Describe electrolytic cells, predict electrode products, and apply Faraday's relationships to calculate amounts in electrolysis.

What this dot point is asking

You must describe how an electrolytic cell works, predict the products at each electrode, and carry out quantitative electrolysis calculations.

How an electrolytic cell differs from a galvanic cell

A galvanic cell produces electricity from a spontaneous reaction; an electrolytic cell consumes electricity to drive a non-spontaneous one. The electrode definitions by process are unchanged, but the signs swap.

Predicting electrode products

In molten ionic compounds, the only ions present are the cation and anion, so the metal is deposited at the cathode and the non-metal formed at the anode.

In aqueous solutions, water can also be oxidised or reduced, so it competes with the dissolved ions:

• At the cathode, the more easily reduced species wins. Water is reduced to hydrogen unless a less reactive metal ion (e.g. $\text{Cu}^{2+}$, $\text{Ag}^+$) is present, in which case the metal is deposited.
• At the anode, water is oxidised to oxygen unless a halide ion is present in reasonable concentration, in which case the halogen tends to form.

Quantitative electrolysis

The charge passed is:

where $Q$ is charge (C), $I$ is current (A) and $t$ is time (s). The moles of electrons are:

Then use the half-equation to relate moles of electrons to moles of product.

Applications

Electrolysis is central to managing resources: extracting reactive metals (aluminium from molten alumina), purifying copper, and electroplating objects with a thin protective or decorative metal layer.