Chapter U6, Problem 3STP

Interpretation Introduction

Interpretation:

The change in color of solution by addition of H+ ions to the solution for the given equilibrium needs to be determined.

$\begin{array}{l}{\text{H}}_{\text{(aq)}}^{\text{+}}{\text{ + In}}_{\text{(aq)}}^{\text{-}}\rightleftharpoons {\text{ HIn}}_{\text{(aq)}}^{}\\ \text{ yellow red}\end{array}$

Concept introduction:

On the basis of direction, a chemical reaction can be classified as reversible and irreversible reaction. An irreversible reaction can proceed in one direction only, whereas a reversible reaction proceeds in both directions.

In the rerversible reaction, reaction moves in both forward and backward directions. When reaction reaches at equilibrium position, the rate of forward reaction is equal to rate of backward direction.

Answer to Problem 3STP

Option (B) is correct option.

Explanation of Solution

Reason for correct option:

The direction of an equilibrium reaction is predicted with the help of Le Châtelier's principle. It states if equilibrium is disturbed by changes in determining factors like temperature, pressure, and concentration, the reaction will tend to shift its equilibrium position in that direction which can counteract the effect of the disturbance. 

$\begin{array}{l}{\text{H}}_{\text{(aq)}}^{\text{+}}{\text{ + In}}_{\text{(aq)}}^{\text{-}}\rightleftharpoons {\text{ HIn}}_{\text{(aq)}}^{}\\ \text{ yellow red}\end{array}$

In the given reaction, by the addition of H+ ions to the solution, the equilibrium will move to that direction which can decrease the concentration of H+ ions. Thus, it will move in forward direction and the color of solution will change from yellow to red.

Conclusion

Reasons for incorrect options:

The addition of H+ ions shifts the equilibrium towards forward direction thus yellow color changes to red that makes all other options incorrect.