Chemical changes
Acids and bases
Acid: a substance that produces H⁺ ions (protons) in aqueous solution. Examples: HCl, H₂SO₄, HNO₃, CH₃COOH. Base: a substance that reacts with an acid to form a salt and water only. Metal oxides and metal hydroxides are bases. Alkali: a base that dissolves in water to produce OH⁻ ions. Examples: NaOH, KOH, Ca(OH)₂.
pH scale
pH runs from 0–14:
- pH 0–6 = acidic (more H⁺ than OH⁻)
- pH 7 = neutral
- pH 8–14 = alkaline (more OH⁻ than H⁺)
Universal indicator and pH meters measure pH. A change of 1 pH unit = 10× change in H⁺ concentration (logarithmic).
Neutralisation
Acid + alkali → salt + water HCl + NaOH → NaCl + H₂O H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
The ionic equation for all neutralisation reactions: H⁺(aq) + OH⁻(aq) → H₂O(l)
Reactions of acids
| Reaction | Equation pattern | Example |
|---|---|---|
| Acid + metal | → salt + hydrogen | HCl + Mg → MgCl₂ + H₂↑ |
| Acid + metal oxide | → salt + water | H₂SO₄ + CuO → CuSO₄ + H₂O |
| Acid + metal hydroxide | → salt + water | HCl + NaOH → NaCl + H₂O |
| Acid + metal carbonate | → salt + water + CO₂ | HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑ |
Naming salts: the metal/base provides the metal name; the acid determines the suffix: hydrochloric → chloride; sulfuric → sulfate; nitric → nitrate.
Edexcel Core Practical CP2 — Investigating neutralisation
Aim: determine the volume of acid needed to neutralise an alkali. Method: add acid from a burette to a fixed volume of alkali + indicator; record colour change at end-point. Or use a pH probe and plot a pH curve (S-shape).
Electrolysis
Electrolysis is the decomposition of an ionic compound (electrolyte) using electricity. Requires ions to be free to move: ionic compound in solution or molten.
| Electrode | Charge | What happens |
|---|---|---|
| Anode (+) | Positive | Negative ions (anions) attracted; oxidation occurs |
| Cathode (−) | Negative | Positive ions (cations) attracted; reduction occurs |
Memory aid: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons).
Electrolysis of molten ionic compounds
Pure/molten NaCl: Na⁺ moves to cathode → reduced to Na metal; Cl⁻ moves to anode → oxidised to Cl₂ gas. Cathode: Na⁺ + e⁻ → Na Anode: 2Cl⁻ → Cl₂ + 2e⁻
Electrolysis of aqueous solutions
When water is present, water can also be discharged at electrodes: Cathode: if metal ion is less reactive than hydrogen, the metal is deposited (e.g. Cu²⁺ → Cu). Cathode: if metal ion is more reactive than hydrogen, hydrogen is produced (2H⁺ + 2e⁻ → H₂). Anode: if halide ions present, halogen gas is produced. Otherwise, oxygen is produced from water.
Edexcel Core Practical CP3 — Electrolysis of copper sulfate
Cathode: Cu²⁺ + 2e⁻ → Cu (copper deposits — cathode gains mass) Anode: Cu → Cu²⁺ + 2e⁻ (copper dissolves — anode loses mass) Using copper electrodes keeps the Cu²⁺ concentration constant (used industrially to purify copper).
⚠Common mistakes
- Anode/cathode confusion: anode is + (an-i-ons go to anode); cathode is − (cat-ions go to cathode).
- Discharge in aqueous solution: halide ions over oxygen (unless halide is very dilute); metal ion vs hydrogen depends on reactivity.
- Forgetting to balance half-equations: always balance charge and atoms.
- Weak acids: hydrochloric, sulfuric and nitric are STRONG acids (fully ionised); ethanoic (vinegar) is a WEAK acid.
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