Reactions of Acids, pH and Neutralisation
Acids, Bases and the pH Scale
An acid is a substance that produces hydrogen ions (\(H^+\)) when dissolved in water. A base is a substance that can neutralise an acid. An alkali is a soluble base that produces hydroxide ions (\(OH^-\)) in water.
The pH scale runs from 0 to 14. Acids have a pH below 7, neutral solutions have a pH of 7, and alkalis have a pH above 7. pH can be measured with universal indicator or a pH probe.
Acids produce \(H^+\) ions in water; alkalis produce \(OH^-\) ions. The pH scale: 0–6 = acidic, 7 = neutral, 8–14 = alkaline. Neutralisation: \(H^+ + OH^- \rightarrow H_2O\).
Neutralisation Reactions
Neutralisation occurs when an acid reacts with a base. The ionic equation is:
$$H^+ + OH^- \rightarrow H_2O$$
The key reactions of acids are:
- Acid + metal → salt + hydrogen. E.g. \(Mg + 2HCl \rightarrow MgCl_2 + H_2\)
- Acid + metal oxide → salt + water. E.g. \(CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O\)
- Acid + metal hydroxide → salt + water. E.g. \(NaOH + HCl \rightarrow NaCl + H_2O\)
- Acid + metal carbonate → salt + water + carbon dioxide. E.g. \(CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\)
The salt produced depends on which acid is used: hydrochloric acid (\(HCl\)) makes chlorides, sulfuric acid (\(H_2SO_4\)) makes sulfates, and nitric acid (\(HNO_3\)) makes nitrates.
To make a pure, dry salt: add an excess of insoluble base or metal oxide to the acid, filter off the excess solid, then evaporate the water from the filtrate to leave salt crystals. In Combined Science this is Required Practical 8.
HCl → chlorides. \(H_2SO_4\) → sulfates. \(HNO_3\) → nitrates. To predict the salt name: metal name + acid type (e.g. magnesium + sulfuric acid = magnesium sulfate).
Strong acids (e.g. hydrochloric, sulfuric, nitric acid) fully ionise in water — every acid molecule releases its \(H^+\) ions. Weak acids (e.g. ethanoic acid, citric acid) only partially ionise — an equilibrium is established between ionised and un-ionised molecules. At the same concentration, a strong acid has a lower pH than a weak acid.