Chapter 21, Problem 48A

Interpretation Introduction

Interpretation: The electrode at which hydrogen is produced needs to be determined.

Concept Introduction: In an electrochemical cell, there are two electrodes: anode and cathode. At the anode, oxidation takes place and at the cathode reduction takes place.

Explanation of Solution

The given diagram is as follows:

  

The oxidation and reduction reactions in the electrolysis of water are represented as follows:

Oxidation: $2{\text{H}}_{\text{2}}\text{O}\left(l\right)\to {\text{O}}_{\text{2}}\left(g\right)+4{\text{H}}^{+}\left(aq\right)+4e$

Reduction: $2{\text{H}}_{\text{2}}\text{O}\left(l\right)+2e\to {\text{H}}_{\text{2}}\left(g\right)+2{\text{OH}}^{-}\left(aq\right)$

In this process, the anode region turns acidic due to an increase in hydrogen ions. On the other hand, the cathode region turns basic as the number of hydroxide ions increases. The above two reactions can be added to get the overall cell reaction:

  $\begin{array}{l}2{\text{H}}_{\text{2}}\text{O}\left(l\right)\to {\text{O}}_{\text{2}}\left(g\right)+4{\text{H}}^{+}\left(aq\right)+4e\\ 2\left(2{\text{H}}_{\text{2}}\text{O}\left(l\right)+2e\to {\text{H}}_{\text{2}}\left(g\right)+2{\text{OH}}^{-}\left(aq\right)\right)\\ \overline{\underset{¯}{{\text{6H}}_{\text{2}}\text{O}\left(l\right)\to {\text{O}}_{\text{2}}\left(g\right)+4{\text{H}}^{+}\left(aq\right)+2{\text{H}}_{\text{2}}\left(g\right)+4{\text{OH}}^{-}\left(aq\right)}}\\ \text{Or,}\\ {\text{2H}}_{\text{2}}\text{O}\left(l\right)\to {\text{O}}_{\text{2}}\left(g\right)+2{\text{H}}_{\text{2}}\left(g\right)\end{array}$

In the given diagram, the region where reduction takes place or the cathode is the region where hydrogen gas is produced.

Here, electrode A is the cathode; thus, it is the region where hydrogen is produced. This is because, for the same number of moles of water, the number of moles of hydrogen gas produced is more than the number of moles of oxygen gas produced and the gas produced in electrode A is more as compared to that in B.

Therefore, the electrode at which hydrogen is produced is electrode A.