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
- Chemist (öffnet einen neuen Tab)
- Chemical engineer (öffnet einen neuen Tab)
- Forensic scientist (öffnet einen neuen Tab)
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).
Diese Lernzettel sind auf Englisch, weil sie britischen Prüfungslehrplänen folgen.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses