Chromate ions, CrO42−\mathrm{CrO_{4}^{2-}}CrO42−, are yellow. Dichromate ions, Cr2O72−\mathrm{Cr_{2}O_{7}^{2-}}Cr2O72−, are orange.
In solution, the two ions are in a dynamic equilibrium.
2CrO42−(aq)+2H+(aq)⇌Cr2O72−(aq)+H2O(l) \mathrm{2CrO_{4}^{2-}(aq) + 2H^{+}(aq) \rightleftharpoons Cr_{2}O_{7}^{2-}(aq) + H_{2}O(l)} 2CrO42−(aq)+2H+(aq)⇌Cr2O72−(aq)+H2O(l)A student has a beaker of an orange-yellow solution that is at equilibrium.
The student adds a few drops of dilute sulfuric acid to the solution.
Predict the colour change. Explain your answer.
The student then adds sodium hydroxide solution until it is in excess.
Predict the colour of the solution. Explain your answer.
Another student says, "Increasing the pressure above the solution would change its colour."
Evaluate this statement.
A third student says:
"When the colour of the solution stops changing, there must be equal concentrations of chromate ions and dichromate ions, and the reactions have stopped."
Evaluate this statement.
47 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.