Chapter 20.3, Problem 26SP

Interpretation Introduction

Interpretation: Using the half-reaction method the reaction in the basic solution is to be balanced.

Concept Introduction: The method of balancing a redox reaction in which balancing of the equation is done by half-reaction i.e., both oxidation and reduction half-reactions are balanced separately before combining them into a redox reaction.

Answer to Problem 26SP

The balanced chemical equation:

  $\text{4Zn(s)}+N{O}_3^{-}(aq)+7O{H}^{-}(aq)+6H_{2}O(l)\to 4Zn{(OH)}_4{}^{2-}(aq)+N{H}_3\text{(aq)}$

Explanation of Solution

Given,

  $Zn\text{(s)}+N{O}_3^{-}(aq)\to N{H}_3(aq)+Zn{\left(OH\right)}_4^{2-}(aq)$

The steps of the method are as follows:

Step 1: Write the ionic form of the unbalanced chemical equation.

  $Zn\text{(s)}+N{O}_3^{-}(aq)\to N{H}_3(aq)+Zn{\left(OH\right)}_4^{2-}(aq)$

Step 2: Identify and write oxidation/reduction half-reactions separately.

  $\begin{array}{l}\text{Oxidation half-reaction :}\\ \text{0 +2 (-2 +1) }\\ \text{Zn(s)}\to Z{\text{n(OH)}}_4{}^{2-}\text{(aq)}\end{array}$

  $\begin{array}{l}\text{Reduction half-reaction:}\\ +5-2\text{ -3 +1}\\ N{O}_3^{-}(aq)\to N{H}_3(aq)\end{array}$

Step 3: Balance atoms other than oxygen on both sides of the half-reactions.

  $Zn\text{(s)}+N{O}_3^{-}(aq)\to N{H}_3(aq)+Zn{\left(OH\right)}_4^{2-}(aq)$

Step 4: As the reaction takes place in a basic medium so, $H_{2}O$ and $O{H}^{-}$ should be used to balance the reaction. Balance the oxygen atom by the addition of $H_{2}O$

  $\begin{array}{l}\text{Oxidation half-reaction : }\\ \text{Zn(s)}+4H_{2}O\to Z{\text{n(OH)}}_4{}^{2-}\text{(aq)}\end{array}$

  $\begin{array}{l}\text{Reduction half-reaction:}\\ N{O}_3^{-}(aq)\to N{H}_3(aq)+3H_{2}O(l)\end{array}$

Step 5: Combine both equations and balance the hydrogen atom by addition of $O{H}^{-}$ .

  $\text{4Zn(s)}+N{O}_3^{-}(aq)+7O{H}^{-}(aq)+6H_{2}O(l)\to 4Zn{(OH)}_4{}^{2-}(aq)+N{H}_3\text{(aq)}$

The balanced chemical equation:

  $\text{4Zn(s)}+N{O}_3^{-}(aq)+7O{H}^{-}(aq)+6H_{2}O(l)\to 4Zn{(OH)}_4{}^{2-}(aq)+N{H}_3\text{(aq)}$