A student dissolves excess solid silver chloride, AgCl(s), in water at 25 °C and allows the system to reach equilibrium. The student then adds a small amount of concentrated sodium chloride solution to the equilibrium mixture. Explain what happens to the position of equilibrium and to the value of Ksp after the sodium chloride is added.

IB DP Chemistry Standard 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).

The dissolution equilibrium for silver chloride can be represented as: AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)

Model answer (4 marks)

Adding NaCl increases the concentration of Cl⁻(aq) ions. The reaction quotient Qsp therefore becomes larger than Ksp, so the equilibrium shifts to the left, forming more AgCl(s). This reduces the concentrations of Ag⁺(aq) and Cl⁻(aq) until Qsp again equals Ksp and equilibrium is re‑established. The value of Ksp does not change because the temperature remains 25 °C.

Examiner tips

  • State that Cl⁻ concentration rises, Qsp > Ksp, shift left, Ksp unchanged
  • Use the correct symbols (Qsp, Ksp) and mention temperature

Common mistakes

  • Saying the equilibrium shifts to the right
  • Claiming Ksp changes with NaCl addition
  • Using the wrong symbol for the equilibrium constant

Mark scheme (4 marks)

  1. Adding sodium chloride increases the concentration of Cl⁻(aq) ions in the mixture.
  2. The reaction quotient Qsp becomes greater than Ksp, so the equilibrium position shifts to the left (towards the reactants / towards more AgCl(s)).
  3. The shift to the left reduces the concentration of Cl⁻(aq) (and Ag⁺(aq)) until Q equals Ksp again / until equilibrium is re-established.
  4. The value of Ksp remains unchanged because temperature has not changed.

Key terms in this question

Ksp

Related

More How far? The extent of chemical change questions

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