6.4Chemical Reactions
Redox — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 10 parts Lesson 5 of 7 1 quiz
Lesson summary
6.4 Redox — Notes — IGCSE Chemistry notes: Define oxidation and reduction in terms of oxygen, electrons and oxidation numbers, and identify oxidising and reducing agents.Roman numeral: gives the oxidation number of an element with variable valency, so iron(III) shows that iron ha…
- Oxidising agent
- Fe2+ is iron(II) and Fe3+ is iron(III), so FeO is iron(II) oxide and Fe2O3 is iron(III) oxide
- Copper(II) sulfate contains Cu2+, CoCl2 is cobalt(II) chloride and AgCl is silver(I) chloride
- Do not confuse it with charge, because +2 is an oxidation number and 2+ is a charge
- Redox reaction: oxidation and reduction happen together in the same reaction
- Oxidation is gain of oxygen, as in Mg + O2 → MgO
Part 1 of 8Roman numerals and oxidation numbers
1
Roman numeral: gives the oxidation number of an element with variable valency, so iron(III) shows that iron has oxidation number +3.
2
Fe2+ is iron(II) and Fe3+ is iron(III), so FeO is iron(II) oxide and Fe2O3 is iron(III) oxide.
3
Copper(II) sulfate contains Cu2+, CoCl2 is cobalt(II) chloride and AgCl is silver(I) chloride.
4
Oxidation number: shows the electrons an atom has lost (positive) or gained (negative) compared with the uncombined element.
5
Do not confuse it with charge, because +2 is an oxidation number and 2+ is a charge.
Part 2 of 8Redox in terms of oxygen
1
Redox reaction: oxidation and reduction happen together in the same reaction.
2
Oxidation is gain of oxygen, as in Mg + O2 → MgO.
3
Reduction is loss of oxygen, as in CuO + H2 → Cu + H2O, where CuO is reduced and H2 is oxidised.
4
Iron extraction: in Fe2O3 + 3CO → 2Fe + 3CO2, iron(III) oxide loses oxygen and is reduced, while carbon monoxide gains oxygen and is oxidised.
5
Redox occurs in metal extraction, displacement reactions, combustion, respiration, rusting and electrolysis.
Part 3 of 8Redox in terms of electrons (Supplement)
1
Oxidation is loss of electrons, with an increase in oxidation number.
2
Reduction is gain of electrons, with a decrease in oxidation number.
3
OIL RIG: Oxidation Is Loss of electrons and Reduction Is Gain of electrons.
4
Half-equations: electrons appear on the right for oxidation and on the left for reduction.
- OxidationZn → Zn2+ + 2e− and Ag → Ag+ + e−.
- ReductionCu2+ + 2e− → Cu and O2 + 4e− → 2O2−.
5
In electrolysis 2Cl− → Cl2 + 2e− occurs at the anode and Na+ + e− → Na at the cathode.
Part 4 of 8Displacement as redox
Displacement as redox
Displacement reactions are redox reactions.
Fe + CuSO4 → FeSO4 + Cu has the ionic equation Fe + Cu2+ → Fe2+ + Cu after removing the spectator ions.
- Iron atoms lose electrons, so they are oxidised.
- Copper ions gain electrons, so they are reduced.
Wording: say that copper(II) ions are reduced to copper atoms, not just that copper is reduced.
Part 5 of 8Rules for oxidation numbers (Supplement)
| Species | Oxidation number |
|---|---|
| Uncombined element (Na, O2, Cl2) | 0 |
| Monatomic ion | Equal to its charge (Na+ = +1, Cl− = −1) |
| Group I / Group II / fluorine | +1 / +2 / −1 |
| Oxygen in most compounds | −2 (−1 in peroxides such as H2O2) |
| Hydrogen in most compounds | +1 (−1 in hydrides such as NaH) |
| Sum in a neutral compound | 0 |
| Sum in a polyatomic ion | Equal to the charge on the ion |
Rules for oxidation numbers (Supplement)
The more electronegative element takes the negative value.
