Oxidation and Reduction

• Oxidation: loss of electrons or gain of oxygen
• Reduction: gain of electrons or loss of oxygen

Example

2Mg + O₂ → 2MgO

Magnesium becomes oxidized when it reacts with oxygen to form magnesium oxide since it has gained oxygen.

CuO + H₂ → Cu + H₂O

Copper(II) oxide becomes reduced when it reacts with hydrogen to form copper since it has lost oxygen.

Redox Reactions

Usually in a reaction, both oxidation and reduction happens at the same time. We call these reactions redox reactions. Oxidizing agents and reducing agents are involved in redox reactions.

• Oxidizing agent: Substance that causes oxidation of other chemicals by gaining electrons or losing oxygen. Oxidizing agent itself is reduced.
• Reducing agent: Substance that causes reduction of other chemicals by losing electrons or gaining oxygen. Reducing agent itself is oxidized.

Example

2Al + Fe₂O₃ → 2Fe + Al₂O₃ 

In a thermite reaction,

• Aluminum is oxidized to aluminum oxide, and iron(III) oxide is reduced to iron.
• Aluminum is the reducing agent, and iron(III) oxide is the oxidizing agent. 

👉Let's take a look at some questions from IGCSE Chemistry exams👈

Question 1

The chemical equations for two reactions that occur during the extraction of aluminium are
A: Al³⁺ + 3e^- → Al
B: C + O₂ → CO₂
For each of these reactions, explain whether the underlined species is being oxidised or reduced.

A: Al³⁺ is being reduced since it gains electrons.

B: C is being oxidized since it gains oxygen.

Question 2

The ionic equation for the reaction between zinc and copper(II) sulfate is

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Explain why this is described as a redox reaction.

• Zn loses electrons and becomes Zn²⁺, so Zn is oxidized in this reaction.
• Cu²⁺ gains electrons and becomes Cu, so Cu²⁺ is reduced in this reaction.
• Zn is the reducing agent, and Cu²⁺ is the oxidizing agent.
• This is a redox reaction because oxidation and reduction occur at the same time.

Question 3

A violent reaction occurs when a mixture of chromium(III) oxide and aluminium is ignited with a magnesium fuse as shown.

The equation for the reaction is shown.

Cr₂O₃ + 2Al → 2Cr + Al₂O₃

Which substance is reducing agent in the reaction?

• Chromium(III) oxide has lost oxygen to form chromium, therefore chromium(III) oxide is reduced.
• Aluminum has gained oxygen and formed aluminum oxide, therefore aluminum is oxidized.
• Chromium(III) oxide is an oxidizing agent, and aluminum is a reducing agent in the reaction.

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