Chapter 8.1, Problem 5RQ

Interpretation Introduction

Interpretation :

A spectator ion and its role in the net ionic equation must be explained.

Concept Introduction :

When two electrolytes react to produce new compounds then net ionic equation can be written for the reaction. Some of the ions remain unchanged. Those are spectator ions.

Answer to Problem 5RQ

A spectator ion is an ion which is present both in the reactant and product side. In a net ionic equation spectator ion is cancelled from both the sides of the chemical equation.

Explanation of Solution

Let’s consider the reaction between a strong acid (HNO₃) and a strong base (KOH).

  ${\text{HNO}}_{\text{3}}\text{(aq)}\to {\text{H}}^{\text{+}}{\text{(aq)+NO}}_{\text{3}}{}^{\text{-}}\text{(aq)}$ .

From the above equation it can be seen that HNO₃ is completely dissociated to produce H+ and NO₃- ions.

  $\text{KOH(aq)}\to {\text{K}}^{\text{+}}{\text{(aq)+OH}}^{\text{-}}\text{(aq)}$ .

Similarly KOH solution produces K+ and OH- ions.

As these are strong electrolytes so there is complete dissociation. No undissociated molecule can be seen.

Overall reaction will be

  $\begin{array}{l}\text{HCl(aq)+KOH(aq)}\to {\text{KCl(aq)+H}}_{\text{2}}\text{O(l)}\text{.}\\ {\text{H}}^{\text{+}}\text{(aq)}+{\text{Cl}}^{\text{-}}{\text{(aq)+K}}^{\text{+}}{\text{(aq)+OH}}^{\text{-}}\text{(aq)}\to {\text{K}}^{\text{+}}{\text{(aq)+Cl}}^{\text{-}}{\text{(aq)+H}}_{\text{2}}\text{O(l)}\text{.}\\ {\text{H}}^{\text{+}}{\text{(aq)+OH}}^{\text{-}}\text{(aq)}\to {\text{H}}_{\text{2}}\text{O(l)}\text{.}\end{array}$

It can be seen that K+ (aq) and Cl^-(aq) are present in both the sides of the reaction and so these are spectator ligands. In the net ionic equation spectator ions are cancelled.