Equilibrium Shifts: A Qualitative View

The above discussion treats changes in pH quantitatively. These changes in pH can be described qualitatively, also. A qualitative view is very useful for predicting how the pH will change in response to external conditions (such as exercise). The principle used for this qualitative view is known as Le Ch â telier's Principle.

Le Châtelier's Principle

When a reactant or product of an equilibrium reaction is added to a solution that is at equilibrium, the added species will react to change the concentrations of the reactants and products in the solution until a new equilibrium is established (but the ratio of the concentrations given in the mass action expression (Equation 6) is the same, because the equilibrium constant, K, is a constant at a given temperature). This process is known as a shift in the equilibrium. In 1884, Henri Le Châtelier developed a rule to predict how a system in equilibrium will shift when the conditions of the system are changed. Although this rule can oversimplify the changes that occur in certain situations, it is a powerful and useful tool for predicting the direction of an equilibrium shift. Le Châtelier's Principle states that "if a change in conditions ([an external] 'stress') is imposed on a system at equilibrium, the equilibrium position will shift in a direction that tends to reduce that change in conditions" (Zumdahl, 208). For example, if the concentration of one of the products of an equilibrium reaction is increased in a solution that was at equilibrium (prior to the concentration increase), the equilibrium will shift so as to reduce the concentration of the product, i.e.,more reactant will be generated. Of course, the reverse equilibrium shift would occur when the concentration of a product is decreased. The effect of a temperature change can also be predicted by Le Châtelier's Principle. (In the case of a change in temperature, the equilibrium constant actually changes.) If a reaction is exothermic, "heat" is treated as a "product" (e.g., A + B -> C + D + "heat"). If a reaction is endothermic, "heat" is treated as a "reactant" (e.g., "heat" + A + B -> C + D). Increasing the temperature can be thought of as increasing the amount of "heat" in the reaction. Examples of external stresses and the equilibrium shifts predicted by Le Châtelier's Principle are shown in the purple table below.

External Stress	Predicted Equilibrium Shift
Change in pressure	Affects gases only.
Concentration of reactant(s) increased	Equilibrium shifts toward products (to the right).
Concentration of product(s) increased	Equilibrium shifts toward reactants (to the left).
Temperature increased (exothermic reaction)	Equilibrium shifts toward reactants (to the left).
Temperature increased (endothermic reaction)	Equilibrium shifts toward products (to the right).

Example: Removal of Bicarbonate by the Kidneys

Le Châtelier's Principle can be used to explain how the kidneys help prevent excessively high pH (a condition known as alkalosis). When the pH of the blood is too high, the kidneys remove bicarbonate ion (HCO₃^-) from the blood. As the kidneys decrease the blood's concentration of HCO₃^-, the equilibrium reaction in Equation 10 is shifted toward the left to compensate for the loss in HCO₃^-, according to Le Châtelier's Principle. When the equilibrium shifts to the left, more H⁺ ions are generated together with HCO₃^- ions. As a result, the pH decreases.