1.9.1Physical Chemistry
Oxidation and Reduction — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 50 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Electron Definitions
Remember OIL RIGOxidation Is Loss, Reduction Is Gain of electrons.
Electron Definitions
Oxidation
- Oxidation A species is oxidised when it loses electrons, so its oxidation state increases, for example Mg → Mg²⁺ + 2e⁻.
Reduction
- Reduction A species is reduced when it gains electrons, so its oxidation state decreases, for example Cl₂ + 2e⁻ → 2Cl⁻.
Redox
- Redox Oxidation and reduction always occur together, because electrons lost by one species must be gained by another.
Oxidation state link
- Oxidation state link A rise in oxidation state shows oxidation, and a fall shows reduction, even when no ions are obviously formed.
Part 2 of 5Oxidising and Reducing Agents
Common oxidising agents
- Common oxidising agents Halogens, oxygen and acidified manganate(VII) accept electrons easily and are themselves reduced in the process.
Common reducing agents
- Common reducing agents Group 1 and 2 metals, hydrogen and iodide ions lose electrons readily and are oxidised in the process.
Reactivity link
- Reactivity link Reactive metals are strong reducing agents because they have low ionisation energies and lose electrons easily.
| Term | Does what to others | What happens to the agent |
|---|---|---|
| Oxidising agent | Takes electrons from another species | It is reduced |
| Reducing agent | Gives electrons to another species | It is oxidised |
Part 3 of 5Half-Equations and Full Equations
Combining half-equations gives the overall ionic equation with electrons cancelled.
1
Write the oxidation half-equation, for example Zn → Zn²⁺ + 2e⁻, showing electrons on the product side.
2
Write the reduction half-equation, for example Cu²⁺ + 2e⁻ → Cu, showing electrons on the reactant side.
3
Multiply half-equations if needed so that the number of electrons lost equals the number gained.
4
Add the half-equations and cancel the electrons to obtain Zn + Cu²⁺ → Zn²⁺ + Cu.
5
State what is oxidised (Zn, 0 to +2) and what is reduced (Cu²⁺, +2 to 0) using oxidation state changes.
Part 4 of 5Worked Example
Question In the reaction Mg + 2H⁺ → Mg²⁺ + H₂, identify what is oxidised, what is reduced and the reducing agent.
- Magnesium goes from oxidation state 0 to +2, so it has lost two electrons.
- Hydrogen goes from +1 in H⁺ to 0 in H₂, so it has gained electrons.
- The species that is oxidised is the reducing agent because it supplies the electrons.
Answer
- Mg is oxidised and is the reducing agent;
- H⁺ is reduced and is the oxidising agent.
Part 5 of 5Exam Checklist & Tips
1.9.1 – Key Points for the Exam
- Oxidation Loss of electrons, or an increase in oxidation state.
- Reduction Gain of electrons, or a decrease in oxidation state.
- Oxidising agent A species that accepts electrons and so is reduced.
- Reducing agent A species that donates electrons and so is oxidised.
- Half-equation An equation showing only the oxidation or only the reduction, with electrons included.
Common Mistakes
- Confusing the agent with the species changed; the oxidising agent is the one that is reduced.
- Writing electrons on the wrong side of a half-equation.
- Forgetting to balance charge as well as atoms in half-equations.
- Stating that oxidation always involves oxygen, when electron loss is the general definition.
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