Balance redox reaction using ion-electron method in acidic medium

Question

Balance the following reaction using the ion-electron (half-reaction) method in acidic medium:

\text{MnO}_4^- + \text{Fe}^{2+} \to \text{Mn}^{2+} + \text{Fe}^{3+}

(NCERT Class 11, Chapter 8)

Solution — Step by Step

Reduction half: $\text{MnO}_4^- \to \text{Mn}^{2+}$

Oxidation half: $\text{Fe}^{2+} \to \text{Fe}^{3+}$

Reduction half:

Balance Mn: already balanced (1 on each side)
Balance O: add 4H $_2$ O to the right → $\text{MnO}_4^- \to \text{Mn}^{2+} + 4\text{H}_2\text{O}$
Balance H: add 8H $^+$ to the left → $\text{MnO}_4^- + 8\text{H}^+ \to \text{Mn}^{2+} + 4\text{H}_2\text{O}$
Balance charge: left side = $(-1 + 8) = +7$ , right side = $+2$ . Add 5e $^-$ to the left:

\text{MnO}_4^- + 8\text{H}^+ + 5e^- \to \text{Mn}^{2+} + 4\text{H}_2\text{O}

Oxidation half:

\text{Fe}^{2+} \to \text{Fe}^{3+} + e^-

The reduction half needs 5e $^-$ , the oxidation half releases 1e $^-$ . Multiply the oxidation half by 5:

5\text{Fe}^{2+} \to 5\text{Fe}^{3+} + 5e^-

Add both halves (electrons cancel):

\text{MnO}_4^- + 8\text{H}^+ + 5\text{Fe}^{2+} \to \text{Mn}^{2+} + 4\text{H}_2\text{O} + 5\text{Fe}^{3+}

Mn: 1 = 1 (balanced)
Fe: 5 = 5 (balanced)
O: 4 = 4 (balanced)
H: 8 = 8 (balanced)
Charge: left = $-1 + 8 + 10 = +17$ ; right = $+2 + 0 + 15 = +17$ (balanced)

The equation is balanced.

Why This Works

The ion-electron method ensures both mass and charge are balanced simultaneously. By separating oxidation and reduction, we can handle each process independently. The key steps are: balance atoms (using H $_2$ O for O and H $^+$ for H in acidic medium), then balance charge (using electrons).

This reaction (permanganate + ferrous ions) is a classic titration in analytical chemistry — used to determine Fe $^{2+}$ concentration. The endpoint is the permanent pink colour of excess KMnO $_4$ .

Alternative Method — In Basic Medium

For basic medium: first balance as if acidic, then add OH $^-$ to both sides to neutralise H $^+$ (converting H $^+$ + OH $^-$ to H $_2$ O).

For JEE, always verify your answer by checking charge balance on both sides — this catches errors. Also remember: in acidic medium, use H $^+$ and H $_2$ O; in basic medium, use OH $^-$ and H $_2$ O. Never mix H $^+$ and OH $^-$ in the same equation.

Common Mistake

Students often forget to balance oxygen using H $_2$ O and hydrogen using H $^+$ . They try to balance just by adding electrons, which leaves the equation unbalanced for atoms. The correct order is: (1) balance atoms other than O and H, (2) balance O with H $_2$ O, (3) balance H with H $^+$ , (4) balance charge with electrons.

Question

Solution — Step by Step

Why This Works

Alternative Method — In Basic Medium

Common Mistake

Try These Next