6.2.3Additional lessons
Relative Ease of Oxidation and Reduction — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 86 of 98
Part 1 of 6Metals and Ease of Oxidation
The more easily an atom loses electrons, the more reactive it is as a metal.
Reactivity series
- A more reactive metal loses electrons more readily, so it is a stronger reducing agent.
Group 1 trend
- Reactivity increases down the group, since the outer electron is lost more easily as atomic size increases.
Group 2 trend
- Reactivity also increases down Group 2 for the same reason, so calcium reacts faster with water than magnesium.
Example
- 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g): Li is oxidised from 0 to +1.
Part 2 of 6Metal Displacement Reactions
Metal Displacement Reactions
Principle
- A more reactive metal displaces a less reactive one from a solution of its ions.
Zinc and copper(II) sulfate
- Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s); blue colour fades and copper deposits.
No reaction
- Copper does not react with ZnSO₄(aq), because copper is less reactive than zinc.
Ionic form
- Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), where Zn is oxidised and Cu²⁺ is reduced.
Part 3 of 6Oxidising Power of the Halogens
Trend
- Oxidising power decreases down Group 17: F₂ > Cl₂ > Br₂ > I₂.
Reason
- Smaller atoms attract an extra electron more strongly, so chlorine is reduced to Cl⁻ more readily than iodine to I⁻.
Displacement
- A halogen displaces a less reactive halogen from its halide solution.
Observations
- Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ gives an orange solution, and Cl₂ + 2I⁻ → 2Cl⁻ + I₂ gives a brown solution.
| Mixture | Reaction? | Equation |
|---|---|---|
| Cl₂(aq) + KBr(aq) | Yes | Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ |
| Br₂(aq) + KCl(aq) | No | Br₂ is a weaker oxidising agent |
| Br₂(aq) + KI(aq) | Yes | Br₂ + 2I⁻ → 2Br⁻ + I₂ |
Part 4 of 6Explaining the Trends
Explaining the Trends
Group 1 and 2
- Larger atoms hold the outer electrons more weakly, so oxidation to the cation is easier down the group.
Halogens
- Smaller halogen atoms attract electrons more strongly, so oxidising power decreases down Group 17.
Solvent test
- Adding hexane shows the halogen colours more clearly, as Br₂ is orange and I₂ is violet in the organic layer.
Linking ideas
- A strong reducing agent has a weak oxidising ion, so Mg²⁺ is a weaker oxidising agent than Cu²⁺.
Part 5 of 6Worked Example
Question Predict whether a reaction occurs when chlorine water is added to potassium iodide solution, and write the ionic equation.
- Chlorine is above iodine in Group 17, so it is the stronger oxidising agent.
- Chlorine is reduced from 0 to −1 and iodide is oxidised from −1 to 0.
- The iodine formed makes the solution brown.
Answer Yes: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq).
Part 6 of 6Exam Checklist & Tips
6.2.3 – Key Points for the Exam
- Reducing agent A species that readily loses electrons, such as a reactive metal.
- Oxidising agent A species that readily gains electrons, such as chlorine.
- Displacement reaction A reaction in which a more reactive element displaces a less reactive one from a compound.
- Reactivity series A list of metals in order of how readily they lose electrons.
Common Mistakes
- Saying that fluorine is the weakest oxidising halogen; it is the strongest.
- Predicting that bromine displaces chlorine from chloride solution.
- Writing halide displacement equations with Cl instead of Cl₂.
- Describing metal reactivity only as speed of reaction without linking it to electron loss.
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