☰ Contents · Chemistry (General chemistry)

The half-reaction method

Lessons 28 · 1 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
28

Balancing redox reactions by the half-reaction method

Textbook: pp. 127–132
GoalBalance redox reactions by the half-reaction method in acidic and alkaline media.
New words
half-reaction · yarim reaksiyaoxidising agent · oksidlovchireducing agent · qaytaruvchiionic equation · ionli tenglama
Explanation

In the half-reaction method separate half-reactions are written for the oxidising and the reducing agent; they involve particles that really exist in the solution (MnO₄⁻, SO₃²⁻, Cr₂O₇²⁻, H⁺, OH⁻, H₂O), not formal ions such as N⁵⁺. The order: 1) balance the atoms of the changing element; 2) balance oxygen; 3) balance hydrogen; 4) balance charge with electrons; 5) equalise the numbers of electrons and add the half-reactions; 6) cancel identical particles on both sides; 7) convert to the molecular equation. In acidic medium add one H₂O for each missing O to the side lacking oxygen, then H⁺ to the side lacking hydrogen. In alkaline medium add 2OH⁻ for each missing O to the side lacking oxygen and 1 H₂O to the other side. At the end check that atoms and total charge are equal on both sides.

Worked examples
Acidic medium: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O; SO₃²⁻ + H₂O − 2e⁻ → SO₄²⁻ + 2H⁺. Multipliers 2 and 5: 2MnO₄⁻ + 6H⁺ + 5SO₃²⁻ → 2Mn²⁺ + 5SO₄²⁻ + 3H₂O; molecular: 2KMnO₄ + 5Na₂SO₃ + 3H₂SO₄ = 2MnSO₄ + 5Na₂SO₄ + K₂SO₄ + 3H₂O.
Alkaline medium: Cr(OH)₃ + 5OH⁻ − 3e⁻ → CrO₄²⁻ + 4H₂O; Br₂ + 2e⁻ → 2Br⁻. Multipliers 2 and 3: 2Cr(OH)₃ + 10OH⁻ + 3Br₂ → 2CrO₄²⁻ + 8H₂O + 6Br⁻; molecular: 2Cr(OH)₃ + 3Br₂ + 10KOH = 2K₂CrO₄ + 6KBr + 8H₂O.
Class activity

Paper exercise: cover the two examples and balance them again from the list of steps; then check atoms and charge in each equation and mark each other’s work.

Practice
1
Balance the half-reaction Cr₂O₇²⁻ → Cr³⁺ in acidic medium and find the sum of all its coefficients.
2
Write the half-reaction SO₃²⁻ → SO₄²⁻ in alkaline medium.
3
The oxidising agent accepts 5 e⁻ and the reducing agent gives 2 e⁻. What are the multipliers and how many electrons are transferred in total?
4
Why is NO₃⁻ written in a half-reaction rather than N⁵⁺?