Chapter 1 · Question 10

Define oxidation and reduction in terms of gain or loss of oxygen. Using the reaction of copper oxide with hydrogen, identify which substance is oxidised and which is reduced. Explain why this reaction is called a redox reaction.

Q10

Define oxidation and reduction in terms of gain or loss of oxygen. Using the reaction of copper oxide with hydrogen, identify which substance is oxidised and which is reduced. Explain why this reaction is called a redox reaction.

Answer Revealed

Direct Answer:

Oxidation is the gain of oxygen (or loss of hydrogen) by a substance. Reduction is the loss of oxygen (or gain of hydrogen) by a substance. In the reaction

\text{CuO(s)} + \text{H}_2\text{(g)} \xrightarrow{\text{Heat}} \text{Cu(s)} + \text{H}_2\text{O(l)}

: copper oxide loses oxygen and is reduced to copper, while hydrogen gains oxygen (forming water) and is oxidised. It is called a redox reaction because both oxidation and reduction occur simultaneously — one reactant is oxidised while the other is reduced.

Simple Explanation

Oxidation means gaining oxygen — like hydrogen picking up oxygen to become water. Reduction means losing oxygen — like copper oxide giving up its oxygen to become pure copper. In the

\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}

reaction, both happen at the same time: copper oxide gets reduced (loses oxygen) and hydrogen gets oxidised (gains oxygen). Since oxidation and reduction go hand in hand, it is a 'redox' (reduction-oxidation) reaction.

Exam-Ready Structure

The concepts of oxidation and reduction form the basis of redox chemistry, introduced in the NCERT Class 10 chapter: 1. Oxidation (in terms of oxygen): A substance is said to be oxidised if it gains oxygen during a chemical reaction. Example: When copper powder is heated in air, it gains oxygen to form black copper oxide:

2\text{Cu(s)} + \text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{CuO(s)}

(Equation 1.28). Copper is oxidised. 2. Reduction (in terms of oxygen): A substance is said to be reduced if it loses oxygen during a chemical reaction. Example:

\text{CuO} + \text{H}_2 \xrightarrow{\text{Heat}} \text{Cu} + \text{H}_2\text{O}

(Equation 1.29). Copper oxide loses its oxygen to become copper — it is reduced. 3. Applying the definitions to the reaction

\text{CuO(s)} + \text{H}_2\text{(g)} \xrightarrow{\text{Heat}} \text{Cu(s)} + \text{H}_2\text{O(l)}

: (a) Hydrogen (

\text{H}_2

) gains oxygen to form water (

\text{H}_2\text{O}

) → hydrogen is oxidised. (b) Copper oxide (

\text{CuO}

) loses oxygen to form copper metal (

\text{Cu}

) → copper oxide is reduced. 4. Redox (oxidation-reduction) reaction: A reaction in which one reactant gets oxidised while the other gets reduced is called an oxidation-reduction reaction, or redox reaction for short. In the above reaction, oxidation and reduction occur simultaneously — without hydrogen being oxidised, copper oxide cannot be reduced, and vice versa. 5. Two other NCERT redox examples: -

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

: Zinc oxide is reduced to zinc (loses oxygen), carbon is oxidised to carbon monoxide (gains oxygen). -

\text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2

: HCl is oxidised to chlorine while manganese dioxide is reduced to manganese chloride. 6. NCERT also extends the definition: oxidation can also mean loss of hydrogen, and reduction can also mean gain of hydrogen.

Key Points

Oxidation (oxygen-based): gain of oxygen by a substance (e.g., Cu → CuO)
Reduction (oxygen-based): loss of oxygen by a substance (e.g., CuO → Cu)
In $\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$ : H₂ is oxidised, CuO is reduced
Redox reaction: oxidation and reduction occur simultaneously in the same reaction
Extended definition: oxidation = loss of hydrogen; reduction = gain of hydrogen

Common Mistakes

Saying only one reactant is oxidised or reduced — in a redox reaction, both oxidation AND reduction must occur
Confusing which substance is oxidised and which is reduced — the substance that gains oxygen is oxidised; the substance that loses oxygen is reduced

Define oxidation and reduction in terms of gain or loss of oxygen. Using the reaction of copper oxide with hydrogen, identify which substance is oxidised and which is reduced. Explain why this reaction is called a redox reaction.