In MnO4−, Mn + 4(−2) = −1, so Mn is +7.
An element is oxidised if its oxidation number increases, for example Fe2+ → Fe3+, and reduced if it decreases.
Part 6 of 8Oxidising and reducing agents (Supplement)
Oxidising and reducing agents (Supplement)
Oxidising agent: oxidises another substance and is itself reduced, so it gains electrons.
- Examples are hydrogen peroxide, chlorine, fluorine and copper(II) oxide.
Reducing agent: reduces another substance and is itself oxidised, so it loses electrons.
- Examples are carbon, hydrogen and potassium iodide.
| Definition | Oxidation | Reduction | Example | Oxidising agent | Reducing agent |
|---|---|---|---|---|---|
| Oxygen | Gains O | Loses O | CuO + H2 → Cu + H2O | CuO | H2 |
| Electrons | Loses e− | Gains e− | Zn + Cu2+ → Zn2+ + Cu | Cu2+ | Zn |
- Identifying the agents
- Write the balanced equation.
- Assign oxidation numbers to every atom on both sides.
- The element whose oxidation number increases is oxidised, and the one that decreases is reduced.
- The reactant containing the oxidised element is the reducing agent, and the one containing the reduced element is the oxidising agent.
Part 7 of 8Colour-change tests
| Reagent | Tests for | Colour change | Reagent itself is |
|---|---|---|---|
| Acidified aqueous potassium manganate(VII) | Reducing agent | Purple to colourless | Reduced (Mn +7 to +2) |
| Aqueous potassium iodide | Oxidising agent | Colourless to brown (red-brown) as iodine forms | Oxidised (I− to I2) |
Colour-change tests
Method
- Place the unknown solution in a test tube.
- Add dilute sulfuric acid to acidify it.
- Add acidified potassium manganate(VII) dropwise, and purple to colourless shows a reducing agent.
- To test for an oxidising agent, add aqueous potassium iodide, and colourless to brown shows one is present.
In the tests KMnO4 is the oxidising agent and KI is the reducing agent.
Part 8 of 8More from the specification
More from the specification
Redox electron transfer and oxidized product
- Redox electron transfer and oxidized product oxidation involves loss of electrons from reducing agents, yielding an oxidized product alongside reduced chemical species.
Redox in Terms of Electron Transfer (OIL RIG)
- Oxidation is the loss of electrons and reduction is the gain of electrons;
- in redox reactions, electron transfer occurs simultaneously between conjugate species.
Redox in Terms of Oxidation State Changes
- Oxidation involves an increase in oxidation state (e.g. Fe2+ to Fe3+), whereas reduction involves a decrease in oxidation state.
Oxidising and Reducing Agents
- An oxidising agent readily accepts electrons and undergoes reduction, while a reducing agent donates electrons and undergoes oxidation.
Formula
- Symbols and numbers showing the atoms in a compound.
Key terms
| Term | Meaning |
|---|---|
| Oxidation | gain of oxygen, loss of electrons or an increase in oxidation number. |
| Reduction | loss of oxygen, gain of electrons or a decrease in oxidation number. |
| Redox | a reaction in which oxidation and reduction occur together. |
| Oxidising agent | a substance that oxidises another and is itself reduced. |
| Reducing agent | a substance that reduces another and is itself oxidised. |
| Oxidation number | the number of electrons an atom has lost or gained compared with the uncombined element. |
Avoid these mistakes
1
Saying oxidation is loss of oxygenit is gain of oxygen or loss of electrons.
2
Saying the oxidising agent is oxidisedit is reduced because it gains electrons.
3
Confusing oxidation number (+2) with charge (2+).
4
Writing Zn + 2e− → Zn2+: in oxidation the electrons are on the right, Zn → Zn2+ + 2e−.
5
Saying KMnO4 goes clear: it goes from purple to colourless.
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