IGCSE Chemistry 0620: Concentration Effects in Halide Electrolysis and Fuel Cells

A water-treatment company investigates the electrolysis of aqueous calcium bromide, $\text{CaBr}_2$ , using inert electrodes, and separately trials a hydrogen–oxygen fuel cell to power its delivery vans.

(a) A concentrated solution of calcium bromide is electrolysed using inert electrodes.

(i) State the product formed at the anode. [1]

(ii) Write an ionic half-equation for this reaction at the anode. [2]

(b) The company repeats the electrolysis using a dilute solution of calcium bromide instead.

(i) State the product now formed at the anode. [1]

(ii) Explain why this product differs from the one formed when the concentrated solution was used. [1]

(c) State one advantage and one disadvantage of using a hydrogen–oxygen fuel cell, rather than a petrol engine, to power a delivery van. [2]

Part (a): Concentrated calcium bromide solution

Concentrated aqueous calcium bromide contains a high concentration of bromide ions, $\text{Br}^-$ , compared with the hydroxide ions supplied by water. At the anode, bromide ions are discharged preferentially.

(i) The product at the anode is bromine.

(ii) The ionic half-equation, showing the loss of electrons (oxidation): $2\text{Br}^-(aq) \rightarrow \text{Br}_2(aq) + 2e^-$

Part (b): Dilute calcium bromide solution

(i) In the dilute solution, the product at the anode changes to oxygen.

(ii) Diluting the solution greatly reduces the concentration of bromide ions, while the concentration of water molecules (and the hydroxide ions they supply) stays comparatively high. With bromide ions now present in much lower relative concentration, hydroxide ions/water are discharged preferentially instead, releasing oxygen rather than bromine.

Part (c): Hydrogen–oxygen fuel cell versus a petrol engine

Advantage: a hydrogen–oxygen fuel cell produces only water as its chemical product, so a van powered this way emits no carbon dioxide or other polluting exhaust gases while running, unlike a petrol engine.

Disadvantage: hydrogen gas is difficult and expensive to store and transport safely. It is highly flammable and must be kept in reinforced, high-pressure tanks, and there are currently far fewer places to refuel with hydrogen than with petrol.

Final answers

(a)(i) Bromine. (a)(ii) $2\text{Br}^-(aq) \rightarrow \text{Br}_2(aq) + 2e^-$
(b)(i) Oxygen. (b)(ii) Bromide ions are in much lower relative concentration than hydroxide ions/water in the dilute solution, so hydroxide ions are discharged instead.
(c) Advantage: only water is produced, so no polluting emissions. Disadvantage: hydrogen is hard/expensive to store, transport and refuel safely.