Oxidation states, half-equations and redox
Oxidation states are a bookkeeping system for electrons. They show what has been oxidised and reduced in any reaction, and they guide you in balancing the half-equations that combine to make a full redox equation.
Part 1 of 3: Learn it
In short
- Oxidation is loss of electrons, or an increase in oxidation state; reduction is the reverse.
- The oxidation states in a compound add to zero; in an ion, to its charge.
- To combine half-equations, multiply so the electrons match, add, and cancel.
Where this is in your specification
Spec points: AQA 7405 3.1.7, OCR A H432 2.1.5, Edexcel 9CH0 Topic 3
| Board | Topic: Redox and electrode potentials |
|---|---|
| AQA 7405 | 3.1.7 and 3.1.11 |
| Edexcel 9CH0 | Topics 3 and 14 |
| OCR H432 | 2.1.5 and 5.2.3 |
| Higher C813 76 | 1(c) Oxidising and reducing agents |
Rules for oxidation states
- An uncombined element is 0, such as Fe or O₂.
- In a simple ion it equals the charge: Na⁺ is +1, Cl⁻ is −1.
- Group 1 metals are +1 and group 2 metals +2 in their compounds.
- Oxygen is usually −2 (but −1 in peroxides and +2 in OF₂).
- Hydrogen is usually +1 (but −1 in metal hydrides such as NaH).
- Fluorine is always −1 in compounds.
Roman numerals in names give an oxidation state: iron(III) is Fe at +3.
Agents
An oxidising agent takes electrons from something else, so it is itself reduced. A reducing agent gives electrons away, so it is itself oxidised.
Half-equations in acid
- Balance the atom that changes oxidation state.
- Balance oxygen by adding H₂O.
- Balance hydrogen by adding H⁺.
- Balance charge by adding electrons to the more positive side.
For manganate(VII) to manganese(II): MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Check: Mn goes from +7 to +2, which matches 5 electrons gained.
What is the oxidation state of nitrogen in NH₄⁺?
Show the answer
−3 (N + 4 = +1).
Part 2 of 3: See it worked
Worked examples
Example 1
Find the oxidation state of chromium in Cr₂O₇²⁻.
- Oxygen: 7 × (−2) = −14
- 2Cr − 14 = −2 (the charge on the ion)
- 2Cr = +12
Answer: +6.
Example 2
Combine MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O with Fe²⁺ → Fe³⁺ + e⁻ into a full equation.
- Multiply the iron half-equation by 5 so both have 5 electrons
- Add them: MnO₄⁻ + 8H⁺ + 5Fe²⁺ + 5e⁻ → Mn²⁺ + 4H₂O + 5Fe³⁺ + 5e⁻
- Cancel the electrons
Answer: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺.
Common mistakes
- Forgetting that the oxidation states in an ion add up to the charge, not zero.
- Saying the oxidising agent is oxidised.
- Putting electrons on the wrong side of a half-equation.
- Leaving electrons in the final redox equation.
What is the oxidation state of sulfur in H₂SO₄?
Show the answer
+6 (2 + S − 8 = 0).
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What is the oxidation state of nitrogen in NH₄⁺?
−3 (N + 4 = +1).
What is the oxidation state of sulfur in H₂SO₄?
+6 (2 + S − 8 = 0).
In Zn + Cu²⁺ → Zn²⁺ + Cu, which species is the oxidising agent?
Cu²⁺, because it gains electrons and is reduced.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses