In electrolysis of a solution, how do I know whether hydrogen or the metal forms at the cathode?

Compare the metal's reactivity with hydrogen's. That one comparison decides it, and it is the step that separates electrolysis of a solution from electrolysis of a molten compound.

Molten first, because it is simpler

A molten ionic compound contains only two kinds of ion, so there is no competition.

• Cathode (negative): the metal
• Anode (positive): the non-metal

Molten lead bromide gives lead at the cathode and bromine at the anode. Nothing else is present to interfere.

Solutions add water, and water changes everything

In a solution there are also H⁺ and OH⁻ ions from the water itself. Now two species compete at each electrode, and you need a rule.

At the cathode, compare the metal with hydrogen in the reactivity series.

• Metal more reactive than hydrogen (potassium through to aluminium, and zinc and iron in dilute solutions) → hydrogen is produced. The more reactive metal prefers to stay as an ion in solution.
• Metal less reactive than hydrogen (copper, silver, gold) → the metal is deposited.

Electrolysing copper sulfate solution gives a copper coating. Electrolysing sodium chloride solution gives hydrogen, not sodium — and that is the result students most often get wrong.

At the anode, a different rule applies.

• A halide present in reasonable concentration → that halogen (chlorine, bromine, iodine)
• No halide → oxygen, from the hydroxide ions

So sodium chloride solution gives chlorine; sodium sulfate solution gives oxygen, because sulfate is not a halide and stays in solution.

The half equations

These carry marks on their own and are predictable once you know the products.

Cathode — reduction, electrons gained:

2H+  +  2e-   ->  H2
Cu2+ +  2e-   ->  Cu

Anode — oxidation, electrons lost:

2Cl-        ->  Cl2  +  2e-
4OH-        ->  O2   +  2H2O  +  4e-

Reduction happens at the cathode; oxidation at the anode. Both are true for every electrolysis, and a question asking which electrode is the site of oxidation is checking that one fact.

A practical point worth remembering: the cathode is negative in electrolysis, which attracts the positive ions. Students who memorise "cathode is positive" from a different context get every product backwards.