How does the values of equilibrium constant predict the extent of reaction?

Explanation

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 `K_(c) or K_(p)` 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 denomina tor): This implies that a high value of:K is suggestive of a high concentration of products and vice. versa.
We can make the following generalisation concerning the composition of equilibrium mixtures : If `K_(c) gt 10^(3)` products predominate over reactants, i.e., if `K_(c)` is very large, the reaction proceeds nearly to completion. Consider the following examples:
(a) The reacțion.of. `H_(2)` with `O_(2)` at 500 K has a very large equilibrium constant `K_(c)=2.4xx10^(47)`
(b) `H_(2_((g))+Cl_(2_((g))) 2HCl(g)` at 300 K has `K_(c)=4.0xx10^(31)`
(c) `H_(2_((g))) +Br_(2_((g)) hArr 2HBr_((g))` at 300 k, `K_(c)=5.4xx10^(18)`
If `K_(c) lt 10^(-3)` reactants predominate over products i.e, if `K_(c)` is very small the reaction proceeds rarely. Consider the following examples:
(a) The decomposition of `H_(2)O` into `H_(2) and O_(2)` at 500 K has a very small equilibrium constant.
`K_(c)=4.1xx10^(-48)`
(b) `N_(2_((g))+O_(2_((g))) hArr 2NO_((g))`
at 298 K has `K_(c)=4.8xx10^(-31)`
If `K_(c)` is in the range of `10^(-3)` to `10^(3)` appreciable concentrations of both reactants and products are present. Consider the following examples :
(a) For reactiion of `H_(2)` with `I_(2)` to give HI.

(b) Also, gas phase decomposition of `N_(3)O_(4)` to `No_(2)` is another reaction with a value of `K_(c)=4.64xx10^(-3)` at `25^(@)C` which is neither too small nor too large. Hence equilibrium mixtures contain appreciable concentrations of both `N_(2)O_(4)and NO_(2)`
These generarlisations are illustrated.