Chapter 17.3, Problem 3RQ

Interpretation Introduction

Interpretation: Effect of change in volume over gaseous system at equilibrium must be explained.

Concept introduction: When the rate at which forward and backward reaction occur become equal such that concentration of reactants and products remains to be constant, that state is described as equilibrium. It is indicated by double sided arrow $\rightleftharpoons$ . It can be physical or chemical in nature.

For an equilibrium reaction:

  $a\text{A}+b\text{B}\rightleftharpoons c\text{C}+d\text{D}$

The expression for equilibrium constant is:

  $K=\frac{{\left[\text{C}\right]}^c{\left[\text{D}\right]}^d}{{\left[\text{A}\right]}^a{\left[\text{B}\right]}^b}$

Where,

  $K$ is equilibrium constant.

  $\left[\text{A}\right]$ and $\left[\text{B}\right]$ are reactant concentrations for A and B.

  $\left[\text{C}\right]$ and $\left[\text{D}\right]$ are product concentrations for C and D.

a, b, c and d are coefficients of reactants.

Answer to Problem 3RQ

In a gaseous system at equilibrium, change in volume has 2 possible effects as listed below:

1. If the volume is compressed or pressure of the system is escalated, equilibrium will shift in direction to oppose volume compression or lower the escalation of pressure.
2. If the volume is expanded or pressure of the system is lowered, equilibrium will shift in direction to oppose volume expanded or escalate the pressure.

Explanation of Solution

In a gaseous system at equilibrium, change in volume has 2 possible effects as listed below:

1. If the volume is compressed or pressure of the system is escalated, equilibrium will shift in direction to oppose volume compression or lower the escalation of pressure.
2. If the volume is expanded or pressure of the system is lowered, equilibrium will shift in direction to oppose volume expanded or escalate the pressure.

Such as in following reaction: ${\text{PCl}}_3\left(g\right)+{\text{Cl}}_2\left(g\right)\rightleftharpoons {\text{PCl}}_{\text{5}}\left(g\right)$ , lowering the volume will move the equilibrium in forward direction that is will fasten the process of product formation. This is because, 1 gaseous molecule on the product side and 2 on reactant side are present.

The expression for equilibrium constant for above reaction is:

  $K=\frac{\left[{\text{PCl}}_3\right]}{\left[{\text{Cl}}_2\right]\left[{\text{PCl}}_5\right]}$

If the number of gaseous molecules is equal on both sides, volume change will not effect any alteration in equilibrium constant.

Conclusion

In a gaseous system at equilibrium, change in volume has 2 possible effects as listed below:

1. If the volume is compressed or pressure of the system is escalated, equilibrium will shift in direction to oppose volume compression or lower the escalation of pressure.
2. If the volume is expanded or pressure of the system is lowered, equilibrium will shift in direction to oppose volume expanded or escalate the pressure.