Explain how the addition of a small amount of concentrated hydrochloric acid to an aqueous equilibrium mixture of chromate(VI) ions and dichromate(VI) ions affects the position of equilibrium and the value of Kc. The equilibrium is: 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l)
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Adding HCl increases the concentration of H⁺ ions in the mixture.
The system responds by shifting the equilibrium position to the right (forward direction) to oppose the increase in H⁺ concentration, in accordance with Le Chatelier’s principle.
Consequently [CrO₄²⁻] decreases and [Cr₂O₇²⁻] increases, so the solution changes from yellow to orange.
The value of Kc remains unchanged because temperature has not changed; Kc depends only on temperature.
The system responds by shifting the equilibrium position to the right (forward direction) to oppose the increase in H⁺ concentration, in accordance with Le Chatelier’s principle.
Consequently [CrO₄²⁻] decreases and [Cr₂O₇²⁻] increases, so the solution changes from yellow to orange.
The value of Kc remains unchanged because temperature has not changed; Kc depends only on temperature.
Examiner tips
- Use the exact reaction and show the shift direction clearly; mention Le Chatelier’s principle. State the colour change as evidence of the shift. Remember Kc is temperature‑dependent only – no change here.
Common mistakes
- Saying the equilibrium shifts to the left. Claiming Kc changes without a temperature change. Omitting the colour change as evidence.
Mark scheme (4 marks)
- Adding HCl increases the concentration of H⁺ ions in the mixture.
- The system responds by shifting the equilibrium position to the right (forward direction) to oppose/reduce the increase in H⁺ concentration, in accordance with Le Chatelier's principle.
- As a result, [CrO₄²⁻] decreases and [Cr₂O₇²⁻] increases, so the solution changes from yellow to orange.
- The value of Kc remains unchanged because temperature has not changed; Kc depends only on temperature.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More How far? The extent of chemical change questions
- For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), explain how an increase in total p…
- Explain why the value of the equilibrium constant, Kc, for a reversible reaction…
- A student claims that adding a catalyst to a reaction mixture at equilibrium wil…
- A saturated aqueous solution of calcium fluoride, CaF₂, is in equilibrium with e…