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What is a redox reaction?

Class 11 ChemistryNCERT chapter: Redox Reactionsइसे हिंदी में पढ़ें

Exam-ready answer

A redox reaction is a reaction in which oxidation and reduction take place simultaneously. In electronic terms, oxidation is the loss of electrons and reduction is the gain of electrons by a species; in terms of oxidation number, oxidation is an increase and reduction a decrease. Example: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), where zinc is oxidised and Cu²⁺ is reduced.

Explanation

Classically, oxidation meant adding oxygen or an electronegative element to a substance, or removing hydrogen or an electropositive element from it; reduction meant the opposite. In 2Mg(s) + O₂(g) → 2MgO(s), magnesium is oxidised. In 2Na(s) + Cl₂(g) → 2NaCl(s), each sodium atom gives one electron to chlorine, forming Na⁺ and Cl⁻. This shows that the real change in both cases is a transfer of electrons.

So, in the electronic concept, oxidation is the loss of electrons and reduction is the gain of electrons. The two must happen together, because electrons lost by one species are taken up by another. The substance that gains electrons is the oxidising agent (oxidant) and is itself reduced; the substance that loses electrons is the reducing agent (reductant) and is itself oxidised. When a zinc rod is dipped in copper sulphate solution, zinc gives electrons to Cu²⁺ ions, so copper deposits on the rod.

For covalent compounds, where electrons are not fully transferred, oxidation number is used: oxidation is an increase in oxidation number and reduction is a decrease. Rusting of iron is an everyday redox reaction: iron atoms lose electrons and are oxidised to iron(III), while oxygen gains electrons and is reduced, in the presence of water. Burning of fuels, respiration and the working of cells and batteries are also redox reactions.

Points to remember

  1. Redox reaction: oxidation and reduction occurring simultaneously in the same reaction.
  2. Oxidation: loss of electrons or increase in oxidation number; reduction: gain of electrons or decrease in oxidation number.
  3. Oxidising agent: electron acceptor, gets reduced; reducing agent: electron donor, gets oxidised.
  4. Example: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s); Zn is the reducing agent and Cu²⁺ the oxidising agent.
  5. Common redox processes: combustion, rusting of iron, respiration and reactions in electrochemical cells.

Exam tip

Write the reaction, put oxidation numbers above each element and show the change with arrows marked 'oxidation' and 'reduction'. Then name the oxidising and reducing agents; this layout gets full marks for both definition and example.

Questions students also ask

Why must oxidation and reduction always occur together?
Free electrons cannot simply be lost into the surroundings during a chemical reaction. Electrons given up by one species must be accepted by another. So whenever one substance is oxidised by losing electrons, another must be reduced by gaining them, and the two half reactions always go together.
Is rusting of iron a redox reaction?
Yes. In rusting, iron loses electrons and is oxidised to iron(III), while oxygen from the air gains electrons and is reduced, with water present. The product is hydrated iron(III) oxide, the reddish-brown rust. Moisture speeds up the process.
How do you identify the oxidising and reducing agents in a reaction?
Assign oxidation numbers to every atom on both sides. The element whose oxidation number increases is oxidised, and the substance containing it is the reducing agent. The element whose oxidation number decreases is reduced, and the substance containing it is the oxidising agent.

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