Grade 10-11 (age 15-17)

high school Oxidation-Reduction (Redox) Reaction

Oxidation-Reduction Reaction

Iron rusting, wood burning and a battery pushing out current are all redox reactions, in which electrons move from one substance to another. Losing electrons is oxidation and gaining them is reduction, and since every electron one substance loses must be taken by another, the two always happen together. When you cannot see where the electrons went, oxidation numbers keep the account: an increase means oxidation and a decrease means reduction. The upper diagram follows zinc handing two electrons to a copper ion, and the lower one tracks the oxidation numbers in three reactions.

Redox Means Electrons Moving

Reaction progress50%

💡 What the Diagram Shows

①Before the reaction there are a zinc atom, Zn, and a copper ion, Cu²⁺; the oxidation number of Zn is 0 and that of Cu²⁺ is +2

②As the reaction proceeds, Zn gives up two electrons and becomes Zn²⁺; losing electrons is oxidation

③Cu²⁺ takes those two electrons and becomes a copper atom, Cu; gaining electrons is reduction

④The number of electrons Zn loses equals the number Cu²⁺ gains, so oxidation and reduction cannot happen separately

⑤Dip a zinc strip into copper sulfate solution and reddish copper really does coat the zinc while the blue color of the solution fades

Keeping Track with Oxidation Numbers

Reaction2Mg + O₂

Oxidation-Number Rules

element = 0, H in compounds = +1, O = −2, monatomic ion = its charge

In a compound the oxidation numbers of all atoms add up to 0; in a polyatomic ion they add up to the ion's charge

🔍 Checking the Three Reactions

①2Mg + O₂ → 2MgO: Mg goes 0 → +2 and is oxidized, O goes 0 → −2 and is reduced; the 4 electrons lost by two Mg atoms are taken by two O atoms

②Fe₂O₃ + 3CO → 2Fe + 3CO₂: Fe goes +3 → 0 and is reduced, C goes +2 → +4 and is oxidized; 6 electrons change hands

③2Na + Cl₂ → 2NaCl: Na goes 0 → +1 and is oxidized, Cl goes 0 → −1 and is reduced; 2 electrons change hands

④In every reaction the electrons lost equal the electrons gained

Oxidizing and Reducing Agents

Oxidizing Agent

oxidizes something else and is itself reduced

Takes electrons from its partner (its own oxidation number falls)

Reducing Agent

reduces something else and is itself oxidized

Gives electrons to its partner (its own oxidation number rises)

💡 Keeping Them Straight

①An oxidizing agent oxidizes another substance, so it is reduced itself

②A reducing agent reduces another substance, so it is oxidized itself

③The name describes what the agent does to its partner; its own change is the opposite, a favorite exam trap

④In Zn + Cu²⁺, Zn gives up electrons, so it is the reducing agent (and is oxidized), while Cu²⁺ takes electrons, so it is the oxidizing agent (and is reduced)

Work It Out

Example 1

Find the oxidation number of sulfur (S) in sulfuric acid (H₂SO₄).

Put in H = +1 and O = −2 and set the total for the compound equal to 0.

2 × (+1) + S + 4 × (−2) = 0

Solve for S.

S = +6

▸ S = +6

Two H and four O add up to +2 − 8 = −6, so S must be +6 for the total to be 0.

Example 2

In Zn + Cu²⁺ → Zn²⁺ + Cu, which element is oxidized and which is reduced?

Write the oxidation number of each element before and after the reaction.

Zn: 0 → +2, Cu: +2 → 0

An increase in oxidation number is oxidation (electrons lost); a decrease is reduction (electrons gained).

Zn oxidized (loses 2 e⁻), Cu²⁺ reduced (gains 2 e⁻)

▸ Zn oxidized / Cu²⁺ reduced

This is the reaction in the upper diagram: Cu²⁺ takes both of the electrons Zn loses.

Summary

Three Views of Redox

①electrons: oxidation = loss, reduction = gain ②oxidation number: oxidation = increase, reduction = decrease ③oxygen: oxidation = gain, reduction = loss

All three read the same reaction on different scales, so they always agree

2021 KICE mock Chemistry I type, adapted

In which of these is the oxidation number of nitrogen (N) +5?

①NH₃

②N₂

③NO

④NO₂

⑤HNO₃

▸ ⑤ HNO₃

In a compound the oxidation numbers add up to 0. Find N in each choice.

N is −3 in NH₃, 0 in N₂, +2 in NO and +4 in NO₂; in HNO₃, H = +1 and three O = −6, so N is +5.

(+1) + N + 3 × (−2) = 0 → N = +5

🎯 Exam Points

①Finding oxidation numbers: put in H = +1 and O = −2, then make the total 0 for a compound or equal to the charge for an ion

②Oxidizing and reducing agents change in the opposite direction to their names

③Electrons lost = electrons gained

④Rusting is the oxidation of iron, and in photosynthesis carbon dioxide is reduced to glucose

⑤Questions often ask you to pick out the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent from oxidation-number changes

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Last updated 2026-10-17

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