Chemical reactions Study Guide

Electrolysis of an aqueous ionic solution involves passing an electric current through the solution. Since the ionic compound is dissolved in water, there are always four types of ions present in the solution that can move towards the electrodes: cations from the ionic compound, hydrogen ions (H+) from the dissociation of water, anions from the ionic compound, and hydroxide ions (OH-) from the dissociation of water. Both cations will move towards the negative electrode (cathode), and both anions will move towards the positive electrode (anode). At each electrode, there is a competition between the two types of ions to be discharged. The outcome of this competition depends on the reactivity series of metals for cations, and the concentration and type of anion for anions.

At the cathode (negative electrode), if the metal in the ionic compound is more reactive than hydrogen (e.g., sodium, potassium, calcium), then hydrogen gas will be produced from the reduction of H+ ions (2H+ + 2e- → H2). If the metal is less reactive than hydrogen (e.g., copper, silver), then the metal itself will be produced from the reduction of its cations (e.g., Cu2+ + 2e- → Cu).

At the anode (positive electrode), if halide ions (Cl-, Br-, I-) are present and are in sufficient concentration, then the corresponding halogen will be produced (e.g., 2Cl- → Cl2 + 2e-). If no halide ions are present, or if they are very dilute, or if other anions like sulfate (SO42-) or nitrate (NO3-) are present, then oxygen gas will be produced from the oxidation of hydroxide ions (4OH- → O2 + 2H2O + 4e-). This competition determines the products of electrolysis. PAG C2 explores these principles in practice.

In chemistry, oxidation and reduction are fundamental processes that always occur together in what is known as a redox reaction. These processes can be defined in terms of the transfer of electrons. Oxidation is defined as the loss of electrons by a species. When an atom or ion loses electrons, its charge becomes more positive or less negative. For example, if a neutral metal atom loses electrons to form a positive ion, it has been oxidised. Reduction, conversely, is defined as the gain of electrons by a species. When an atom or ion gains electrons, its charge becomes more negative or less positive. For instance, if a non-metal atom gains electrons to form a negative ion, it has been reduced. A helpful mnemonic to remember these definitions is OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). In any redox reaction, one species will always be oxidised while another is simultaneously reduced. The species that causes another species to be oxidised is called the oxidising agent (it itself gets reduced), and the species that causes another to be reduced is called the reducing agent (it itself gets oxidised). Electrons cannot simply appear or disappear; they must be transferred from one reactant to another.