7.6.1 Predicting the Extent of a Reaction

The numerical value of the equilibrium constant for a reaction indicates the extent of the reaction. But it is important to note that an equilibrium constant does not give any information about the rate at which the equilibrium is reached.

The magnitude of Kc or Kp is directly proportional to the concentrations of products (as these appear in the numerator of equilibrium constant expression) and inversely proportional to the concentrations of the reactants (these appear in the denominator).

This implies that a high value of K is suggestive of a high concentration of products and vice-versa.

We can make the following generalizations concerning the composition of equilibrium mixtures

If Kc > 103, products predominate over reactants, i.e., if Kc is very large, the reaction proceeds nearly to completion. Consider the following examples

(a) The reaction of H2 with O2 at 500 K has a very large equilibrium constant, Kc = 2.4 × 1047.

(b) H2(g) + Cl2(g) 2HCl(g) at 300K has Kc = 4.0 × 1031.

(c) H2(g) + Br2(g) 2HBr (g) at 300 K, Kc = 5.4 × 1018

If Kc < 10–3, reactants predominate over products, i.e., if Kc is very small, the reaction proceeds rarely. Consider the following examples:

(a) The decomposition of H2O into H2 and O2 at 500 K has a very small equilibrium constant, Kc = 4.1 × 10-48

(b) N2(g) + O2(g) 2NO(g), at 298 K has Kc = 4.8 ×10-31

If Kc is in the range of 10-3 to 103 , appreciable concentrations of both reactants and products are present. Consider the following examples

(a) For reaction of H2 with I2 to give HI, Kc = 57.0 at 700K.

(b) Also, gas phase decomposition of N2O4 to NO2 is another reaction with a value of Kc = 4.64 × 10-3 at 25°C which is neither too small nor too large. Hence, equilibrium mixtures contain appreciable concentrations of both N2O4 and NO2.

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