Reactions of Acids (C4.2)

Acids and alkalis

Acid: substance that dissociates in water to produce H⁺ ions (protons). pH < 7.
Alkali: substance that produces OH⁻ ions in water. pH > 7.
Neutral: pH = 7.

Strong acids fully dissociate (ionise) in water: HCl, H₂SO₄, HNO₃.
Weak acids only partially dissociate: ethanoic acid (CH₃COOH), carbonic acid, citric acid.

Same concentration: strong acid has a lower pH than a weak acid (more H⁺ ions).

The pH scale and indicators

pH is a measure of H⁺ ion concentration. pH 0 = most acidic; pH 14 = most alkaline.
Each unit change in pH = 10× change in H⁺ concentration.

Indicators: litmus (red in acid, blue in alkali), universal indicator (colour range), phenolphthalein (colourless in acid, pink in alkali).

Reactions with metals

acid + metal → salt + hydrogen
Example: 2HCl + Mg → MgCl₂ + H₂

Test for hydrogen: place a lit splint at the test tube mouth → squeaky pop.

Reactions with metal oxides (neutralisation)

acid + metal oxide → salt + water
Example: H₂SO₄ + CuO → CuSO₄ + H₂O
Ionic equation: H⁺ + OH⁻ → H₂O (for all acid-alkali neutralisations)

Reactions with metal hydroxides

acid + metal hydroxide → salt + water
Example: HCl + NaOH → NaCl + H₂O

Reactions with metal carbonates

acid + metal carbonate → salt + water + carbon dioxide
Example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

Test for CO₂: bubble through limewater → turns milky/cloudy.

Making soluble salts

1. Add excess metal/metal oxide/carbonate to acid (ensures all acid reacts).
2. Filter off excess solid.
3. Evaporate filtrate to crystallise the salt.

Naming salts

Hydrochloric acid → chloride salt (e.g. NaCl — sodium chloride).
Sulfuric acid → sulfate salt (e.g. MgSO₄ — magnesium sulfate).
Nitric acid → nitrate salt (e.g. Ca(NO₃)₂ — calcium nitrate).

Common exam errors

1. Saying strong acid = concentrated acid — strength refers to degree of dissociation; concentration refers to amount per volume.
2. Forgetting CO₂ is produced from carbonate reactions (not just water + salt).
3. Saying neutralisation produces water and an acid — it produces water and a salt.