The effect of a change in overall pressure or volume
| Kc = | [C]c[D]d |
| [A]a[B]b | |
| reaction quotient |
Such changes are most commonly discussed for equilibria involving gases, but the same reasoning applies to diluting an aqueous equilibrium.
The effect on the magnitude of the reaction quotient with respect to K depends on total powers of concentration in the numerator and the denominator of the reaction quotient.
If the total powers are the same, changing all components by the same factor has no effect on the reaction quotient.
If the total powers are different, changing all components by the same factor makes the reaction quotient unequal to K.
If the total powers are different, changing all components by the same factor makes the reaction quotient unequal to K.
| Kc = | 22 × [HI]2 |
| 22 × [H2][I2] | |
| reaction quotient |
2HI(g)Doubling the concentration by an increase in pressure changes numerator and denominator by same factor, and results in no change to the reaction quotient.
Therefore the reaction quotient is still equal to K.
| Kc = | 22 × [NO2]2 |
| 23 × [NO]2[O2] | |
| reaction quotient |
2NO2(g)Doubling the concentration by an increase in pressure changes the numerator and denominator by different factors so that the reaction quotient no longer equals K.
The direction of the change in the reaction quotient can be predicted because the effect of an increase or decrease is greatest when more powers of concentration are multiplied together.
Thus the denominator of the reaction quotient above increases more. This makes the reaction quotient less than K and reaction occurs to form more NO2. Likewise if the concentration were halved, the denominator decreases more.
Because all components were changed in the same way, when equilibrium is restored, the concentration of all components are be changed in the same direction (increase or decrease depending on the nature of the change). However the degree of change is greater for the products of the reaction to restore equilibrium.
The reaction to restore equilibrium consumes NO and O2. Thus when equilibrium is restored, the increase in NO and O2 concentrations will be less than a factor of two. The NO2 concentration will increase by MORE than a factor of two.