A concentration sensor measures the concentration of C in a closed reaction vessel.
A(aq)+B(aq)⇌C(aq) \mathrm{A(aq) + B(aq) \rightleftharpoons C(aq)} A(aq)+B(aq)⇌C(aq)At equilibrium, the sensor reading is 0.40 mol dm-3.
Some concentrated A is injected through a self-sealing port. The temperature stays constant and the volume of the mixture changes by a negligible amount.
What is observed after equilibrium is reached again?
The sensor remains at 0.40 mol dm−30.40\text{ mol dm}^{-3}0.40 mol dm−3 because equilibrium fixes the product concentration.
The sensor settles below 0.40 mol dm−30.40\text{ mol dm}^{-3}0.40 mol dm−3 as the equilibrium shifts towards the reactants.
The sensor settles above 0.40 mol dm−30.40\text{ mol dm}^{-3}0.40 mol dm−3 as the equilibrium shifts to form more C.
The sensor does not settle because C is continually formed and used at dynamic equilibrium.
124 exam-style questions on OCR GCSE Chemistry 5.3 Equilibria, covering 5.3.1 Reversible reactions and dynamic equilibrium and 5.3.2 Changing the position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.