Predicting reactions — reactivity series, neutralisation, electrolysis and qualitative analysis

The reactivity series

The reactivity series ranks metals (and hydrogen) by their tendency to lose electrons and form positive ions. The order (most to least reactive):

K > Na > Li > Ca > Mg > Al > Zn > Fe > Sn > Pb > (H₂) > Cu > Ag > Au > Pt

Key uses:

• Predictions: a metal reacts with an acid only if it is above hydrogen in the series; a metal displaces another metal from a salt solution only if it is higher in the series.
• Extraction method: metals above carbon in the series (K, Na, Ca, Mg, Al) cannot be reduced by carbon and must be extracted by electrolysis; metals below carbon (Zn, Fe, Cu, Sn) can be reduced by carbon in a blast furnace.

Metal displacement example: Mg + CuSO₄ → MgSO₄ + Cu (magnesium is more reactive than copper so it displaces copper from solution; magnesium is oxidised, copper ions are reduced — this is a REDOX reaction).

Neutralisation and pH

Acids release H⁺ ions in aqueous solution; alkalis release OH⁻ ions. The pH scale runs 0–14: pH < 7 is acidic, pH 7 is neutral, pH > 7 is alkaline. Each pH unit represents a 10-fold change in H⁺ concentration (Higher tier).

Common acids: hydrochloric (HCl), sulfuric (H₂SO₄), nitric (HNO₃). Common bases: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂), ammonia (NH₃, a weak alkali), metal oxides and carbonates.

Reactions forming named salts:

• Hydrochloric acid → chloride salts
• Sulfuric acid → sulfate salts
• Nitric acid → nitrate salts

PAG C1 — making copper sulfate: CuO + H₂SO₄ → CuSO₄ + H₂O. Excess insoluble CuO used to ensure acid is completely neutralised, then filtered off.

Electrolysis

Electrolysis is the decomposition of an ionic compound (the electrolyte) when it is molten or dissolved in water, using an electric current.

• Anode (+): anions (negative ions) are attracted here and are oxidised (lose electrons).
• Cathode (−): cations (positive ions) are attracted here and are reduced (gain electrons).

Mnemonic: PANIC — Positive Anode, Negative Is Cathode.

Electrolysis of molten ionic compounds

All ions discharge at the relevant electrode.

• Molten lead bromide: Pb²⁺ → Pb (cathode); 2Br⁻ → Br₂ (anode).

Electrolysis of aqueous solutions

Water provides H⁺ and OH⁻ ions as competition. Discharge preference:

• At the cathode: if the metal is below hydrogen in the reactivity series (e.g. Cu), metal ions are reduced; otherwise H₂ is produced.
• At the anode: if Cl⁻ ions are present in high concentration, Cl₂ is produced; otherwise O₂ (from OH⁻ ions) is produced.

Electrolysis of brine (NaCl aq):

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

Electrolysis of copper sulfate with copper electrodes: used to purify copper. Anode dissolves (Cu → Cu²⁺ + 2e⁻); cathode gains copper. Mass of each electrode changes by the same amount.

OCR PAG C7 covers electrolysis experiments (e.g. copper plating, electrolysis of dilute sulfuric acid). J258/02 regularly includes a 6-mark extended response on electrolysis.

Qualitative analysis — flame tests and chemical tests

Flame tests (metal cation identification)

Precipitate tests for metal ions (add NaOH solution)

Tests for anions

• Carbonate (CO₃²⁻): add dilute acid → fizzes; limewater turns milky (CO₂ produced).
• Sulfate (SO₄²⁻): add dilute HCl then barium chloride → white precipitate (BaSO₄).
• Chloride (Cl⁻): add dilute HNO₃ then silver nitrate → white precipitate (AgCl), which dissolves in ammonia.
• Nitrate (NO₃⁻): add NaOH + aluminium foil, warm → ammonia gas produced (turns damp red litmus blue). (Higher tier)

OCR PAG C8 covers qualitative analysis experiments.