OCR A Level Chemistry A: Redox (H432) – Revision Notes

These revision notes condense section 2.1.5, Redox, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover every learning outcome, 2.1.5(a) to (f), in Module 2, Foundations in chemistry. H432 is the full A level. The Redox study guide has the full explanations and longer worked examples.

When you are ready to test yourself under pressure, go to the Redox practice questions. Every H432 topic is linked from the OCR chemistry hub. Tick outcomes off on the printable checklist as you go. A short check of the whole course is in the free diagnostics.

2.1.5(a): Oxidation number rules

Apply from the top. A higher rule overrides a lower one.

Rule	Oxidation number
Atom in an uncombined element (Na, Br₂, P₄)	0
Total for a compound with no charge	0
Total for an ion	equals its charge
Simple ion (Cl⁻, Mg²⁺)	the ion’s charge
Fluorine, wherever it is combined	−1
Group 1 metals; Group 2 metals; aluminium (combined)	+1; +2; +3
H in compounds	+1, but −1 in metal hydrides (e.g. KH)
O in compounds	−2, but −1 in peroxides (e.g. Li₂O₂)
Cl in compounds	−1, unless bonded to O or F

The specification names the peroxide and metal hydride cases, so check for them first.

Method in steps: finding an unknown oxidation number

1. Write x for the element you want, times its number of atoms.
2. Add the fixed contributions from the rules.
3. Make the sum 0 for a neutral compound, or equal to the ionic charge.
4. Solve for x and write it with a sign: +3, not 3 or 3+.

Worked reminder: phosphorus in P₄O₆

4x + 6(−2) = 0
4x = +12
x = +3

Worked reminder: chromium in CrO₃

x + 3(−2) = 0
x = +6

So CrO₃ is chromium(VI) oxide.

2.1.5(b) and (c): Names, numerals and formulae

A Roman numeral in brackets gives the oxidation number of the element just before it. For an oxoanion it refers to the central atom.

Name	Formula	Oxidation number shown
iron(II) hydroxide	Fe(OH)₂	Fe +2
iron(III) hydroxide	Fe(OH)₃	Fe +3
manganese(II) chloride	MnCl₂	Mn +2
chromium(VI) oxide	CrO₃	Cr +6
chlorate(I) ion	ClO⁻	Cl +1
chlorate(III) ion	ClO₂⁻	Cl +3
chlorate(V) ion	ClO₃⁻	Cl +5
sulfate(IV) ion	SO₃²⁻	S +4
nitrate(III) ion	NO₂⁻	N +3

Defaults from the specification: “nitrate” alone means NO₃⁻ and “sulfate” alone means SO₄²⁻.

Method in steps: name to formula

1. Read the oxidation number from the numeral.
2. For a metal cation, that number is its charge. For an oxoanion, find the number of oxygens that gives the stated oxidation number and the ion’s charge.
3. Combine ions so the total charge is zero. Use brackets around polyatomic ions when you need more than one.

Method in steps: formula to name

1. Find the oxidation number of the element that can vary.
2. Write it as a Roman numeral straight after that element’s name, with no space.
3. Leave out numerals for elements with a single oxidation number in compounds (Na, Mg, Al).

2.1.5(d): Oxidation and reduction

Process	Electrons	Oxidation number
Oxidation	lost	goes up
Reduction	gained	goes down

OIL RIG (oxidation is loss, reduction is gain) is a quick memory aid. In every redox reaction:

• one species is oxidised and another is reduced;
• total electrons lost = total electrons gained;
• change per atom × number of atoms changing = electrons for that element.

The specification wants examples from all three blocks. Have one ready for each: an s-block metal forming a cation, a p-block element such as Al or a halogen, and a d-block ion changing between oxidation numbers (Fe²⁺/Fe³⁺, Cu/Cu²⁺).

2.1.5(e): Metals with dilute acids

General pattern: metal + acid → salt + hydrogen. The metal is oxidised (0 to positive); hydrogen is reduced (+1 to 0); the anion does not change.

Acid	Salt ending	Anion
hydrochloric acid, HCl	chloride	Cl⁻
sulfuric acid, H₂SO₄	sulfate	SO₄²⁻
nitric acid, HNO₃	nitrate	NO₃⁻
ethanoic acid, CH₃COOH	ethanoate	CH₃COO⁻

Nitric acid is listed for naming salts, but its reactions with metals do not usually give hydrogen. The specification says reactions involving nitric acid or concentrated sulfuric acid could be assessed under outcome (f).

Method in steps: full equation

1. Fix the metal’s oxidation number in the salt (Mg +2, Al +3, Zn +2; Fe +2 with dilute acids).
2. Write the salt formula by balancing charges.
3. Balance the metal, then the anion, then hydrogen.
4. Add state symbols: metal (s), acid and salt (aq), hydrogen (g).

