Question

Balance an equilibrium between chromate (Cr2O72-) and trivalent chromium (Cr3+).

If it was desired to obtain the answer to part a in terms of OH- instead of H+, how would I write the equation?

Answer 1

Equilibrium between Cr₂O7^2- < --- > Cr³⁺

To show this equilibrium perform following steps.

Step 1 :

Cr and oxygen balancing : This is done by using H2O molecule.

There are 7 Oxygens to the left side, so we add 7 H2O molecules.

Cr₂O7^2- < --- > 2Cr³⁺ + 7 H2O

Step 2 :

Hydrogen balancing :

This is done by addition 2H+ for each H2O molecule.

Cr₂O7^2- + 14H⁺ < --- > 2Cr³⁺ + 7 H2O

Step 3 :

Charge balance:

Charge is balanced by addition of electrons.

Cr₂O7^2- + 14H⁺ + 6 e^- < --- > 2Cr³⁺ + 7 H2O

Step 4:

Here the condition is to add OH-

We add 14 OH- to both side.

Cr₂O7^2- + 14H⁺ +14 OH- + 6 e^- < --- > 2Cr³⁺ + 7 H2O + 14 OH-

We know OH- +H+ will form H2O

Cr₂O7^2- + 14H2O + 6 e^- < --- > 2Cr³⁺ + 7 H2O + 14 OH-

H2O molecules from both side are cancelled.

Cr₂O7^2- + 7 H2O + 6 e^- < --- > 2Cr³⁺ + 14 OH-