SPEC 4.4.2.1 – 4.4.2.2 • MODULE 4 • HT FOR ACID+METAL REDOX
Reactions of Acids & Salt Production
Four reaction patterns. One naming rule. The salt's name is decided by the acid; the gas product depends on what the acid reacts with. Get the pattern right and every Paper 1 acid question becomes routine.
Why This Lesson Matters
Acid reactions are the most-tested topic in Module 4 — and the errors are very predictable. Students lose marks not because the chemistry is hard, but because they confuse which gas comes out and which salt is named.
4
Reaction Patterns
3
Acids Named in Spec
2/3
Scored Both on Q01.1 Jun 2024
Spec Scope (Important)
4.4.2.1 (acid + metal): spec limits this to magnesium, zinc and iron with hydrochloric and sulfuric acids.
4.4.2.2 (neutralisation/salt production): covers acid + base, alkali, and carbonate.
NOT in this lesson — the ionic equation H⁺ + OH⁻ → H₂O is in 4.4.2.4 (Lesson 4.05); the crystallisation method is in 4.4.2.3 (RP1, Lesson 4.04).
Key Definitions
Acid
A substance that produces H⁺ ions in aqueous solution. The three acids you must know: HCl, H₂SO₄, HNO₃.
Salt
A compound formed when the H⁺ ions in an acid are replaced by metal (or ammonium) ions.
Base
A substance that neutralises an acid. Includes metal oxides, metal hydroxides, and metal carbonates.
Alkali
A soluble base — i.e. a base that dissolves in water (e.g. NaOH, KOH).
Neutralisation
The reaction between an acid and a base (or alkali) to produce a salt and water. Carbonates also produce CO₂ as a third product.
The Four Reaction Patterns
Acids react with four types of substance. Each gives a salt — but the second product differs each time. Memorise this table.
Reactant TypeProductsExample
1. Metal (above H)salt + hydrogenMg + 2HCl → MgCl₂ + H₂
2. Metal oxide (base)salt + waterCuO + 2HCl → CuCl₂ + H₂O
3. Metal hydroxide (alkali if soluble; base if not)salt + waterNaOH + HCl → NaCl + H₂O
4. Metal carbonatesalt + water + CO₂CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
The Key Distinction
Hydrogen is ONLY produced from acid + metal. Carbonates give CO₂, not hydrogen. Oxides and hydroxides give only water. Confusing these is the most common mistake on this topic.
Pattern 1 — Acid + Metal
Carries forward from 4.01: a metal above hydrogen in the reactivity series displaces hydrogen from acid.
metal + acid → salt + hydrogen
Spec Scope — Limited to These Combinations Only
Spec 4.4.2.1 verbatim: "Knowledge of reactions limited to those of magnesium, zinc and iron with hydrochloric and sulfuric acids ."
Metal+ HCl+ H₂SO₄
MgMg + 2HCl → MgCl₂ + H₂Mg + H₂SO₄ → MgSO₄ + H₂
ZnZn + 2HCl → ZnCl₂ + H₂Zn + H₂SO₄ → ZnSO₄ + H₂
FeFe + 2HCl → FeCl₂ + H₂Fe + H₂SO₄ → FeSO₄ + H₂
Reactivity Reminder (4.01)
Vigour decreases down the group: Mg reacts fastest, then Zn, then Fe. Cu, Ag, Au are below H — they do not react with dilute acids.
Why Not HNO₃ + Metal?
Nitric acid IS in the spec for reactions with oxides, hydroxides, and carbonates (Patterns 2–4). But it's excluded from acid + metal at GCSE because HNO₃ is an oxidising acid: with most metals it produces nitrogen oxides (NO, NO₂) rather than reliably giving H₂. So the spec sticks to HCl and H₂SO₄ for clean H₂-producing reactions.
Pattern 2 — Acid + Metal Oxide
Metal oxides are bases — they neutralise acids to give salt and water. No gas produced.
metal oxide + acid → salt + water
Examples
CuO + 2HCl → CuCl₂ + H₂OCuO + H₂SO₄ → CuSO₄ + H₂OMgO + 2HNO₃ → Mg(NO₃)₂ + H₂O
Why Insoluble Oxides Are Useful
Metals that don't react with acid (e.g. Cu) can still form salts via their oxides. CuO + acid gives copper salts even though Cu metal can't.
