10.2.1 The Equilibrium Constant

• A homogenous system is one in which all components are in the same phase (state). For example, a gas phase system will have all its reactants and products in the gaseous state (g).
• A heterogenous system is one where at least two different phases (states) are present. For example, a gaseous and solid phase system.
• For reversible reactions, there is an equilibrium constant, Kc, that indicates the position of equilibrium for a reaction at a certain temperature.
• Kc for a reaction always has the same value unless the reaction conditions are changed, which in turn changes the position of equilibrium and therefore Kc.
• Kc equals the concentration of the products divided by the concentration of the reactants. Any variation in the number of moles raises that substance to a power with the same value as the number of moles.
• The equilibrium constant has varying units depending on the chemical reaction. It can be calculated by subbing the units into the Kc expression.
• The value of Kc is not affected by concentration change or use of a catalyst. However, it is affected by the changing reaction temperature, as the equilibrium position change results in different concentrations of reactants and products.