Chemical equilibria and Le Chatelier's principleAQA A-Level Chemistry: Topic test

20 questions, 54 marks

AQA A-Level Chemistry

Chemical equilibria and Le Chatelier's principle topic test

Total 54 marks

Name

Class

Date

1
Iron(III) ions, Fe³⁺(aq), which are pale yellow, react reversibly with colourless thiocyanate ions, SCN⁻(aq), to form the blood-red complex ion [Fe(SCN)]²⁺(aq): Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq). A dilute mixture of the two solutions is left to reach equilibrium at constant temperature and appears pale red.
(a)
A few drops of concentrated iron(III) nitrate solution are added to the equilibrium mixture. What is observed?
[1 mark]
AThe solution becomes paler
BThe red colour deepens
CThe solution becomes colourless
DThe colour is unchanged because a catalyst is needed to restore equilibrium
(b)
What happens to the value of Kc for this equilibrium when the iron(III) nitrate is added at constant temperature?
[1 mark]
AIt increases because more product forms
BIt decreases because the concentration of Fe³⁺ is higher
CIt increases because the forward rate is greater
DIt is unchanged because Kc depends only on temperature
(c)
A few drops of aqueous silver nitrate are added to the original equilibrium mixture. This removes thiocyanate ions as an insoluble precipitate. State the colour change you would expect and explain it.
[2 marks]
Total for question 1: 4 marks
2
Nitrosyl chloride decomposes reversibly in a closed container: 2NOCl(g) ⇌ 2NO(g) + Cl₂(g). The forward reaction is endothermic, with ΔH = +77 kJ mol⁻¹.
(a)
The temperature of the equilibrium mixture is increased. What happens to the position of equilibrium and to the value of Kc?
[1 mark]
AThe position moves to the left and Kc decreases
BThe position moves to the right and Kc is unchanged
CThe position moves to the right and Kc increases
DThe position moves to the left and Kc increases
(b)
What are the units of Kc for this equilibrium?
[1 mark]
Amol dm⁻³
Bmol² dm⁻⁶
Cmol⁻¹ dm³
DThere are no units
(c)
The container is compressed at constant temperature. State and explain the effect on the position of equilibrium.
[2 marks]
Total for question 2: 4 marks
3
Phosphorus pentachloride dissociates when heated: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). In an experiment, 1.00 mol of PCl₅ is placed in a rigid 5.00 dm³ vessel and heated to 500 K. At equilibrium the vessel contains 0.40 mol of PCl₅.
(a)
Write an expression for Kc for this equilibrium and calculate its value at 500 K, including units.
[3 marks]
(b)
In a second experiment at 500 K, the equilibrium concentrations of PCl₃ and Cl₂ are each 0.060 mol dm⁻³. Calculate the equilibrium concentration of PCl₅. At 600 K the value of Kc is 3.0 mol dm⁻³. Deduce, with a reason, whether the forward reaction is exothermic or endothermic.
[4 marks]
Total for question 3: 7 marks
4
Hydrogen is produced industrially by the water-gas shift reaction: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), ΔH = −41 kJ mol⁻¹. The reaction uses an iron-based catalyst at about 700 K and pressures of up to about 30 atm. The values of Kc are 124 at 500 K, 9.1 at 700 K and 2.1 at 900 K.
(a)
Explain why the plant operates at a compromise temperature of 700 K with a catalyst, and why changing the pressure does not change the equilibrium yield of hydrogen.
[6 marks]
(b)
A mixture of 1.00 mol of CO and 1.00 mol of H₂O is left to reach equilibrium at 700 K in a rigid vessel. Calculate the amount of hydrogen at equilibrium. Then state and explain how the percentage of CO converted would change if the starting mixture contained 1.00 mol of CO and 3.00 mol of H₂O at the same temperature.
[6 marks]
Total for question 4: 12 marks
5
In a car engine cylinder, nitrogen and oxygen react reversibly: N₂(g) + O₂(g) ⇌ 2NO(g), ΔH = +180 kJ mol⁻¹. At 2000 K the value of Kc is 4.0 × 10⁻⁴. At equilibrium at this temperature, the concentrations of N₂ and O₂ are 0.080 mol dm⁻³ and 0.020 mol dm⁻³ respectively.
(a)
During compression in the cylinder the pressure of the equilibrium mixture rises sharply at constant temperature. What is the effect on the position of equilibrium?
[1 mark]
AThere is no effect, because there are equal numbers of moles of gas on each side
BThe position moves to the right, because NO is the only product
CThe position moves to the left, because the reaction is endothermic
DThe position moves to the right, because the forward reaction has a higher rate
(b)
What is the equilibrium concentration of NO at 2000 K, in mol dm⁻³?
[1 mark]
A6.4 × 10⁻⁷
B5.7 × 10⁻³
C8.0 × 10⁻⁴
D2.8 × 10⁻³
(c)
The value of Kc at 2500 K is greater than at 2000 K. Explain this, with reference to the enthalpy change of the forward reaction.
[2 marks]
Total for question 5: 4 marks
6
Hydrogen is manufactured by steam reforming: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g), ΔH = +206 kJ mol⁻¹. The process uses a nickel catalyst at about 1100 K.
(a)
Which change would increase the equilibrium yield of hydrogen?
[1 mark]
AAdding more nickel catalyst
BIncreasing the total pressure at constant temperature
CDecreasing the temperature
DDecreasing the total pressure at constant temperature
(b)
Which is the correct expression for Kc for this equilibrium?
[1 mark]
A[CO][H₂] / ([CH₄][H₂O])
B[CO][H₂]³ / ([CH₄][H₂O])
C[CH₄][H₂O] / ([CO][H₂]³)
D[CO][H₂]³ / [CH₄]
(c)
State and explain how the value of Kc changes when the temperature is raised from 900 K to 1100 K, and state the effect of the nickel catalyst on the value of Kc.
[2 marks]
Total for question 6: 4 marks
7
Butane and methylpropane are structural isomers that interconvert in the presence of a catalyst: butane(g) ⇌ methylpropane(g). At a fixed temperature, an equilibrium mixture contains 0.150 mol dm⁻³ butane and 0.60 mol dm⁻³ methylpropane.
(a)
Write an expression for Kc and calculate its value at this temperature, with units if appropriate.
[3 marks]
(b)
At the same temperature, 2.00 mol of pure butane is placed in a rigid 4.00 dm³ vessel with the catalyst and left to reach equilibrium. Calculate the equilibrium concentration of methylpropane.
[4 marks]
Total for question 7: 7 marks
8
Sulfuryl chloride decomposes reversibly in a sealed flask: SO₂Cl₂(g) ⇌ SO₂(g) + Cl₂(g), ΔH = +68 kJ mol⁻¹. The flask has reached equilibrium at constant temperature. A student then investigates how the equilibrium mixture responds to changes, starting by injecting extra chlorine gas quickly into the flask.
(a)
Explain what is meant by dynamic equilibrium, and describe what happens to the rates of the forward and reverse reactions and to the position of equilibrium after the chlorine is injected, until the new equilibrium is established.
[6 marks]
(b)
The student then states: "Because Kc is a constant, changing the conditions can never change the amounts present in an equilibrium mixture." Evaluate this statement by considering the effect on the equilibrium mixture and on Kc of increasing the pressure by compression, raising the temperature, and adding a catalyst.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).