C10

Question 1

Explain the effect of increasing pressure on the rate of reaction (2)

Answer 1

• increased rate AND greater concentration of molecules / more molecules per unit volume
• more collisions per second / more frequent collisions

Question 2

State le Chateliers principle (1)

Answer 2

The position of a dynamic equilibrium shifts to minimise the effect of any change

Question 3

Given reaction
Given conditions
Explain this statement in terms of le Chateliers principle (3)

Answer 3

Pressure mark:
Right hand side has ‘fewer gaseous moles’ OR vice versa

Temperature mark:
Forward reaction is exothermic

Equilibrium mark:
Lower temperature AND increasing pressure shifts equilibrium to the right

Question 4

Explain how catalyst increases rate of reaction (2)

Answer 4

Catalyst lowers the activation energy by providing an alternative route

With a catalyst a greater proportion of molecules have energy greater than activation energy / equal to activation energy

Question 5

Equation given: 2SO2 (g) +O2 (g) ⇌ 2SO3 (g)
Catalyst used

Explain the conditions of temperature and pressure that could be used to obtain the maximum equilibrium yield of sulfur trioxide

Discuss the importance of a compromise between equilibrium yield and reaction rate when deciding the operational conditions for this process (6)

Note: YERC

Answer 5

Indicative scientific points may include
Yield
• Increasing pressure increases yield of SO3
• Decreasing temperature increases yield of SO3
Explanation
• (pressure) more moles/molecules on the
reactant side ORA
• (temp.) the forward reaction is exothermic ORA
Rate
• Increasing pressure increases rate
• Increasing temperature increases rate
Compromise
• Choose a higher temperature which creates a
reduced yield but in a shorter space of time
IGNORE reference to increase pressure leading to safety/cost issues

Level 3 (5–6 marks)
All/most points covered and clearly linked.
Must have points taken across all of the headings in the indicative points for Level 3.
The explanations show a well-developed line of reasoning linked to appropriate suggestions which is clear and logically structured. The compromises are relevant and well thought out and clearly linked to the explanations.
Level 2 (3–4 marks)
Suggests correct conditions with explanations OR comments on compromises with reference to yield AND rate effect.
The explanations are linked to appropriate suggestions and show a line of reasoning with some structure. The compromises are relevant but may not be clearly linked to the explanation.
Level 1 (1–2 marks)
Comments on conditions with some explanation OR comments on
compromise with reference to yield OR rate.
The comments about yield/rate with explanation are basic and communicated in an unstructured way. The compromises may not be relevant with lack of reasoning.
0 marks
No response or no response worthy of credit.