Topic 4: Electrochemistry

Electrochemistry studies the link between electrical energy and chemical change. When an electric current passes through a molten or dissolved ionic compound, the compound breaks down in a process called electrolysis. This topic explains where the ions go, what forms at each electrode, and how these ideas are used in industry and energy.

The basics: electrolytes and electrodes

Electrolysis is the breakdown of an ionic compound, when molten or in aqueous solution, by the passage of electricity. The liquid that conducts and is decomposed is the electrolyte; it conducts because its ions are free to move and carry charge. The two electrodes are rods, usually made of inert graphite or platinum, that dip into the electrolyte and connect to the power supply. The electrode joined to the positive terminal is the anode, and the electrode joined to the negative terminal is the cathode. During electrolysis, positive ions (cations) move toward the negative cathode, while negative ions (anions) move toward the positive anode. A solid ionic compound cannot be electrolysed because its ions are locked in a lattice and cannot move.

Electrolysis of molten compounds

Molten binary ionic compounds give the simplest case because only the two ions of the compound are present. For molten lead(II) bromide, the lead ions are attracted to the cathode where they gain electrons and form liquid lead metal, while the bromide ions move to the anode where they lose electrons and form brown bromine vapour. The general rule for any molten compound is straightforward: the metal forms at the cathode and the non-metal forms at the anode. This is exactly how reactive metals such as aluminium are extracted industrially, since they are too reactive to obtain by heating with carbon.

Electrolysis of aqueous solutions: the rules

Aqueous solutions are more complex because water also provides hydrogen ions and hydroxide ions, so there is competition at each electrode. At the cathode, hydrogen gas is produced unless the metal present is less reactive than hydrogen, in which case the metal is deposited instead. So copper or silver will plate out, but the ions of reactive metals such as sodium or potassium stay in solution and hydrogen forms. At the anode, oxygen gas is usually produced from hydroxide ions, except when the solution contains a high concentration of a halide ion (chloride, bromide or iodide), in which case that halogen is released instead.

Products and observations

Applying the rules predicts the products. Electrolysis of dilute sulfuric acid gives hydrogen at the cathode and oxygen at the anode, effectively splitting water, with twice the volume of hydrogen formed. Concentrated aqueous sodium chloride (brine) gives hydrogen at the cathode, chlorine at the anode, and leaves sodium hydroxide in solution, an important industrial process. Electrolysis of copper(II) sulfate solution with inert electrodes deposits a pink-brown layer of copper at the cathode and bubbles oxygen at the anode, and the blue colour of the solution fades as copper ions are removed. Watch for clues such as gas bubbles, colour changes and metal deposits.

Electroplating and purification of copper

Electroplating coats a cheaper metal object with a thin layer of another metal to improve its appearance or to protect it from corrosion. The object to be plated is made the cathode, the plating metal is made the anode, and the electrolyte is a solution of a salt of the plating metal. Copper is purified by electrolysis on a large scale: a thick block of impure copper is the anode and a thin sheet of pure copper is the cathode, with copper(II) sulfate solution as the electrolyte. Copper dissolves from the impure anode and pure copper deposits on the cathode, while insoluble impurities drop to the bottom as anode sludge.

Hydrogen-oxygen fuel cells

A hydrogen-oxygen fuel cell uses the reaction of hydrogen and oxygen to produce electrical energy directly, with water as the only product. Hydrogen is supplied to one electrode and oxygen to the other; the overall reaction is the same as burning hydrogen, but the energy is released as electricity rather than heat. Compared with petrol or diesel engines, fuel cells produce no carbon dioxide or polluting gases at the point of use and can be more efficient. Drawbacks include the high cost, the difficulty of storing flammable hydrogen safely, and the energy needed to produce the hydrogen in the first place.

Writing half equations

Half equations show what happens to electrons at a single electrode. At the cathode, reduction occurs because positive ions gain electrons; for example, the cathode reaction for sodium is Na+ + e- gives Na, and for hydrogen it is 2H+ + 2e- gives H2. At the anode, oxidation occurs because negative ions lose electrons; for example chloride forms chlorine by 2Cl- gives Cl2 + 2e-, and hydroxide forms oxygen by 4OH- gives O2 + 2H2O + 4e-. A reliable memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. Always check that both the atoms and the charges balance on each side.