Chemical reactions, Year 10: electrolysis and extracting metals

An electrolyte

HintThe word shares its opening with the name of the process that uses it.

The whyIonic compounds conduct when molten or dissolved in water, because then their ions can move. Passing a current through such a liquid breaks the compound down, which is electrolysis.

The cathode

HintIts name begins with the third letter of the alphabet; the name of the positive one begins with the first.

The whyThe negative electrode has this name and the positive electrode is the anode. Positively charged ions are attracted to the negative electrode, and negatively charged ions to the positive one.

The negatively charged ions

HintUnlike charges attract.

The whyThe positive electrode, the anode, attracts ions with the opposite charge. In molten lead bromide, for example, the bromide ions move to the anode and the lead ions move to the cathode.

It is discharged, becoming an atom or molecule of an element

HintIt arrives carrying a charge and does not keep it.

The whyAt the electrode the ion gains or loses electrons, so it loses its charge. Metal ions become metal atoms; non-metal ions become atoms that usually pair up into molecules, such as Br2. This is how electrolysis turns a compound into its elements.

A half equation

HintIt tells only part of the story of the whole reaction, and its name says what fraction.

The whyEach electrode has its own, for example Pb²⁺ + 2e⁻ → Pb at the negative electrode, where e⁻ stands for an electron. Higher tier candidates are expected to write these, or to complete and balance ones that are supplied.

Lead

HintDecide which of the two ions in the compound carries a positive charge.

The whyLead bromide contains metal ions, Pb²⁺, and bromide ions, Br⁻. The positive metal ions are attracted to the negative electrode, where they are discharged and collect as a molten metal.

A brown gas (bromine)

HintThe non-metal is released there, and at this temperature it is not a liquid.

The whyBromide ions are attracted to the positive electrode and are discharged there. The melt is hot enough for the element, a liquid at room temperature, to come off as a coloured vapour. Because that vapour is toxic, the demonstration is done in a fume cupboard.

Melting frees its ions to move

HintThink about what carries the charge through the liquid, and where those particles are stuck in a solid.

The whyIn the solid, the ions are held in fixed positions in a lattice. Once the compound is molten they can travel to the electrodes, so a current can pass and the compound can be broken down.

They do not react themselves

HintTheir only job is to carry the current into and out of the liquid.

The whyInert electrodes, usually graphite or platinum, conduct the current but take no part in the chemical changes. That way, whatever forms comes only from the electrolyte.

Chlorine

HintThe negative ions in the melt are the ones attracted there.

The whySodium chloride contains Na⁺ and Cl⁻ ions. The negative ions move to the positive electrode and are discharged as a gas, while sodium forms at the negative electrode.

It is more reactive than carbon

HintCompare where the two metals sit in the reactivity series.

The whyCarbon can take oxygen only from the oxides of metals below it in the reactivity series. Aluminium is above carbon, so carbon cannot reduce aluminium oxide and electricity has to be used instead.

Cryolite

HintIts name starts like the word for the science of extreme cold, though it is used molten.

The whyAluminium oxide melts at over 2000 °C, which would take a huge amount of energy. Dissolved in this molten compound it can be electrolysed below 1000 °C, which saves energy and money.

They react with the oxygen formed there and burn away

HintThink about which gas is released at that electrode, and what hot carbon does in that gas.

The whyOxide ions are discharged at the positive electrodes. At the working temperature the gas that forms reacts with the carbon to make carbon dioxide, so the electrodes are gradually used up.

melt; current

HintOne is a change of state; the other is what flows through the cell.

The whyThe compound must be molten so that its ions can move, and a very large flow of charge must be kept up for a long time to discharge them. Both cost a great deal of energy, which is one reason metals such as aluminium are worth recycling.

Oxygen

HintThe compound holds only two elements, and the metal goes to the other electrode.

The whyAluminium oxide contains aluminium ions and oxide ions. The negative oxide ions travel to the positive electrode and are discharged as a gas, which then reacts with the hot carbon electrode to give carbon dioxide.