This is the basis of RP1 (Lesson 4.04) — making soluble salts from acid + insoluble base.
Pattern 3 — Acid + Metal Hydroxide
Metal hydroxides are also bases. If they dissolve in water, they're called alkalis. Either way: salt + water.
metal hydroxide + acid → salt + water
Soluble Hydroxides (Alkalis)
NaOH, KOH, Ca(OH)₂ — dissolve in water, give alkaline solutions.NaOH + HCl → NaCl + H₂O2KOH + H₂SO₄ → K₂SO₄ + 2H₂O
Insoluble Hydroxides (Bases)
Cu(OH)₂, Mg(OH)₂ — don't dissolve, but still neutralise acids.Cu(OH)₂ + 2HCl → CuCl₂ + 2H₂O
Vocabulary Distinction
All alkalis are bases, but not all bases are alkalis. The difference is solubility. The reaction with acid is the same in both cases: salt + water.
Pattern 4 — Acid + Metal Carbonate
Carbonates are also bases — but they give three products: salt, water, AND carbon dioxide.
metal carbonate + acid → salt + water + CO₂
Examples
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
What You Observe
Effervescence (fizzing) — bubbles of CO₂ escape.
If you bubble the gas through limewater, the limewater turns cloudy / milky white — confirming CO₂.
(MS: "turns cloudy/milky"; do NOT write "turns white".)
Naming Salts — Match Anion to Acid
The salt's name has two parts: the metal (from the metal/oxide/hydroxide/carbonate) and the anion (from the acid). The acid determines the suffix.
AcidAnion producedExamples
HCl (hydrochloric)chloride (Cl⁻)NaCl (sodium chloride), MgCl₂ (magnesium chloride), ZnCl₂
H₂SO₄ (sulfuric)sulfate (SO₄²⁻)CuSO₄ (copper sulfate), MgSO₄, K₂SO₄
HNO₃ (nitric)nitrate (NO₃⁻)NaNO₃ (sodium nitrate), Ca(NO₃)₂
Don't Confuse These Endings
Examiners flag confusion between similar-sounding names:
chloride (Cl⁻) ≠ chlorate (ClO₃⁻) ≠ chlorite (ClO₂⁻)
sulfate (SO₄²⁻) ≠ sulfite (SO₃²⁻) ≠ sulfide (S²⁻)
nitrate (NO₃⁻) ≠ nitrite (NO₂⁻) ≠ nitride (N³⁻)
Predicting Products From Reactants
Spec skill: "predict products from given reactants" and "use the formulae of common ions to deduce the formulae of salts".
3-Step Method
Identify the acid → tells you the anion (Cl⁻ / SO₄²⁻ / NO₃⁻)
Identify the metal compound → tells you the metal cation (and its charge)
Combine cation and anion using balanced charges (criss-cross / lowest whole numbers)
Example — Copper Carbonate + Sulfuric Acid
Step 1: H₂SO₄ → anion is SO₄²⁻.
Step 2: CuCO₃ contains Cu²⁺.
Step 3: Cu²⁺ + SO₄²⁻ → charges balance 1:1 → CuSO₄.
Pattern 4 (carbonate): add water + CO₂.CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂
Deducing Salt Formulae — Charge Balance
Spec skill: "use the formulae of common ions to deduce the formulae of salts". The total positive charge must equal the total negative charge — adjust subscripts until they balance.
Common Ion Charges (Quick Reference)
Cations (positive):
1+: Na⁺, K⁺, Li⁺, Ag⁺, NH₄⁺
2+: Mg²⁺, Ca²⁺, Cu²⁺, Zn²⁺, Fe²⁺
3+: Al³⁺, Fe³⁺
Anions (negative):
1−: Cl⁻, NO₃⁻, OH⁻
2−: SO₄²⁻, CO₃²⁻, O²⁻
AQA's periodic table (provided in exam) shows charges for elemental ions. The exam often gives the formula of polyatomic ions in the question.
