A change in concentration of one component of a system at equilibrium, causes reaction to occur to reestablish equilibrium. Knowing the direction of the change in one component and the reaction quotient expression (
Q), the direction of change in the other components can be predicted because
- for any system at equilibrium, the reaction quotient Q equals the equilibrium constant K
- concentrations multiplied by one another in Q change in the opposite directions.
Consider how the changes in [H3O+] affect [conjugate base] for proton transfer equilibria.
Decrease in [H3O+] | Ka = | [H3O+][conjugate base] | | [acid] |
Decrease in [H3O+] causes reaction resulting in
increased [conjugate base]
decreased [acid]
when equilibrium is reestablished | acid + H2O → H3O+ + conjugate base | Qa = | [H3O+][conjugate base] | | [acid] |
For a system at equilibrium
Qa = Ka
| Increase in [H3O+] | Ka = | [H3O+][conjugate base] | | [acid] |
Increase in [H3O+] causes reaction
resulting in
decreased [conjugate base]
increased [acid]
when equilibrium is reestablished |
- concentrations divided by one another change in the same direction.
Consider how the changes in [H3O+] affect [acid] above.
As might have been expected
addition of H3O+ (decreasing pH) increases the "acidity", and [acid] is increased
removal of H3O+ (increasing pH) decreases the "acidity", and [conjugate base] is increased.