AQA A-Level Chemistry Paper 2, 2022: Question 2

11 marks · Easy difficulty · State/Explain/Numerical

Tetrachloroethene Equilibrium and Environmental Impact This question explores equilibrium concepts in the production of tetrafluoroethene from chlorodifluoromethane. It involves calculations of equilibrium amounts and the equilibrium constant (Kc), with its expression and units. Additionally, it examines the effect of temperature on equilibrium yield and discusses the environmental issues caused by chlorodifluoromethane, contrasting its impact with pentane as a refrigerant alternative.

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Question

AQA A-Level Chemistry Paper 2, 2022: Question 2
Question text

02 Tetrafluoroethene is made from chlorodifluoromethane in this reversible reaction.

2 CHClF (g) ⇌ C F (g) + 2 HCl(g) ΔH = +128 kJ mol−1

22 4

A 2.00 mol sample of CHClF is placed in a container of volume 23.2 dm3 and heated.

When equilibrium is reached, the mixture contains 0.270 mol of CHClF2

02.1 Calculate the amount, in moles, of C2F4 and of HCl in the equilibrium mixture.

[2 marks]

Amount of C2F4 mol

Amount of HCl mol

02.2 Give an expression for Kc for this equilibrium.

[1 mark]

Kc

02.3 Calculate a value for Kc

Give its units.

[3 marks]

Kc Units

02.4 State and explain the effect of using a higher temperature on the equilibrium yield of

tetrafluoroethene.

[3 marks]

Effect on yield

Explanation

02.5 Chemists provided evidence that was used to support a ban on the use of

chlorodifluoromethane as a refrigerant.

Many refrigerators now use pentane as a refrigerant.

State the environmental problem that chlorodifluoromethane can cause.

Give one reason why pentane does not cause this problem.

[2 marks]

Environmental problem

*09* Reason why pentane does not cause this problem

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2022: Question 2

Question Answers Additional Comments/Guidelines Mark

C2F4 = 0.865 mol Award 1 mark if HCl = 2 × C2F4 M1

02.1

HCl = 1.73 mol M2

(2 x AO2)

K = [C F ] [HCl]2 Penalise round brackets 1

c 2 4

02.2 2

[CHClF2] (AO2)

K = [0.865/23.2][1.73/23.2]2 Allow ecf for use of their answer(s) to Q2.1 and M1

c

[0.27/23.2]2 Q2.2

M1 for dividing by volume

02.3 Kc = 1.5(3) must be at least 2sf Allow 1.53-1.54 If no use of volume allow M2 for 35.5 M2

If upside down can allow all 3 marks as ECF to

−3 Q2.2 Leads to an answer of 0.65(3) mol–1dm3

Units = mol dm M3

(3 x AO2)

Yield would increase M1

Equilibrium opposes temperature increase Shifts /moves to reduce temperature M2

02.4

Moves in the endothermic direction Ignore favours M3

(3 x AO2)

Causes ozone depletion/decomposition/damage Accept hole in the ozone layer M1

Pentane does not have C-Cl bonds Accept does not produce Cl radicals M2

02.5

Accept does not contain chlorine (2 x AO1)

How to answer it

Equilibrium and Environmental Chemistry Guide

Part (a)

What to do: Use the stoichiometry of the reaction to deduce the amounts of the other species in the equilibrium mixture.

Amount of C2F4 = Amount of CHClF2 reacted ÷ 2
Amount of HCl = 2 × Amount of C2F4

Common errors:

  • Incorrectly calculating the amount of CHClF2 reacted.
  • Not recognising that HCl is produced in a 2:1 ratio with C2F4.
Why students get it wrong: Some students mistakenly think the amount of HCl equals the amount of CHClF2 reacted, ignoring the stoichiometric ratio.

Part (b)

What to do: Write the equilibrium constant expression using the concentrations of the species.

Kc = [C2F4][HCl]2 ÷ [CHClF2]2

Common errors:

  • Using round brackets instead of square brackets for the expression.
  • Forgetting to square the concentrations of HCl and CHClF2.
Teacher Tip: Remember to use square brackets to represent concentration and ensure all exponents match the stoichiometry of the equation.

Part (c)

What to do: Substitute the concentrations into the Kc expression and include the volume in the calculations.

Kc = ([C2F4] ÷ volume) × ([HCl] ÷ volume)2 ÷ ([CHClF2] ÷ volume)2

Common errors:

  • Forgetting to divide the amounts by the volume of the container to calculate concentrations.
  • Incorrectly rearranging the expression.
Why students get it wrong: Students often assume the given amounts are already concentrations and skip the division by volume.

Part (d)

What to do: Apply Le Chatelier’s principle to predict the effect of temperature on the equilibrium yield of tetrafluoroethene.

Common errors:

  • Incorrectly stating that a higher temperature would decrease the yield without linking this to the endothermic nature of the forward reaction.
  • Failing to mention that the equilibrium shifts to oppose the increase in temperature.
Teacher Tip: Always relate the direction of equilibrium shift to the sign of ΔH and the principle of opposing the change.

Part (e)

What to do: Identify the environmental impact of chlorodifluoromethane (ozone depletion) and explain why pentane does not cause the same issue.

Common errors:

  • Stating general environmental problems like “global warming” instead of the specific issue of ozone depletion.
  • Failing to explain that pentane does not contain chlorine and therefore does not release chlorine radicals.
Why students get it wrong: Some students confuse the role of chlorofluorocarbons in ozone depletion with their role in global warming, leading to vague or incorrect answers.

Final Notes

This question set covers equilibrium constants, stoichiometry, and environmental chemistry. Pay attention to units, stoichiometric ratios, and the precise wording of environmental impacts. Practice writing equilibrium constant expressions and using Le Chatelier’s principle to strengthen your understanding.

Topics

Physical Chemistry · Organic Chemistry · 3.1.6 Chemical Equilibria, Le Chatelier's Principle, and Kc · 3.3.3 Halogenoalkanes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.