Types of Chemical Reactions and Electrolysis
Oxidation and Reduction
Oxidation is the gain of oxygen. Reduction is the loss of oxygen. These always happen together in what are called redox reactions. At Higher tier you also define them by electron transfer, and a useful mnemonic is OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
For example, when copper oxide is reduced by carbon: \(2CuO + C \rightarrow 2Cu + CO_2\). The copper oxide loses oxygen (it is reduced) and the carbon gains oxygen (it is oxidised). Carbon is the reducing agent (it causes the reduction of copper oxide) and copper oxide is the oxidising agent (it causes the oxidation of carbon). At Higher Tier, you also need to explain redox in terms of electrons: the substance that is oxidised loses electrons, and the substance that is reduced gains electrons. For example, in the formation of magnesium oxide: \(Mg \rightarrow Mg^{2+} + 2e^-\) (oxidation) and \(O + 2e^- \rightarrow O^{2-}\) (reduction).
Oxidation = gain of oxygen OR loss of electrons. Reduction = loss of oxygen OR gain of electrons. In terms of electrons (Higher): the substance that is oxidised loses electrons; the substance that is reduced gains electrons.
Acids and Alkalis
Acids produce hydrogen ions (\(H^+\)) when they dissolve in water. Solutions of alkalis contain hydroxide ions (\(OH^-\)). The pH scale measures how acidic or alkaline a solution is, running from 0 (very strong acid) to 14 (very strong alkali). pH 7 is neutral, below 7 is acidic, and above 7 is alkaline.
You can measure pH using universal indicator (which changes colour across a range: red for strong acids, green for neutral, purple for strong alkalis) or a pH meter (which gives a precise numerical reading). Litmus paper gives a simple acid/alkali test but cannot distinguish between different strengths.
Dilute and concentrated refer to the amount of acid dissolved in a given volume of water. Strong and weak refer to the degree of ionisation. A strong acid (e.g. hydrochloric acid) fully ionises in water — every molecule produces \(H^+\) ions. A weak acid (e.g. ethanoic acid) only partially ionises — only some molecules produce \(H^+\) ions. When the hydrogen ion concentration increases by a factor of ten, the pH decreases by one.
Neutralisation and Making Salts
Neutralisation occurs when an acid reacts with a base or alkali to form a salt plus water. The ionic equation for neutralisation is:
$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$
Key reactions to know:
Acid + metal \(\rightarrow\) salt + hydrogen. For example: \(Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)\).
Acid + metal carbonate \(\rightarrow\) salt + water + carbon dioxide. For example: \(CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\).
Acid + alkali \(\rightarrow\) salt + water. For example: \(NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)\).
The name of the salt depends on the acid used: hydrochloric acid \(\rightarrow\) chloride salts, sulfuric acid \(\rightarrow\) sulfate salts, nitric acid \(\rightarrow\) nitrate salts.
Electrolysis
Electrolysis is the process of using an electric current to decompose (break down) an ionic compound that is either molten or dissolved in water. The ionic compound must be in a liquid state (molten or dissolved in water) so that the ions are free to move and carry the electrical current. Solid ionic compounds cannot be electrolysed because their ions are held in fixed positions within the lattice.
The ionic compound is placed in a container with two electrodes connected to a power supply. Positive ions (cations) move towards the cathode (negative electrode), where they gain electrons and are reduced. Negative ions (anions) move towards the anode (positive electrode), where they lose electrons and are oxidised. Metals (or hydrogen) form at the cathode; non-metals form at the anode.
For molten binary compounds, predicting products is straightforward. For example, molten sodium chloride produces sodium at the cathode and chlorine at the anode. For aqueous solutions, there are competing reactions because water also contains \(H^+\) and \(OH^-\) ions. At the cathode, hydrogen is produced instead of the metal if the metal is more reactive than hydrogen. At the anode, oxygen is usually produced (from \(OH^-\) ions) unless halide ions are present, in which case the halogen is produced. For example, electrolysing aqueous copper sulfate with inert (carbon) electrodes produces copper at the cathode (because copper is less reactive than hydrogen) and oxygen at the anode (because no halide ions are present). The electrodes used matter: inert electrodes (such as carbon or platinum) do not take part in the reaction, whereas non-inert electrodes (such as copper) can themselves dissolve or be deposited during electrolysis.
At the cathode: \(\text{Na}^+ + e^- \rightarrow \text{Na}\) (reduction). At the anode: \(2\text{Cl}^- \rightarrow Cl_2 + 2e^-\) (oxidation). Half equations show what happens at each electrode separately.