Chemical changes — section overview

Section C4 covers acids and bases, metal reactions, and electrolysis — the chemistry of transformation.

Acids, bases and neutralisation

Acid: substance that releases H⁺ ions in solution. pH < 7. Base: substance that neutralises an acid. pH > 7. Alkali: a base that dissolves in water.

Neutralisation: Acid + Base → Salt + Water Example: HCl + NaOH → NaCl + H₂O

Salt production:

• Metal + acid → salt + hydrogen
• Metal oxide/hydroxide + acid → salt + water
• Metal carbonate + acid → salt + water + CO₂

Naming salts: HCl → chloride; H₂SO₄ → sulfate; HNO₃ → nitrate.

pH and the H⁺ concentration

The pH scale (0–14):

• Strong acids: fully ionise in water → high [H⁺] → low pH
• Weak acids: partially ionise → lower [H⁺] → higher pH (than strong acid at same concentration)

pH changes by 1 = ×10 change in [H⁺].

Metal reactions

Reactivity series (most to least reactive): K, Na, Li, Ca, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au

• Metals above hydrogen in the reactivity series react with dilute acids to produce hydrogen gas.
• More reactive metals displace less reactive metals from solutions.

Displacement: Zn + CuSO₄ → ZnSO₄ + Cu (zinc is more reactive than copper)

Extraction of metals

Reduction with carbon: metals below carbon in reactivity can be extracted by heating ore with carbon (e.g. iron from iron oxide). Fe₂O₃ + 3CO → 2Fe + 3CO₂

Electrolysis: used for metals above carbon in reactivity series (e.g. aluminium) — too reactive to extract with carbon.

Electrolysis

Passing electric current through an ionic compound (molten or dissolved) to break it down.

At cathode (−): cations (+) reduced (gain electrons). Metal deposited. At anode (+): anions (−) oxidised (lose electrons). Non-metal released.

Electrolysis of brine (NaCl solution):

• Cathode: 2H⁺ + 2e⁻ → H₂ (hydrogen gas)
• Anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas)
• Remaining solution: NaOH (sodium hydroxide)

OIL RIG

Oxidation Is Loss of electrons Reduction Is Gain of electrons

Common exam mistakes in C4

1. Strong acid ≠ concentrated acid — strong = fully ionises; concentrated = high amount per unit volume
2. Cathode is negative — positive ions are attracted to the NEGATIVE electrode (cathode) and are REDUCED
3. Displacement — more reactive displaces less reactive — not the other way around