Equilibrium constants have been determined for many reactions over a wide range of temperatures. One obvious use is to calculate the concentrations of the various substances at equilibrium. In the ammonia example, if you had been given the equilibrium constant K and the pressures of ammonia and hydrogen, you could have calculated the pressure of nitrogen. (You may want to attempt this simple calculation.)
Another important use of equilibrium constants is to predict the initial direction of a reaction. Most commonly, the original concentrations of substances are unstable with respect to equilibrium; this is necessarily the case if any substance is absent. The reaction will proceed in one direction until equilibrium is attained. Referring to the ammonia example, if initially
ammonia decomposes to nitrogen and hydrogen with decreasing and increasing and until the equilibrium pressures are reached. In the converse case, where initially
nitrogen and hydrogen will combine to form ammonia with decreasing and and increasing until the values satisfy the equilibrium constant. Notice that only concentrations at equilibrium are stable and unchanging at a given temperature.
Although equilibrium calculations involve the concentrations of dissolved substances and gases, the values for pure liquids and solids are virtually constant and so are usually not incorporated into the equilibrium constant. As an illustration of this point, the reaction \
contains a liquid (H 2O) that has a fixed composition. The equilibrium may be expressed by either of the two equations:
where the concentration of H 2O does not appear because, as a constant, it is included in the value of the equilibrium constant. As was mentioned earlier, there are different equilibrium constants depending on whether the concentrations are in molarity ( K c) or pressure ( K p).
As one more example of the nonappearance of pure phases in the equilibrium equation, examine the reaction of lithium bromide with hydrogen:
The equilibrium equation is
Constant concentrations of the two solids (lithium bromide and lithium hydride) are not included in the value of K. Although pure liquids and solids do not appear in the equilibrium expression as commonly written, they must be present as real substances in the actual reaction for the two opposing reactions to be at equilibrium.
The earlier calculations for both acid dissociations and solubility products are special applications of finding concentrations from equilibrium constants.
- At 100°C, the halogen reaction