A student electrolysed aqueous copper(II) sulfate using two copper electrodes. A meter labelled A was connected in the circuit, as shown in Fig. 11.1. (a) N...

This is electrolysis of copper(II) sulfate with copper electrodes and an ammeter in the circuit. It tests how the current is measured and controlled and how to improve the experiment.

(a) Meter A is an ammeter [1]; it measures the (electric) current, in amperes [1]. It is connected in series so the same current flows through it as through the cell.

(b) The value shown on meter A is 0.40 A (read from the label on Fig. 11.1) [1].

(c) At the cathode copper is deposited (a copper layer forms as \( \mathrm{Cu^{2+} + 2e^- \rightarrow Cu} \)) [1]; at the anode the copper dissolves and loses mass (\( \mathrm{Cu \rightarrow Cu^{2+} + 2e^-} \)) [1].

(d) To increase the mass of copper deposited, any two of [2]: increase the current; pass the current for a longer time. Both increase the total charge passed, and mass deposited is proportional to the charge.

(e) To keep the current constant, put a variable resistor (rheostat) in the circuit [1] and adjust it whenever the ammeter reading changes so the reading stays the same [1]. This is needed because the resistance of the cell drifts as the experiment runs.

(f) Two sources of error (any two) [2]: some copper flakes off the cathode before weighing; the electrode is not fully dry when reweighed; the copper anode contains impurities that fall off; heat or evaporation changes the solution. Each of these makes the measured mass change differ from the true value.

Exam tip: mass deposited depends on charge (current x time), so quoting both a bigger current and a longer time is the safe pair of answers.