CombinationCharge ratioFormulaSalt example
Na⁺ + Cl⁻1+ : 1−NaClsodium chloride
Mg²⁺ + Cl⁻2+ : 1− → need 2 Cl⁻MgCl₂magnesium chloride
Na⁺ + SO₄²⁻1+ : 2− → need 2 Na⁺Na₂SO₄sodium sulfate
Cu²⁺ + SO₄²⁻2+ : 2− → balance 1:1CuSO₄copper sulfate
Ca²⁺ + NO₃⁻2+ : 1− → need 2 NO₃⁻Ca(NO₃)₂calcium nitrate
Bracket Rule
When you need more than one of a polyatomic ion (like NO₃⁻, SO₄²⁻, OH⁻), put it in brackets with the subscript outside: Ca(NO₃)₂ — NOT CaNO₃₂. Single-atom ions (Cl⁻) don't need brackets: MgCl₂.
Examiner Insight — P1 Jun 2024 Q01.1
The actual question: "A student produced a salt by reacting copper carbonate with sulfuric acid... Complete the word equation for the reaction: copper carbonate + sulfuric acid → ___ + ___ + carbon dioxide"2 MARKS
"Two thirds of students gained both marks, with nearly all scoring one mark. A few incorrect salts were named, such as copper chloride and copper sulfide, and sometimes hydrogen was given as the third product instead of water." — AQA Examiner Report, P1 Jun 2024 Q01.1
✗ Wrong Salt
"copper chloride"
The acid is sulfuric — anion must be sulfate.
✗ Wrong Salt
"copper sulfide"
Sulfide (S²⁻) is the wrong anion. Sulfate is SO₄²⁻.
✗ Wrong Gas
"copper sulfate + hydrogen + carbon dioxide"
Hydrogen comes from acid + METAL. Carbonate gives water.
✓ Both Marks
"copper sulfate + water + carbon dioxide"
MS allows: CuSO₄ for "copper sulfate"; H₂O for "water".
Which Gas? Match It To The Pattern
The single most common error on this topic is mixing up the gas products. Here's the diagnostic:
If the acid reacted with…Then the gas product is…
Metal (Mg, Zn, Fe)Hydrogen (H₂) — squeaky pop test
Metal oxide (CuO, MgO)No gas — only salt + water
Metal hydroxide (NaOH, Cu(OH)₂)No gas — only salt + water
Metal carbonate (CaCO₃, CuCO₃)Carbon dioxide (CO₂) — turns limewater milky
Quick Diagnostic
Bubbles? → either hydrogen (metal) or CO₂ (carbonate). Look at the reactant.
No bubbles? → it's an oxide or hydroxide. Just salt and water.
Acid + Metal as a Redox Reaction
HT only — Spec 4.4.2.1
HT students must explain that acid + metal reactions are redox reactions in terms of electron transfer.
Worked Example — Mg + 2HCl → MgCl₂ + H₂
Strip out the spectator Cl⁻ ions to see the redox at work:Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)Half-equations:Mg → Mg²⁺ + 2e⁻ (magnesium loses 2 electrons → OXIDISED)2H⁺ + 2e⁻ → H₂ (hydrogen ions gain 2 electrons → REDUCED)
The Pattern
For any acid + metal reaction in the spec (Mg, Zn, Fe + HCl or H₂SO₄): the metal is oxidised (loses electrons), and the H⁺ ions are reduced (gain electrons to form H₂). Same logic as displacement (4.02) — the metal more readily forms positive ions.
Identifying Oxidised & Reduced Species
HT only — Spec 4.4.2.1
Spec skill: "identify which species are oxidised and which are reduced in given chemical equations". Apply OIL RIG using the same charge-tracking method from 4.02.
Example — Zn + H₂SO₄ → ZnSO₄ + H₂
Track each species' charge change:
Zn: starts as Zn (charge 0); ends as Zn²⁺ in ZnSO₄ (charge +2). Charge increased → lost 2 electrons → OXIDISED.
H: starts as H⁺ in H₂SO₄ (charge +1); ends as H in H₂ (charge 0). Charge decreased → gained electrons → REDUCED.
SO₄²⁻: unchanged on both sides (in H₂SO₄ and in ZnSO₄). It's a spectator ion — neither oxidised nor reduced.
Mark-Scheme Rule From 4.02
If the question says "in terms of electrons", the MS will reject any reference to oxygen. Stick with electron transfer: "Zn loses 2 electrons" / "H⁺ ions gain electrons".
