In principle, any reaction that can be represented by a balanced chemical equation can take place. There are, however, two situations which may inhibit the reaction from occurring.
- The thermodynamic tendency (the combination of entropy and enthalpy) for the reaction to occur may be so small that the quantity of products is very low, or even negligible. This type of chemical reaction is said to be thermodynamically inhibited.
- The rate at which the reaction proceeds may be so slow that many years are required to detect any product at all, in which case we say the reaction is kinetically inhibited.
As a reaction proceeds, the quantities of the components on one side of the reaction equation will decrease and those on the other side will increase. As the concentrations of the components on one side of the equation decrease, that reaction rate slows down. As the concentrations of the components on the other side of the equation increase, that reaction rate speeds up. Eventually the two reaction rates become equal and the composition of the system stops changing. At this point, the reaction is in it's equilibrium state and no further change in composition will occur, as long as the system is left undisturbed.
In many reactions, the equilibrium state occurs when significant amounts of both reactants and products are present. Such a reaction is said to be reversible. The equilibrium composition is independent of the direction from which it is approached. The labeling of substances as reactants and products is entirely a matter of convenience.
The law of mass action states that any chemical change is a competition between a forward reaction (left–to-right) and a reverse reaction (right-to-left). The rates of these two reactions are governed by the concentrations of the substances reacting, and the temperature. As the reaction proceeds, these two reaction rates approach each other in magnitude and at equilibrium, they become equal.
Since the reactions continue at equilibrium (at equal rates), equilibrium is referred to as dynamic equilibrium. At equilibrium, microscopic changes (the forward and reverse reactions) continue but macroscopic changes (changes in quantities of substances) cease.
When a chemical system is at equilibrium, any disturbance of the system, such as a change in temperature, or the addition or removal of a reactant or product, will cause the equilibrium to shift to a new equilibrium state (different quantities of reaction components). The disturbance in the system causes changes in the reaction rates and quantities of components change until the reaction rates again become identical, and a new equilibrium position is established.