For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), explain how an increase in total pressure at constant temperature affects the position of equilibrium and the value of Kc.

IB DP Chemistry Higher Level (2023 syllabus) — R2.3 How far? The extent of chemical change · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

An increase in total pressure shifts the equilibrium to the side with fewer moles of gas, i.e. to the right (towards NH₃). This is because the system responds to the pressure rise by reducing the number of gas molecules, in accordance with Le Chatelier’s principle. The equilibrium constant Kc is unchanged, as it depends only on temperature and not on pressure; at constant temperature the value of Kc remains the same even though the concentrations of the individual species change.

Examiner tips

  • State the shift of equilibrium (right) and the reason (fewer moles of gas).
  • Mention Le Chatelier’s principle explicitly.
  • Note that Kc is constant at constant temperature.
  • Use correct chemical equation and symbols.

Common mistakes

  • Saying the equilibrium shifts to the left or to the side with more moles.
  • Claiming that Kc changes with pressure.
  • Using Kp instead of Kc without justification.

Mark scheme (4 marks)

  1. An increase in pressure shifts the position of equilibrium to the right / towards the side with fewer moles of gas.
  2. Because increasing pressure increases the concentration (or partial pressure) of all gaseous species, the system responds to reduce the pressure / number of moles of gas, in accordance with Le Chatelier's principle.
  3. The value of Kc remains unchanged / constant.
  4. Kc depends only on temperature; since temperature is constant, Kc is unchanged even though the concentrations of individual species change.

Key terms in this question

position of equilibrium · Kc · total pressure

Related

More How far? The extent of chemical change questions

▶ Try answering this question with AI marking (free) →