7.4 Oxidizing and Reducing Agents

This topic explains oxidizing agents and reducing agents — the substances that cause oxidation and reduction in redox reactions.

Definition

Oxidizing Agent

An oxidizing agent is a substance that causes another substance to oxidize by taking electrons from it. It is often called an electron acceptor because it accepts electrons during a reaction.

Simple explanation: An oxidizing agent takes electrons from another substance, causing it to be oxidized.

Reducing Agent

A reducing agent is a substance that causes another substance to be reduced by donating electrons to it. It is often called an electron donor because it loses electrons during the reaction.

Simple explanation: A reducing agent gives electrons to another substance, causing it to be reduced.

Main Idea

In a redox reaction:

• The oxidizing agent is reduced (it gains electrons).
• The reducing agent is oxidized (it loses electrons).

Simple explanation: The oxidizing agent takes electrons; the reducing agent gives electrons.

Core Concept

Detailed Explanation

Oxidizing Agents

Definition: A substance that causes another substance to oxidize by taking electrons from it.

• Often called an electron acceptor.
• Oxidizing agents themselves are reduced in the process (they gain electrons).

• Oxygen (O₂)
• Hydrogen peroxide (H₂O₂)
• Chlorine (Cl₂)
• Potassium permanganate (KMnO₄)

Simple explanation: Oxidizing agents are "electron hungry" — they take electrons from other substances.

Reducing Agents

Definition: A substance that causes another substance to be reduced by donating electrons to it.

• Often called an electron donor.
• Reducing agents themselves are oxidized in the process (they lose electrons).

• Hydrogen gas (H₂)
• Metal hydrides (such as NaBH₄)
• Carbon monoxide (CO)
• Metals such as zinc (Zn) and aluminium (Al)

Simple explanation: Reducing agents are "electron generous" — they give electrons to other substances.

Example from the Book

Reaction:

2Na + Cl₂ → 2NaCl

• Na is the reducing agent because it is oxidized (loses electrons).
• Cl₂ is the oxidizing agent because it is reduced (gains electrons).

Simple explanation: Sodium gives electrons (reducing agent); chlorine takes electrons (oxidizing agent).

Activity 7.1 (from the book)

Experiment 1: Reaction between FeSO₄ and KMnO₄

• Prepare solutions of ferrous sulphate (FeSO₄) and potassium permanganate (KMnO₄) in separate beakers.
• Transfer about 10 cm³ of ferrous sulphate solution in a test tube.
• Add about 10 cm³ of dilute H₂SO₄ in it.
• Now add a few drops of KMnO₄ solution in the test tube.

Observation: The purple colour of KMnO₄ is discharged.

• FeSO₄ reduces KMnO₄, so its purple colour is discharged.
• KMnO₄ oxidizes FeSO₄ in this reaction.
• FeSO₄ is the reducing agent, whereas KMnO₄ is the oxidizing agent.

• MnO₄⁻ ions (purple) are reduced to Mn²⁺ ions (colourless).
• During this reaction, the oxidation number of Mn changes from +7 to +2.

Simple explanation: KMnO₄ is an oxidizing agent used to test for reducing agents. When it oxidizes something, it changes from purple to colourless.

Experiment 2: Reaction between KI and H₂O₂

• Prepare an aqueous solution of potassium iodide (KI) and transfer it to a 10 cm³ test tube.
• Add about 5 cm³ of hydrogen peroxide (H₂O₂) to it.

Observation: The solution turns reddish-brown, indicating the formation of iodine (I₂).

• Potassium iodide (KI) is a reducing agent.
• It is often used to test for the presence of oxidizing agents.
• When KI solution is added to an acidified hydrogen peroxide solution, the solution turns reddish-brown.
• The appearance of this colour is due to the formation of iodine (I₂).
• KI is oxidized and H₂O₂ is reduced.

Equation:

H₂O_2(aq) + 2KI_(aq) + 2H⁺_(aq) → 2K⁺_(aq) + I_2(aq) + 2H₂O_(l)

Colour change: Colourless → reddish-brown

Simple explanation: KI is a reducing agent used to test for oxidizing agents. When it is oxidized, it forms iodine, which has a reddish-brown colour.