON CALCULATING Kc AND EQUILIBRIUM
CONCENTRATIONS! Be sure to review the homework questions on these sections.
p. 7