# 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)

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

## Mark scheme (4 marks)

1. Adding HCl increases the concentration of H⁺ ions in the mixture.
2. 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.
3. As a result, [CrO₄²⁻] decreases and [Cr₂O₇²⁻] increases, so the solution changes from yellow to orange.
4. The value of Kc remains unchanged because temperature has not changed; Kc depends only on temperature.

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