Worked reminder: aluminium with ethanoic acid

Al is +3, so the salt is (CH₃COO)₃Al.

2Al(s) + 6CH₃COOH(aq) → 2(CH₃COO)₃Al(aq) + 3H₂(g)

Two Al atoms lose 3 electrons each (6). Six H atoms go from +1 to 0, gaining 6. Balanced.

The specification states that ionic equations are not required for (e), so give the full equation.

2.1.5(f): Unfamiliar redox reactions

1. Label every atom’s oxidation number on both sides.
2. Circle the atoms whose number changes.
3. Find electrons lost and gained (change × atoms).
4. Use the balance to predict a ratio, a missing product or a new oxidation number.

Worked reminder: hydrogen sulfide with sulfur dioxide

2H₂S + SO₂ → 3S + 2H₂O

S in H₂S is −2 and goes to 0: two atoms lose 2 each (4 electrons). S in SO₂ is +4 and goes to 0: one atom gains 4. The same element is oxidised in one compound and reduced in another.

Concentrated sulfuric acid and nitric acid can act like this with metals: sulfur or nitrogen is reduced instead of hydrogen, so SO₂ or NO₂ forms instead of H₂.

Must-know distinctions

• Oxidation number vs ionic charge. +2 (sign first) for oxidation numbers; 2+ (number first) for charges.
• Per atom vs total. In P₄O₆ each P is +3; the four P atoms together are +12.
• Iron(II) vs iron(III). Dilute acids give iron(II) salts. Chlorine turns iron(II) into iron(III).
• Chlorate(I) vs chlorate(V). ClO⁻ and ClO₃⁻ are different ions; the numeral, not the word “chlorate”, tells you which.
• Hydride vs hydroxide. H is −1 in KH but +1 in KOH.
• Dilute vs concentrated acid. Dilute acids with reactive metals give hydrogen. Concentrated sulfuric acid and nitric acid can give SO₂ or NO₂.

Quick self-test

1. State the oxidation number of sulfur in H₂S.
2. State the oxidation number of hydrogen in SrH₂.
3. State the oxidation number of oxygen in K₂O₂.
4. State the oxidation number of chlorine in ClF₃.
5. Write the formula of lead(II) nitrate.
6. Name KClO₂, showing the oxidation number of chlorine.
7. Name MnO₂.
8. Write a full equation, with state symbols, for nickel reacting with dilute sulfuric acid to give nickel(II) sulfate. Name what is oxidised.
9. In 2K + 2H₂O → 2KOH + H₂, which element is reduced, and from what to what?
10. 0.0100 mol of a metal M reacts with excess dilute acid and gives 0.0150 mol of H₂. Deduce the oxidation number of M in the salt.

Answers

1. −2. Two H at +1 give +2, so S is −2.
2. −1. Sr is +2 in a metal hydride, so each H is −1.
3. −1. K₂O₂ is a peroxide: 2(+1) + 2x = 0.
4. +3. F is always −1, so Cl balances three of them.
5. Pb(NO₃)₂. Pb²⁺ needs two NO₃⁻.
6. Potassium chlorate(III). +1 + x + 2(−2) = 0 gives x = +3.
7. Manganese(IV) oxide. x + 2(−2) = 0.
8. Ni(s) + H₂SO₄(aq) → NiSO₄(aq) + H₂(g). Nickel is oxidised, 0 to +2.
9. Hydrogen, from +1 in H₂O to 0 in H₂. Potassium is oxidised (0 to +1); oxygen stays −2.
10. Electrons gained by hydrogen = 2 × 0.0150 = 0.0300 mol. Per M: 0.0300 ÷ 0.0100 = 3, so M is +3.

Where marks are usually lost

• Treating O in a peroxide as −2 or H in a metal hydride as +1.
• Leaving off the sign, or writing an oxidation number as a charge (3+).
• Giving the total for all atoms of an element instead of the value per atom.
• Writing FeCl₃ or Fe₂(SO₄)₃ as the product of iron with a dilute acid.
• Missing brackets: AlCH₃COO₃ or Pb NO₃ 2 instead of (CH₃COO)₃Al and Pb(NO₃)₂.
• Putting a numeral on a metal with only one oxidation number, such as “magnesium(II) chloride”.
• Calling the acid’s anion oxidised or reduced when its oxidation numbers do not change.
• In an unfamiliar equation, saying “nitrogen is reduced” when only some of the nitrogen atoms change.
• Not checking that electrons lost equal electrons gained before giving a ratio.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 2.1.5 Redox, learning outcomes (a) to (f). Published by OCR.