9.4 Importance of Equilibrium Constant

Knowing the numerical value of equilibrium constant of a chemical reaction, direction as well as extent of the reaction can be predicted.

1. Predicting Direction of a Reaction

Direction of a reaction at a particular moment can be predicted by inserting the concentration of the reactants and products at that particular moment in the equilibrium expression. Consider the gaseous reaction of hydrogen with iodine.

We withdraw the samples from the reaction mixture and determine the concentrations of ${\mathrm{H}}_{2(\mathrm{g})}$$\mathrm{H}_{2(\mathrm{g})}$ , ${\mathrm{I}}_{2(\mathrm{g})}$$\mathrm{I}_{2(\mathrm{g})}$ and ${\mathrm{H}\mathrm{I}}_{(\mathrm{g})}$$\mathrm{HI}_{(\mathrm{g})}$ . Suppose concentrations of the components of the mixture are:

$[{\mathrm{H}}_2{]}_{\mathrm{t}}=0.10\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}[{\mathrm{I}}_2{]}_{\mathrm{t}}=0.20\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}\mathrm{a}\mathrm{n}\mathrm{d}[\mathrm{H}\mathrm{I}{]}_{\mathrm{t}}=0.40\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}\mathrm{.}$$\mathrm{[H_2]_{t} = 0.10 mol dm^{-3}\quad [I_2]_{t} = 0.20 mol dm^{-3}\quad and\quad [HI]_{t} = 0.40 mol dm^{-3}.}$

The subscript 't' with the concentration symbols means that the concentrations are measured at some time t, not necessarily at equilibrium. When we put these concentrations into the equilibrium constant expression, we obtain a value called the reaction quotient $Q_{c}$ . The reaction quotient for this reaction is calculated as:

As the numerical value of $Q_{c}$ (8.0) is less than $K_{c}$ (57.0), the reaction is not at equilibrium. It requires more concentration of product. Therefore, reaction will move in the forward direction.