Question

Consider a galvanic cell based in the reaction Fe²⁺ + Cr₂O₇^2- → Fe³⁺ + Cr³⁺ in acidic solution. What is the coefficient of Fe³⁺in the balanced equation?

Reference: Ref 18-2

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Select one:

a.

3

b.

4

c.

none of these

d.

6

e.

2

Answer 1

Fe in Fe+2 has oxidation state of +2

Fe in Fe+3 has oxidation state of +3

So, Fe in Fe+2 is oxidised to Fe+3

Cr in Cr2O7-2 has oxidation state of +6

Cr in Cr+3 has oxidation state of +3

So, Cr in Cr2O7-2 is reduced to Cr+3

Reduction half cell:

Cr2O7-2 + 6e- --> 2 Cr+3

Oxidation half cell:

Fe+2 --> Fe+3 + 1e-

Balance number of electrons to be same in both half reactions

Reduction half cell:

Cr2O7-2 + 6e- --> 2 Cr+3

Oxidation half cell:

6 Fe+2 --> 6 Fe+3 + 6e-

Lets combine both the reactions.

Cr2O7-2 + 6 Fe+2 --> 2 Cr+3 + 6 Fe+3

Balance Oxygen by adding water

Cr2O7-2 + 6 Fe+2 --> 2 Cr+3 + 6 Fe+3 + 7 H2O

Balance Hydrogen by adding H+

Cr2O7-2 + 6 Fe+2 + 14 H+ --> 2 Cr+3 + 6 Fe+3 + 7 H2O

This is balanced chemical equation in acidic medium

Coefficient of Fe3+ is 6

Answer: 6