Chapter 20, Problem 79A

Interpretation Introduction

Interpretation:

The composition of cell before the reaction is to be determined.

Concept introduction:

The oxidation of any ion at the anode or its reduction at the cathode depends on the reduction potential of the ion.

Answer to Problem 79A

Before the electrolysis began the cell contained a solution of potassium bromide in water.

Explanation of Solution

When a solution of potassium bromide is electrolyzed, the electrolyte contains four different ions of H⁺, OH^-, K+ and Br^-. Out of K+ and H+ the hydrogen ion gets reduced due to its higher reduction potential forming hydrogen gas at the cathode. Out of OH- and Br- the bromine ion gets oxidized due to its lower reduction potential forming bromine gas at the anode. So after the electrolysis only K+ and OH- ions remain forming a concentrated solution of KOH.

Conclusion

The composition of cell before the electrolysis was found using thefinal products to be an aqueous solution of KBr.