The knowledge of equilibrium constant helps us in
• predicting the extent of a reaction.
• predicting the direction in which the net reaction is taking place.
• calculation of equilibrium concentrations and equilibrium pressures.
PREDICTION OF EXTENT OF REACTION
The magnitude of equilibrium constant tells us about the extent to which the reactants are converted into the products before the equilibrium is attained.
Larger the value of K, greater is the extent to which the reactants are converted into the products. On the other hand, if the value of K is small it indicates that equilibrium is mainly in favour of reactants and that only a small fraction of the reactants is converted into products.
We can make the following generalisations concerning the composition of equilibrium mixtures:
• If K c > 103, products predominate over reactants. If K c is very large, the reaction proceeds nearly to completion.
• If K c < 10 -3, reactants predominate over products. If K c is very small, the reaction hardly proceeds.
• If K c is in the range of 10-3 to 103, appreciable concentrations of both reactants and products are present.
For example, consider the following reaction:
K = 2.2 x 1022 at 1000 k
K for this reaction is given by the expression:
K = [CO2]2 / [CO]2 [O2]
Since the value of Kin this case is very small, it indicates that at equilibrium, the concentrations of H2 andO2 are very small. In other words, under the given conditions only a small fraction of H2O dissociates into H2 and O2. Now let us consider some reactions in which the value of K c is neither too small nor too large. In such reactions there are appreciable concentrations of both reactants and products at equilibrium. The two such equilibria are given as follows:
Relationship between K c and the extent of reaction is illustrated in Fig. 21.9.
Fig. 21.9. Relationship between K c and the extent of reaction.
PREDICTING THE DIRECTION OF THE NET REACTION
From the knowledge of equilibrium constant, it is possible to predict the direction in which the net reaction is taking place at a given concentrations or partial pressures of reactions and products.
From the given concentrations, the concentration quotient Qc for the reaction is calculated. By comparing the value of Qc with the equilibrium constant, K c the direction of reaction can be predicted.
• If Qc > K c, the net reaction is taking place in backward direction i.e., direction of reactants.
• If Qc < K c, the net reaction is taking place in forward direction i.e. , direction of products.
• If Qc = K c, the reaction is at equilibrium i.e., no net reaction is taking place.
Fig. 21.10. Relation between Qc, K c and the direction of net reaction.
CALCULATION OF EQUILIBRIUM CONCENTRATIONS AND EQUILIBRIUM “PRESSURES .
If the equilibrium concentrations of various reactants and products are known in a reaction, the equilibrium constant can be calculated. On the other hand, if the equilibrium constant is known, then equilibrium concentration can be calculated.