What Do We Know?

One way to disturb reaction systems that involve acids and bases is to change the pH. The equilibria of acids and bases involve hydronium or hydroxide ions. Therefore, if a substance added to a solution changes the pH of the solution, the speciation of acids and bases already in the solution will be affected. For example, consider the equilibrium involving carbonic acid and hydrogen carbonate ions:

Imagine some HCl_(aq) was added to this system. How would the equilibrium be affected?

First of all, the HCl_(aq) would ionize and produce hydronium ions. This would increase the concentration of hydronium ions in the solution. Increasing the concentrations of the products causes the value of Q to increase, so that Q≠K.

In order to reach equilibrium, the reaction must shift to reduce Q, so that Q approaches K. To reduce Q, the reaction shifts to the left, converting products to reactants. This causes the concentration of carbonic acid to increase and the concentration of hydrogen carbonate ions to decrease. As H₂CO₃ only exists temporarily before breaking down into CO₂ and H₂O, the change in equilibrium could be experimentally verified by the production of CO₂ gas.