Oxidising and Reducing Agents in Redox: Class 10

Quick answer: Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen. In a redox reaction both happen together. The oxidising agent is the reactant that gives oxygen (or removes hydrogen) and is itself reduced; the reducing agent is the reactant that removes oxygen (or gives hydrogen) and is itself oxidised. In $\mathrm{CuO + H_2 \rightarrow Cu + H_2O}$, $\mathrm{CuO}$ is reduced and is the oxidising agent, while $\mathrm{H_2}$ is oxidised and is the reducing agent.

Oxidation and reduction

In Class 11 you will also meet the electron definitions: oxidation is loss of electrons and reduction is gain of electrons. For Class 10, the oxygen and hydrogen definitions are enough.

Oxidising and reducing agents

Key rule: Pick all four answers (substance oxidised, substance reduced, oxidising agent, reducing agent) from the reactants. The substance oxidised is the reducing agent, and the substance reduced is the oxidising agent.

How to identify them: a method

Step 1: For each reactant, find what it becomes in the products.

Step 2: Check whether it gained or lost oxygen or hydrogen.

Step 3: A reactant that gains oxygen or loses hydrogen is oxidised; one that loses oxygen or gains hydrogen is reduced.

Step 4: The reactant that is oxidised is the reducing agent; the one that is reduced is the oxidising agent.

Reaction 1: copper(II) oxide and hydrogen

$\mathrm{CuO(s) + H_2(g) \xrightarrow{heat} Cu(s) + H_2O(l)}$

When hydrogen is passed over heated black copper(II) oxide, the black coating turns brown as copper forms.

• $\mathrm{CuO}$ loses oxygen and becomes Cu: it is reduced, so it is the oxidising agent.
• $\mathrm{H_2}$ gains oxygen and becomes $\mathrm{H_2O}$: it is oxidised, so it is the reducing agent.

Reaction 2: zinc oxide and carbon

$\mathrm{ZnO(s) + C(s) \rightarrow Zn(s) + CO(g)}$

• $\mathrm{ZnO}$ loses oxygen: reduced, oxidising agent.
• C gains oxygen to form CO: oxidised, reducing agent.

This reaction is used to extract zinc from its oxide.

Reaction 3: manganese dioxide and hydrochloric acid

$\mathrm{MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2}$

• $\mathrm{MnO_2}$ loses oxygen: reduced, oxidising agent.
• HCl loses hydrogen and forms $\mathrm{Cl_2}$: oxidised, reducing agent.

Reaction 4: hydrogen sulphide and chlorine

$\mathrm{H_2S + Cl_2 \rightarrow 2HCl + S}$

• $\mathrm{H_2S}$ loses hydrogen to form S: oxidised, reducing agent.
• $\mathrm{Cl_2}$ gains hydrogen to form HCl: reduced, oxidising agent.

Summary table

Exam tips

• Write the full formula of the reactant, for example "HCl is oxidised", not "chlorine is oxidised".
• Remember the swap: oxidised ↔ reducing agent, reduced ↔ oxidising agent.
• In a two-part question, say both what is oxidised and what is reduced; a redox reaction always has both.

Common mistakes

• Naming a product (such as Cu or $\mathrm{H_2O}$) as the oxidising or reducing agent.
• Calling the substance that is oxidised the oxidising agent.
• Missing hydrogen transfer in reactions like $\mathrm{H_2S + Cl_2}$, where no oxygen is involved.
• Writing unbalanced equations, for example $\mathrm{MnO_2 + HCl}$ without the 4.

Frequently Asked Questions

What is an oxidising agent?

A substance that oxidises another substance by giving it oxygen or removing hydrogen from it. The oxidising agent is itself reduced.

What is a reducing agent?

A substance that reduces another substance by removing oxygen from it or giving it hydrogen. The reducing agent is itself oxidised.

Which substance is oxidised in MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂?

HCl is oxidised, because it loses hydrogen to form chlorine. MnO₂ is reduced.

In CuO + H₂ → Cu + H₂O, what is the reducing agent?

Hydrogen (H₂), because it removes oxygen from copper(II) oxide and is itself oxidised to water.

Why are oxidation and reduction always found together?

When one substance gains oxygen, another must lose it, so oxidation of one reactant goes with reduction of another.