Chapter 17.3, Problem 4RQ

Interpretation Introduction

Interpretation: Effect of increase in temperature on exothermic system at equilibrium has to 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 4RQ

For exothermic system at equilibrium, value for $K$ decreases as T increases because it favors the reactant formation or backward reaction. New equilibrium sets in backward direction.

Explanation of Solution

Reactions that during product formation, release heat as product are exothermic in nature. Such system at equilibrium, move in backward direction if more heat energy is added to the system or temperature is increased.

This as per Le Chatelier’s principle follows the theory that, if temperature is increased for exothermic system at equilibrium then system will move in direction which will oppose or lower the further increase in temperature. One such reaction is Haber process:

  ${\text{N}}_2\left(g\right)+{\text{H}}_2\left(g\right)\rightleftharpoons 2{\text{NH}}_3\left(g\right)$

The expression for equilibrium constant for above reaction is:

  $K=\frac{{\left[{\text{NH}}_3\right]}^2}{\left[{\text{N}}_2\right]\left[{\text{H}}_2\right]}$

For above system at equilibrium, value for $K$ decreases as T increases because it favors the reactant formation or backward reaction.

Conclusion

For exothermic system at equilibrium, value for $K$ decreases as T increases because it favors the reactant formation or backward reaction. New equilibrium sets in backward direction.