Chemical reactions, Year 10: electrolysis and extracting metals

An electrolyte

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

为什么Ionic 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

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

为什么The 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

提示Unlike charges attract.

为什么The 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

提示It arrives carrying a charge and does not keep it.

为什么At 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

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

为什么Each 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

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

为什么Lead 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)

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

为什么Bromide 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

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

为什么In 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

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

为什么Inert 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

提示The negative ions in the melt are the ones attracted there.

为什么Sodium 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

提示Compare where the two metals sit in the reactivity series.

为什么Carbon 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

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

为什么Aluminium 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

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

为什么Oxide 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

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

为什么The 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

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

为什么Aluminium 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.