ABSTRACT
Each reversible reaction has two sets of molecules: the set of molecules on the left represents reactants in the forward reaction and products in the reverse reaction; the set of molecules on the right represents products in the forward reaction and reactants in the reverse reaction. Equilibrium can be established for a variety of chemical reactions, physical processes, and phase changes for all states of matter. The equilibrium expression uses a number of different concentration units; for concentrations other than molarity, the proper symbol for the specific concentration unit is used. The techniques for calculations using the equilibrium expression involve determining all variables in terms of one unknown variable to be solved. In all cases, stoichiometric analysis of the balanced equation is required. The common ion effect describes conditions for aqueous equilibria in which a solution already contains a measurable starting concentration of one of the aqueous ion products. This circumstance most often occurs for solubility or acid/base equilibria.
