An equilibrium constant calculated from partial pressures (Kp) is related to K by the ideal gas constant (R), the temperature (T), and the change in the number of moles of gas during the reaction. An equilibrium system that contains products and reactants in a single phase is a homogeneous equilibrium; a system whose reactants, products, or both are in more than one phase is a heterogeneous equilibrium. When a reaction can be expressed as the sum of two or more reactions, its equilibrium constant is equal to the product of the equilibrium constants for the individual reactions.

You will also notice in Table 2.2.2 that equilibrium constants have no units, even though Equation 2.2.5 suggests that the units of concentration might not always cancel because the exponents may vary. In fact, equilibrium constants are calculated using “effective concentrations,” or activities, of reactants and products, which are the ratios of the measured concentrations to a standard state of 1 M. As shown in Equation 2.2.6, the units of concentration cancel, which makes K unitless as well: (2.2.6)[A]m⁢e⁢a⁢s⁢u⁢r⁢e⁢d[A]s⁢t⁢a⁢n⁢d⁢a⁢r⁢ds⁢t⁢a⁢t⁢e=MM=m⁢o⁢lLm⁢o⁢lL Because equilibrium constants are calculated using “effective concentrations” relative to a standard state of 1 M, values of K are unitless.

Although the activities of pure liquids or solids are not written explicitly in the equilibrium constant expression, these substances must be present in the reaction mixture for chemical equilibrium to occur. Whatever the concentrations of CO and CO⁢A2, the system described in Equation 2.3.1 will reach chemical equilibrium only if a stoichiometric amount of solid carbon or excess solid carbon has been added so that some is still present once the system has reached equilibrium. As shown in Figure 2.3.1, it does not matter whether 1 g or 100 g of solid carbon is present; in either case, the composition of the gaseous components of the system will be the same at equilibrium.

You will also notice in Table 2.2.2 that equilibrium constants have no units, even though Equation 2.2.5 suggests that the units of concentration might not always cancel because the exponents may vary. In fact, equilibrium constants are calculated using “effective concentrations,” or activities, of reactants and products, which are the ratios of the measured concentrations to a standard state of 1 M. As shown in Equation 2.2.6, the units of concentration cancel, which makes K unitless as well: (2.2.6)[A]m⁢e⁢a⁢s⁢u⁢r⁢e⁢d[A]s⁢t⁢a⁢n⁢d⁢a⁢r⁢ds⁢t⁢a⁢t⁢e=MM=m⁢o⁢lLm⁢o⁢lL Because equilibrium constants are calculated using “effective concentrations” relative to a standard state of 1 M, values of K are unitless.