Chapter 17.2, Problem 4RQ

Interpretation Introduction

Interpretation: Equilibrium expressions don’t include solids or liquids which must be explained with reasons.

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.

Answer to Problem 4RQ

Concentration of solids and liquids are never incorporated in the expression for equilibrium.

Explanation of Solution

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.

In case of writing expression for equilibrium constant, concentration terms for solids and liquids are not included as their concentration remains to be unchanged through the reaction.

Such as in reaction: ${\text{CaCO}}_3\left(s\right)\rightleftharpoons \text{CaO}\left(s\right)+{\text{CO}}_2\left(g\right)$

  $K=\left[{\text{CO}}_2\right]$

Since ${\text{CaCO}}_3$ and $\text{CaO}$ are pure solids, their concentration term is not included.

Conclusion

Concentration of solids and liquids are never incorporated in the expression for equilibrium.