How to construct and balance redox equations?

How do we construct and balance redox equations?

The way to construct a redox equation is to write two balanced ionic half-reaction equations, one for the oxidising agent and the other one for the reducing agent, and combine them. The method to balance each half-reaction equation is slightly different from that of a normal stoichiometric equation and involves the following steps:-

1. 1. Balance elements other than O and H
   2. Balance O by adding H₂O
   3. Balance H by adding H⁺
   4. Balance excess positive charges by adding electrons, e^–
   5. (For reaction in a basic solution) Balance H⁺ by adding OH^– on both sides, combine H⁺ and OH^– to form H₂O and cancel common terms

For example, the redox equation for the oxidation of acidified iron (II) sulphate by potassium dichromate (VI) is constructed as follows:

1st half-reaction equation

Fe²⁺ → Fe³⁺

Balancing the equation

Fe²⁺ → Fe³⁺ + e^–            eq1

(note that steps 1 through 3 and step 5 do not apply)

2nd half-reaction equation

Cr₂O₇^2- → Cr³⁺

Balancing the equation

Cr₂O₇^2- → 2Cr³⁺

Cr₂O₇^2- → 2Cr³⁺ + 7H₂O

Cr₂O₇^2-  + 14H⁺ → 2Cr³⁺ + 7H₂O

Cr₂O₇^2-  + 14H⁺ + 6e^–→ 2Cr³⁺ + 7H₂O            eq2

(note that step 5 does not apply)

Next, combine the balanced half-reaction equations, cancel common terms and ensure that all electrons are eliminated in the final equation. For this example, we multiply eq1 by 6 throughout before adding to eq2, giving:

6Fe²⁺(aq) + Cr₂O₇^2- (aq) + 14H⁺ (aq) → 6Fe³⁺(aq) + 2Cr³⁺ (aq) + 7H₂O (l)

 

 

 

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