Chapter 21, Problem 102A

(a)

Interpretation Introduction

Interpretation: A balanced equation for the reaction of sodium with water needs to be written.

Concept Introduction: In a redox reaction, reduction and oxidation take place simultaneously. Here, oxidation takes place at the anode and reduction takes place at the cathode.

Explanation of Solution

The reaction of sodium and water is a redox reaction. Here, oxidation of sodium and reduction of water takes place. The two half-reactions are as follows:

Oxidation:

  $\text{Na}\to {\text{Na}}^{+}+e$

Reduction:

  ${\text{2H}}_{\text{2}}\text{O}\left(l\right)+\text{2e}\to {\text{H}}_2\left(g\right)+2{\text{OH}}^{-}\left(aq\right)$

The overall reaction can be represented as follow:

  $\begin{array}{l}2\left(\text{Na}\left(s\right)\to {\text{Na}}^{+}\left(aq\right)+e\right)\\ {\text{2H}}_{\text{2}}\text{O}\left(l\right)+\text{2e}\to {\text{H}}_2\left(g\right)+2{\text{OH}}^{-}\left(aq\right)\\ \overline{\underset{¯}{\text{2Na}\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)\to {\text{2Na}}^{+}\left(aq\right)+{\text{H}}_2\left(g\right)+2{\text{OH}}^{-}\left(aq\right)}}\\ \text{Or,}\\ \text{2Na}\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)\to \text{2NaOH}\left(aq\right)+{\text{H}}_2\left(g\right)\end{array}$

Therefore, the balanced equation for the reaction is as follows:

  $\text{2Na}\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)\to \text{2NaOH}\left(aq\right)+{\text{H}}_2\left(g\right)$

(b)

Interpretation Introduction

Interpretation: A balanced equation for the reaction of calcium with water needs to be written.

Concept Introduction: In a redox reaction, reduction and oxidation take place simultaneously. Here, oxidation takes place at the anode and reduction takes place at the cathode.

Explanation of Solution

The reaction of calcium and water is a redox reaction. Here, oxidation of calcium and reduction of water takes place. The two half-reactions are as follows:

Oxidation:

  $\text{Ca}\to {\text{Ca}}^{2+}+2e$

Reduction:

  ${\text{2H}}_{\text{2}}\text{O}\left(l\right)+\text{2e}\to {\text{H}}_2\left(g\right)+2{\text{OH}}^{-}\left(aq\right)$

The overall reaction can be represented as follow:

  $\begin{array}{l}\text{Ca}\left(s\right)\to {\text{Ca}}^{2+}\left(aq\right)+2e\\ {\text{2H}}_{\text{2}}\text{O}\left(l\right)+\text{2e}\to {\text{H}}_2\left(g\right)+2{\text{OH}}^{-}\left(aq\right)\\ \overline{\underset{¯}{\text{Ca}\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)\to {\text{Ca}}^{2+}\left(aq\right)+{\text{H}}_2\left(g\right)+2{\text{OH}}^{-}\left(aq\right)}}\\ \text{Or,}\\ \text{Ca}\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)\to \text{Ca}{\left(\text{OH}\right)}_{\text{2}}\left(aq\right)+{\text{H}}_2\left(g\right)\end{array}$

Therefore, the balanced equation for the reaction is as follows:

  $\text{Ca}\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)\to \text{Ca}{\left(\text{OH}\right)}_{\text{2}}\left(aq\right)+{\text{H}}_2\left(g\right)$