Reaction equilibria are characterised by a quantity known as the
equilibrium constant (K). The equilibrium constant is related to
amounts of reactants and products present
at equilibrium by an expression known as the
reaction quotient.
The form of the expression depends on the equation for the reaction.
Amounts may be expressed as either concentrations (in
) or pressure (in kPa) because these are both a measure of the number of particles per unit volume
| Kc = | [C]c[D]d |
| [A]a[B]b |
| | reaction
quotient |
The
equilibrium constant Kc uses
concentration as the measure of amount present at equilibrium
.The form of the expression is shown for the general equation below.
aA + bB → cC + dD
A, B, C and D are all solutes or gases
a, b, c and d are the numbers required to balance the equation
!!!Solids and pure liquids do not appear in the reaction quotient because, provided some is present, the amount of such substances does not affect the concentrations of solutes in solutions in which they are in contact.
!!!Solvents, like H2O for reactions in aqueous solution, do not appear in the reaction quotient because they are present in large excess and their concentration does not change as a consequence of reaction.
| Kp= | p(C)cp(D)d |
| p(A)ap(B)b |
| | reaction quotient |
The
equilibrium constant Kp for gas phase reactions uses pressure as the measure of amounts of reactants and products present at equilibrium.
!!!Solids and pure liquids do not appear in the reaction quotient because, provided some is present, the amount of such substances does not affect the pressures of gases above them.
As their name implies, equilibrium constants are constant at a particular temperature; however the concentrations (or pressures) of the individual components in mixtures at equilibrium may differ widely.
Each of the different combinations is referred to as an equilibrium composition. Despite the differences in concentrations of the individual components, the concentrations (or pressures) for any of these will combine, when substituted into the reaction quotient expression to equal K.