What Do We Know?
Near the ocean’s surface, experimental measurements show that the concentration of carbonate ions is about 1.91×10-4 mol L-1, and the concentration of calcium ions is about 1.05×10-2 mol L-1. Therefore, the reaction quotient, Q, is:
Worked Example
The dissolution of calcium carbonate in the ocean is an important reaction because marine animals require calcium carbonate to form their shells.
The solubility product, Ksp, of this reaction is 6.5x10-7 but, in the ocean, the reaction quotient, Q, is 2.0×10-6. Compare the reaction quotient and the solubility product. Is the reaction at equilibrium? If not, in what direction must the reaction shift to reach equilibrium?
Q≠Ksp indicating that the reaction is not at equilibrium. The fact that Q is greater than Ksp tells us that more products are present in the ocean than there would be if the dissolution of calcium carbonate were at equilibrium. In order to decrease Q, the reaction must shift to produce more reactants. This means that in the ocean, calcium ions and carbonate ions should react to form solid calcium carbonate.
Ocean water contains more calcium and carbonate ions than it would at equilibrium and is said to be supersaturated. In order to reach equilibrium, the ions must undergo a precipitation reaction to form solid CaCO3. However, this reaction occurs very slowly, allowing the ocean to remain supersaturated with calcium and carbonate ions in its surface layer. Some marine animals, however, can facilitate this reaction in order to produce their shells.