Mark-Scheme Phrases to Memorise
Verbatim language from real Paper 1 mark schemes. Use these — don't paraphrase.
ConceptMark-scoring phraseDon't write
Acid + metal products"salt + hydrogen ""salt + water"; "salt + CO₂"
Acid + carbonate products"salt + water + carbon dioxide ""salt + hydrogen"; "salt + water" (missing CO₂)
Acid + oxide / hydroxide"salt + water ""salt + hydrogen"
Salt name (HCl)"[metal] chloride "chlorate / chlorite
Salt name (H₂SO₄)"[metal] sulfate "sulfide / sulfite
Salt name (HNO₃)"[metal] nitrate "nitride / nitrite
CO₂ test resultlimewater "turns cloudy / milky ""turns white"
Practice 1 — Foundation
(a) Name the salt produced when nitric acid reacts with potassium hydroxide.1 MARK
(b) Complete the word equation: magnesium + sulfuric acid → ___ + ___2 MARKS
(c) Complete the word equation: calcium carbonate + hydrochloric acid → ___ + ___ + ___3 MARKS
a / 1
Potassium nitrate. (HNO₃ → nitrate; KOH → potassium.)
b / 1
magnesium sulfate (MgSO₄ allowed)
b / 2
hydrogen (H₂ allowed) — acid + METAL gives hydrogen, not water.
c / 1
calcium chloride (CaCl₂)
c / 2
water (H₂O)
c / 3
carbon dioxide (CO₂) — acid + CARBONATE gives all three.
⚠️
Common errors flagged in P1 Jun 2024 Q01.1 ER: writing wrong anion ("chloride" from sulfuric acid) or wrong gas ("hydrogen" from carbonate). Match anion to acid; match gas to reactant type.
Practice 2 — HT
Magnesium reacts with hydrochloric acid:
Mg + 2HCl → MgCl₂ + H₂(a) Identify the species that is oxidised. Explain your answer in terms of electrons.2 MARKS
(b) Identify the species that is reduced. Explain your answer in terms of electrons.2 MARKS
a / 1
Magnesium (Mg) is oxidised.
a / 2
Magnesium atoms lose 2 electrons (to form Mg²⁺).
b / 1
Hydrogen ions (H⁺) are reduced.
b / 2
Hydrogen ions gain electrons (to form H₂). Each H⁺ gains 1 electron; two H⁺ ions together gain 2 electrons.
⚠️
Cl⁻ is unchanged on both sides — it's a spectator ion. Don't mention oxygen: MS rejects oxygen references when "in terms of electrons" is asked. Track the charge change to identify each species.
Summary
The Four Reaction Patterns
Metal: acid + metal → salt + hydrogen (limited to Mg, Zn, Fe + HCl/H₂SO₄ per spec)
Metal oxide: acid + oxide → salt + water
Metal hydroxide: acid + hydroxide → salt + water (alkali if soluble; base if not)
Metal carbonate: acid + carbonate → salt + water + CO₂
Salt Naming — Anion From Acid
HCl → metal chloride (don't confuse with chlorate / chlorite)
H₂SO₄ → metal sulfate (don't confuse with sulfide / sulfite)
HNO₃ → metal nitrate (don't confuse with nitride / nitrite)
Predict products: identify anion from acid + cation from metal compound; combine using balanced charges
HT — Acid + Metal Redox
For Mg/Zn/Fe + HCl/H₂SO₄: metal is oxidised (loses electrons → becomes M²⁺); H⁺ ions are reduced (gain electrons → form H₂)
Cl⁻ / SO₄²⁻ are spectator ions (unchanged)
Mark-scheme rule: "in terms of electrons" rejects any reference to oxygen
The Common Errors (P1 Jun 2024 Q01.1)
Wrong anion: writing "chloride" or "sulfide" when sulfuric acid is the reactant
Wrong gas: writing "hydrogen" from acid + carbonate (it's CO₂)
Confusing similar-sounding salt endings (chloride/chlorate; sulfate/sulfide; nitrate/nitrite)
Exam Tip
Before writing any acid-reaction product, ask two questions: (1) Which acid? — that fixes the anion. (2) Which type of reactant? (metal / oxide / hydroxide / carbonate) — that fixes the second/third products. Two questions, every time.
Next: Why This Lesson Matters