Question

Balance the following electrochemical reaction in an acidic solution:

CrO4^- +S^2- > Cr^3 +(SO4)^2-

Answer 1

reductiion reaction: CrO₄^- ------> Cr³⁺

CrO₄^-   +8H⁺   + 4e^-    ----------> Cr³⁺ + 4H₂O

multiplying the above equation by 4

4CrO₄^-   +32H⁺   + 16e^-    ----------> 4Cr³⁺ + 16H₂O .....................equation (1)

oxidation reaction : S^2- --------> SO₄^2-

S^2- + 4H₂O -----------> SO₄^2- + 8H⁺ + 8e^-

multiplying by 2

2S^2- + 8H₂O -----------> 2SO₄^2- +16H⁺ + 16e^- ..........................equation(2)

Adding equation 1 and 2 and cancelling the common terms

4CrO₄^-   + 2S^2- + 16H+ -------------> 4Cr³⁺   + 2SO₄^2-   + 8H₂O

                                   or

2CrO₄^-   + S^2- + 8H+ -------------> 2Cr³⁺   + SO₄^2-   + 4H₂O