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MHT-CET Chemistry · Redox Reactions

Redox Reactions: Spotting What Is Oxidised, Counting Electrons, and Balancing

In a redox reaction one element's oxidation number rises and another's falls; the electrons lost by the first must equal the electrons gained by the second, and that equality fixes the coefficients.

All Redox Reactions notesMHT-CET Chemistry strategy

Why this matters

12 PYQs, none HARD. Six ask which element is oxidised or reduced, or whether a reaction is redox at all; six ask for electrons transferred or a missing coefficient. Two cards, both built on the oxidation numbers of the page before.

Concept 1 of 2: Which Element Is Oxidised, Which Is Reduced — and Is It Redox at All?

Mark the oxidation number of every element on both sides and look for the two that move. The one that goes up lost electrons and was oxidised; the one that goes down gained them and was reduced. If nothing moves, the reaction is not redox, however much the formulas change.

Definition

  • Oxidised = oxidation number rises (electrons lost); reduced = it falls (electrons gained).
  • The species containing the reduced element is the oxidising agent, and vice versa.
  • Not redox: double displacement (NaCl + KNO₃), acid–base (Mg(OH)₂ + NH₄Cl), and Cr₂O₇²⁻ + H₂O ⇌ 2CrO₄²⁻ + 2H⁺ — Cr stays +6.
  • Redox: displacement of a metal, e.g. Zn + 2AgCN → Zn(CN)₂ + 2Ag (Zn 0 → +2, Ag +1 → 0); combination with an element, e.g. 3Mg + N₂ → Mg₃N₂.

Worked example

In 3H₃AsO₃ + BrO₃⁻ → Br⁻ + 3H₃AsO₄, which element is reduced and which oxidised?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 2 · Q78Moderate

Example 1 · Redox Reactions · Balancing Redox Reactions and Oxidized/Reduced Species

Identify the elements undergoing reduction and oxidation respectively in the following redox reaction? 3H3AsO3(aq)+BrO3−(aq)→Br−(aq)+3H3AsO43H_3AsO_3(aq)+BrO_3^-(aq)\rightarrow Br^-(aq)+3H_3AsO_4

Picking hydrogen or oxygen because they appear on both sides

H (+1) and O (−2) are spectators in almost every aqueous redox reaction. Look for the element whose number actually changes.
Drill 5 more on which element is oxidised, which is reduced — and is it redox at all?

Concept 2 of 2: Electrons Transferred and the Balancing Coefficients

The change in oxidation number per atom is the number of electrons that atom moves. Multiply by the atoms of that element in the formula to get electrons per formula; then choose coefficients so the electrons lost equal the electrons gained. For a half-reaction, balance the charge instead.

Definition

  • Electrons per atom = size of the change: MnO₄⁻ → Mn²⁺ 5; Cr₂O₇²⁻ → 2Cr³⁺ 6 (3 per Cr); NO₃⁻ → NH₄⁺ 8; KMnO₄ → Mn₂O₃ 4 per Mn.
  • Oxidation-number method: electrons lost × coefficient = electrons gained × coefficient.
  • Half-reaction (ion-electron) method: after balancing atoms, add electrons so both sides carry the same charge.
  • Per mole of oxidising agent in Zn + 2HCl: H⁺ + e⁻ → ½H₂, so 1 mol of electrons.

Electron balance

nlost×(coefficient)=ngained×(coefficient)n_{\text{lost}} \times (\text{coefficient}) = n_{\text{gained}} \times (\text{coefficient})

Worked example

Balance Mn²⁺ + x ClO₃⁻ → MnO₂ + x ClO₂. What is x?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 1 · Q68Moderate

Example 2 · Redox Reactions · Balancing Redox Reactions and Oxidized/Reduced Species

What is the value of x to balance: Mn(aq)2++xClO3−(aq)→MnO2(s)+xClO2(aq)\text{Mn}^{2+}_{(aq)}+x\text{ClO}_3^-\text{(aq)}\to\text{MnO}_2\text{(s)}+x\text{ClO}_2\text{(aq)}

Counting electrons per ion instead of per atom

Cr₂O₇²⁻ → 2Cr³⁺ is 3 electrons per Cr but 6 per dichromate ion. Read which the question asks for.

Forgetting that N₂ has two atoms

In CuO + NH₃ → N₂, one N₂ needs 2 N at 3 electrons each — 6 electrons, so 3 Cu (2 each) balance it, not 6.
Drill 5 more on electrons transferred and the balancing coefficients

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Watch out for (3)

Test yourself on Redox Reactions

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Redox Reactions chapter testDrill all 12 questions