Page 14 - 10-Chem-9 Chemical Equilibrium

9. Chemical Equilibrium eLearn.Punjab

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 H2(g) , I2(g) and HI(g) . Suppose concentrations of the components of the mixture are:

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 Qc. The reaction quotient for this reaction is calculated as:

As the numerical value of Qc (8.0) is less than Kc (57.0), the reaction is not at equilibrium.
It requires more concentration of product. Therefore, reaction will move in the forward
direction.
The reaction quotient Qc is useful because it predicts the direction of the reaction by
comparing the value of Qc with Kc .
Thus, we can make the following generalization about the direction of the reaction.

If Qc < Kc ; the reaction goes from left to right, i.e., in forward direction to attain
equilibrium.

If Qc > Kc ; the reaction goes from right to left, i.e., in reverse
direction to attain equilibrium.

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