Chapter 7 - Redox Reactions

Redox Reactions explains chemical changes in which oxidation and reduction occur simultaneously. Redox processes are involved in respiration, combustion, corrosion, bleaching, metallurgy, batteries and electrochemical cells.

The chapter teaches students to recognise electron transfer, calculate oxidation numbers and balance reactions in acidic or basic media.

Classical and electronic ideas

In the classical sense, oxidation may involve addition of oxygen, removal of hydrogen, addition of an electronegative element or removal of an electropositive element. Reduction involves the opposite changes.

The electronic definitions are broader:

• Oxidation is loss of electrons.

• Reduction is gain of electrons.

These processes must occur together because electrons lost by one species are accepted by another.

Oxidising and reducing agents

An oxidising agent causes oxidation and is itself reduced. A reducing agent causes reduction and is itself oxidised.

Students identify these agents by tracking electron transfer or the change in oxidation number.

Oxidation number

Oxidation number is an assigned value used to track electron distribution in compounds and ions.

Students apply rules to:

• Elements in the free state

• Monatomic and polyatomic ions

• Neutral compounds

• Hydrides

• Peroxides and superoxides

• Compounds containing oxygen or halogens

The sum of oxidation numbers equals the overall charge of the species.

Types of redox reactions

Important types include:

• Combination reactions

• Decomposition reactions

• Displacement reactions

• Disproportionation reactions

In disproportionation, the same element in one oxidation state undergoes both oxidation and reduction.

Balancing by oxidation-number method

Students:

1. Assign oxidation numbers.

2. Identify the atoms oxidised and reduced.

3. Calculate the total increase and decrease.

4. Equalise electron change.

5. Balance remaining atoms and charge.

6. Verify the completed equation.

Balancing by ion-electron method

The equation is divided into oxidation and reduction half-reactions. Each half is balanced for atoms and charge, electrons are equalised and the half-reactions are added.

In acidic medium, water and hydrogen ions are used. In basic medium, hydroxide ions are used to remove hydrogen ions from the final equation.

Redox reactions and electrodes

When oxidation and reduction occur at separate electrodes, electrons travel through an external circuit.

• Oxidation occurs at the anode.

• Reduction occurs at the cathode.

This prepares students for Class 12 Electrochemistry.

What can students study on Teachoo?

• Oxidation-number rules

• Oxidised and reduced species

• Oxidising and reducing agents

• Classification of redox reactions

• Balancing in acidic medium

• Balancing in basic medium

• Half-reaction practice

• NCERT solutions

• MCQs, assertion-reason and case-based questions

Common mistakes

• Treating oxidation as addition of oxygen in every reaction

• Confusing the oxidising agent with the species oxidised

• Forgetting that the sum of oxidation numbers equals overall charge

• Balancing atoms but not charge

• Leaving electrons in the final equation

• Using acidic-medium steps unchanged in basic medium

Frequently asked questions

What is a redox reaction?

A redox reaction is one in which oxidation and reduction occur together.

Can oxidation occur without reduction?

No. The electrons lost by one species must be accepted by another.

What is the difference between charge and oxidation number?

Charge is an actual overall electrical property of an ion. Oxidation number is a formal assignment used to track electron movement.

How is the oxidising agent identified?

It is the species that is reduced; its oxidation number decreases.

What is disproportionation?

It is a reaction in which the same element is both oxidised and reduced.

Which balancing method is better?

Both are valid. The oxidation-number method is often quicker for molecular equations, while the half-reaction method is systematic for ionic equations.

Learn Redox Reactions with Teachoo for oxidation-number calculations, balancing methods and exam-oriented NCERT practice.