A student carried out four electrolysis experiments, each using carbon electrodes and a direct current. Table 13.1 lists the electrolyte used in each experi...

This question tests the products of electrolysis for four different electrolytes. Work out the cathode product (a metal or hydrogen) and the anode product (a non-metal) for each.

(a) The completed table:

Exp 1 molten (no water present): lead [1], bromine [1]. Exp 2: hydrogen [1] (sodium too reactive), chlorine [1] (chloride concentrated). Exp 3: copper [1] (copper below hydrogen in reactivity), oxygen [1] (from the sulfate/water). Exp 4: hydrogen [1], oxygen [1] (electrolysis of the water, i.e. "electrolysis of acidified water").

(b) A metal is deposited at the cathode in experiments 1 and 3 [2] (lead and copper). In experiments 2 and 4 the cathode gives hydrogen because the metal ion present is too reactive to be discharged from aqueous solution.

(c) Test for oxygen: insert a glowing splint into the gas [1]; the splint relights [1], because oxygen supports combustion.

(d) Molten lead(II) bromide conducts because the ions are free to move and carry the charge; in the solid the ions are held in fixed positions in the lattice and cannot move [1]. Moving charged particles (ions) are what conduct the current.

(e) Safety precaution for experiment 2: work in a fume cupboard, because chlorine is toxic [1].

Exam tip: "molten" removes water, so the metal itself forms at the cathode; in aqueous solution a reactive metal is replaced by hydrogen.