How to predict the extent of a reaction.

Explanation

The magnitude of the equilibrium constant tells us how far a reaction has proceeded by the time equilibrium has been reached. That is, the value of K of a reversible reaction indicates the extent of the reaction.
Consider a reaction `A+BhArrC+D` and its equilibrium constant `K_(c)`.
`K_(c)=([C][D])/([A][B])`
It can be seen that the value of `K_(c)` is directly proportional to the concentration of the products, and is inversely proportional to the concentration of the reactants. Therefore, a high value of `K_(c)` indicates a high concentration of products and vice versa.
Case I : `K_(c)gtgt1` The reaction is strongly product-favoured. Because, equilibrium concentartion of products and much greater than equilibrium concentration of reactions.
A large value of `K_(c)` suggests that the reaction proceeds nearly to completion.
Examples :
(1) The ozone depletion reaction in the stratosphere.
`NO(g)+O_(3)(g)hArrNO_(2)(g)+O_(2)(g), K_(c)=6xx10^(34)" at "25^(@)C`
Case II : `K_(c)ltlt1` : The reaction is strongly reactant favoured. Because, equilibrium concentrations of reactants are greater than that of products.
A small value of `K_(c)` suggests that the reaction proceeds only to a negligible extent.
Examples :
(1) Ozonisation of oxygen.
`3O_(2)(g)hArr2O_(3)(g)," "K_(c)=6.25xx10^(-58)" at "25^(@)C`
Such low values of `K_(c)` in the above two examples indicate that the reactions do not take plate at `25^(@)C`
Case III : `K_(c)=1` : Equilibrium mixture contains significant concentrations of reactants and products. In this case, `K_(c)` is neither extremely large nor extremely small. Therefore neither the forward reaction nor the backward reaction goes to completion.
Examples:
1. Dissociation of `N_(2)O_(4)` has neither a very large nor a very small equilibrium constant.
`N_(2)O_(4)(g)hArr2NO_(2)(g)," "K_(c)=4.64xx10^(-3)" at "25^(@)C,andK_(c)=1.00" at "118^(@)C`