Chapter 21, Problem 68A

(a)

Interpretation Introduction

Interpretation: The two possible reactions that can take place at the anode in the electrolysis of brine need to be determined.

Concept Introduction: In the electrolysis of brine (NaCl solution), three important chemicals can be obtained that is $\text{NaOH}$ , ${\text{Cl}}_{\text{2}}$ and ${\text{H}}_{\text{2}}$ . Here, reduction occurs at the cathode and oxidation at the anode.

Explanation of Solution

At the anode, oxidation takes place. In the electrolysis of brine (NaCl solution), the two possible oxidation reactions are as follows:

1. $2{\text{Cl}}^{-}\left(aq\right)\to {\text{Cl}}_{\text{2}}\left(g\right)+2e$
2. $2{\text{H}}_{\text{2}}\text{O}\left(l\right)\to {\text{O}}_{\text{2}}\left(g\right)+4{\text{H}}^{+}\left(aq\right)+4e$

(b)

Interpretation Introduction

Interpretation: The two possible reactions that can take place at the cathode in the electrolysis of brine need to be determined.

Concept Introduction: In the electrolysis of brine (NaCl solution), three important chemicals can be obtained that is $\text{NaOH}$ , ${\text{Cl}}_{\text{2}}$ and ${\text{H}}_{\text{2}}$ . Here, reduction occurs at the cathode and oxidation at the anode.

Explanation of Solution

At the cathode, reduction takes place. In the electrolysis of brine, the two possible reduction reactions are as follows:

1. ${\text{Na}}^{+}\left(aq\right)+e\to \text{Na}\left(s\right)$
2. $2{\text{H}}_{\text{2}}\text{O}\left(l\right)+2e\to {\text{H}}_{\text{2}}\left(g\right)+2{\text{OH}}^{-}\left(aq\right)$

(c)

Interpretation Introduction

Interpretation: The reaction that takes place at the anode needs to be identified.

Concept Introduction: The reaction with lower reduction electrode potential than other will undergo oxidation and takes place at the anode.

Explanation of Solution

The reduction electrode potential for the two possible reactions of the anode is as follows:

1. $2{\text{Cl}}^{-}\left(aq\right)\to {\text{Cl}}_{\text{2}}\left(g\right)+2e\text{ 1}\text{.36 V}$
2. $2{\text{H}}_{\text{2}}\text{O}\left(l\right)\to {\text{O}}_{\text{2}}\left(g\right)+4{\text{H}}^{+}\left(aq\right)+4e\text{ 2}\text{.07 V}$

Here, the reduction electrode potential for reaction 1 is lower than for reaction 2. Thus, from the two possible reactions, chlorine ions can easily get oxidized to chlorine gas. Therefore, the following reaction occurs at the anode:

  $2{\text{Cl}}^{-}\left(aq\right)\to {\text{Cl}}_{\text{2}}\left(g\right)+2e$

(d)

Interpretation Introduction

Interpretation: The reaction that takes place at the cathode needs to be identified.

Concept Introduction: The reaction with a higher reduction electrode potential than other will undergo reduction and takes place at the cathode.

Explanation of Solution

The reduction electrode potential for the two possible reactions of the cathode is as follows:

1. ${\text{Na}}^{+}\left(aq\right)+e\to \text{Na}\left(s\right)\text{ -2}\text{.71 V}$
2. $2{\text{H}}_{\text{2}}\text{O}\left(l\right)+2e\to {\text{H}}_{\text{2}}\left(g\right)+2{\text{OH}}^{-}\left(aq\right)\text{ -0}\text{.83 V}$

Here, the reduction electrode potential for reaction 2 is higher than reaction 1 thus, from the two possible reactions, water molecules can easily get reduced to hydrogen gas. Therefore, the following reaction occurs at the cathode:

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