Balancing redox reactions

Two methods for balancing redox reactions: 1. Oxidation number method 2. Half-reaction method.

• Balance redox equations using the oxidation number method.

• Balance redox equations in acidic and basic solutions using the half reaction method.

Why we balance electrons lost and gained in redox equations.

Cu + Cl2 → Cu+ + Cl-

+100 -1

gains 1e-

loses 1e-

2

x 2 = 2e-

2 2

Have to make sure the number of atoms and the

number of electrons transferred are equal.

x 2 atoms = 2e-

K2Cr2O7 + H2O + S → SO2 + KOH + Cr2O3

Step 1: Assign oxidation numbers.

Step 2: Calculate number of electrons lost/gained – watch the number of atoms.

-2+4-2+1-2+6+1 -2+30 +1-2+1

K2Cr2O7 + H2O + S → SO2 + KOH + Cr2O3

-2+4-2+1-2+6+1 -2+30 +1-2+1

lose 4e-

gains 3e- x 2 atoms = 6e-

Step 3: Balance atoms that lose / gain electrons.– Use lowest common multiple. – Massage numbers to make

atoms equal.

Step 4: Balance others by conventional method.

K2Cr2O7 + H2O + S → SO2 + KOH + Cr2O3

+4+6 +30

lose 4e-

gains 3e- x 2 atoms = 6e-

x 3 = 12e-

x 2 = 12e-

2 23 3

K2Cr2O7 + H2O + S → SO2 + KOH + Cr2O32 23 3 42

P + HNO3 + H2O → NO + H3PO4

+2+5 -2+10 -2+5+1-2+1 -2

lose 5e-

gains 3e-

x 3 = 15e-

x 5 = 15e-

3 35 52

H2SeO3 + HClO3 → H2SeO4 + Cl2 + H2O+6+1+5+1-2+4+1 -2+1-2 0-2

lose 2e-

gains 5e-

x 5 = 10e-

x 2 = 10e-

5 52 1

Step 3: Balance atoms that lose / gain electrons.– Use lowest common multiple. – Massage numbers to make

atoms equal.