Oxidation-Reduction (Redox) Reaction

Oxidation-Reduction Reaction

Oxidation is losing electrons (oxidation number up) and reduction is gaining them (oxidation number down); they always happen together. Oxidation numbers follow rules such as 0 for elements, H=+1, O=-2, and sum to zero in a compound. An oxidizing agent oxidizes something else while itself being reduced; a reducing agent does the opposite. Here you change reaction progress and examples to track electron flow and oxidation-number shifts.

What is Redox?

Reaction Progress5

💡 Key: Electron Transfer

①Oxidation = losing electrons (oxidation number ↑)

②Reduction = gaining electrons (oxidation number ↓)

③Oxidation and reduction always happen together

④One side loses → another side must gain

Changes in Oxidation Number

Reaction Example0

Oxidation Number Rules

pure element = 0, H = +1, O = -2, ion = charge

Sum of oxidation numbers in a compound = 0

🔍 Check the Three Examples!

①Mg + O₂: Mg 0→+2 (ox), O 0→-2 (red)

②Fe₂O₃ + CO: Fe +3→0 (red), C +2→+4 (ox)

③Na + Cl₂: Na 0→+1 (ox), Cl 0→-1 (red)

Oxidizing and Reducing Agents

Oxidizing Agent

oxidizes the other = itself is reduced

Takes electrons (oxidation number decreases)

Reducing Agent

reduces the other = itself is oxidized

Gives electrons (oxidation number increases)

💡 Easy Mnemonic

①Oxidizing agent = 'oxidizes the other', itself is reduced

②Reducing agent = 'reduces the other', itself is oxidized

③Name and behavior are opposite! → common trap

④Zn + Cu²⁺: Zn = reducing agent (oxidized), Cu²⁺ = oxidizing agent (reduced)

Work It Out

Example 1

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

Apply the rules H=+1 and O=−2, and use the fact that the oxidation numbers in a compound sum to 0.

2(+1) + S + 4(−2) = 0

Rearrange to solve for the oxidation number of S.

S = +6

▸ S = +6

The two H and four O contribute +2−8=−6, so S must be +6 to make the total 0.

Example 2

In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, identify which element is oxidized and which is reduced.

Track the change in oxidation number of each element.

Zn: 0 → +2, Cu: +2 → 0

A rise in oxidation number is oxidation (loses electrons); a fall is reduction (gains electrons).

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

▸ Zn oxidized / Cu²⁺ reduced

Zn loses electrons so its oxidation number rises; Cu²⁺ gains electrons so its oxidation number falls.

Summary

Three Definitions of Redox

①Electrons: ox=lose, red=gain ②Oxidation number: ox=↑, red=↓ ③Oxygen: ox=combine, red=remove

All three views are consistent

2021 KICE mock Chemistry I type, adapted

In which of the following is the oxidation number of nitrogen (N) equal to +5?

①NH₃

②N₂

③NO

④NO₂

⑤HNO₃

▸ ⑤ HNO₃

The oxidation numbers in a compound sum to 0.

In HNO₃, H=+1 and three O are −2 each, so solve for the oxidation number of N.

(+1) + N + 3(−2) = 0 → N = +5

🎯 Exam Points

①Find oxidation numbers: H=+1, O=-2, sum=0 (compound) or charge (ion)

②Identify ox/red agents: name is opposite to its own change

③Electron balance: electrons lost = electrons gained

④Everyday redox: rusting (ox), photosynthesis (red)

⑤Frequent on exams: identify ox/red and agents from oxidation numbers

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Stoichiometry

Acid-Base Reactions

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