Chemical reactions, Year 10: electrolysis of solutions and half equations

Hydrogen

HintSodium is far too reactive to be released from water, so something from the water takes its place.

The whyThe solution contains positive ions from the water as well as sodium ions. Because sodium is the more reactive, it is the ions from the water that are discharged, and bubbles of gas appear at the electrode.

Copper, as a pink-brown coating

HintThis metal sits below hydrogen in the reactivity series, so it is discharged in preference.

The whyBoth metal ions and hydrogen ions are attracted to the negative electrode. The metal here is less reactive than hydrogen, so its ions are the ones discharged and the electrode becomes plated.

Oxygen

HintNo halide ions are present, so the water supplies the ion that is discharged.

The whyAt the positive electrode a halogen forms only if the solution contains halide ions. Otherwise hydroxide ions from the water are discharged and a colourless gas is given off; the sulfate ions stay in solution.

From water molecules breaking down

HintLook at what the compound was dissolved in.

The whyA tiny fraction of the molecules of the solvent split into hydrogen ions and hydroxide ions. In electrolysis these compete with the ions of the dissolved compound, which is why hydrogen or oxygen is often produced instead of the expected element.

Chlorine

HintThe solution contains halide ions, and this is the halogen they belong to.

The whyWhen halide ions are present they are discharged at the positive electrode, giving the halogen instead of oxygen. This one is recognised because it turns damp litmus paper white.

So that the solution is the only thing that differs

HintThink about what makes a comparison between the results fair.

The whyThe investigation tests a hypothesis about how the dissolved compound affects what forms at each electrode. If the electrodes or the voltage also changed, a different product could not be put down to the dissolved compound alone.

Anode

HintOxidation is loss of electrons; ask which electrode the negative ions travel to in order to give theirs up.

The whyNegative ions move to the positive electrode and lose electrons there, which is oxidation. At the negative electrode positive ions gain electrons, which is reduction.

2H⁺ + 2e⁻ → H2

HintThe gas is made of molecules holding two atoms, so start by taking two ions.

The whyEach hydrogen ion needs one electron to become a hydrogen atom, and the atoms pair up as H2. Two ions therefore take two electrons. Because electrons are gained, this is a reduction.

2

HintEach ion carries one extra electron and has to give it up.

The whyTwo chloride ions each lose one electron, and the two chlorine atoms join as a Cl2 molecule. The charges balance: two negative charges on the left, two electrons on the right.

4OH⁻ → O2 + 2H2O + 4e⁻

HintBesides the gas, the ions leave behind a familiar liquid; it takes four ions to make one molecule of the gas.

The whyFour hydroxide ions give up four electrons and rearrange into one oxygen molecule and two water molecules. It can also be written 4OH⁻ − 4e⁻ → O2 + 2H2O. Because electrons are lost, this is an oxidation.

3

HintThe ion has to end up with no overall charge.

The whyAn Al³⁺ ion has three more protons than electrons, so it needs three electrons to become a neutral atom: Al³⁺ + 3e⁻ → Al. Gaining electrons means the aluminium ions are reduced.