Reaction equilibria are characterised by a quantity known as the equilibrium constant (K). Equilibrium constants are 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.
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
Because use of [A] means concentration of A in , the subscript c is sometimes omitted from Kc when the full reaction quotient expression is shown.
In general, the expression has concentrations of products in the numerator, concentrations of reactants in the denominator each raised to the power equal to their coefficient in the balanced equation.
The number of different concentrations appearing in the numerator and in the denominator depends on the number of different reactants and products.
For aA bB + cC, there will be two concentrations in the numerator and one in the denominator.
!!!Note that some substances appearing in the reaction equation do NOT appear in the reaction quotient.
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.
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.
aA + bB
cC + dDA, B, C and D are all solutes or gases
a, b, c and d are the numbers required to balance the equation
Because use of [A] means concentration of A in , the subscript c is sometimes omitted from Kc when the full reaction quotient expression is shown.
| Kc = | [C]c[D]d |
| [A]a[B]b | |
| reaction quotient |
In general, the expression has concentrations of products in the numerator, concentrations of reactants in the denominator each raised to the power equal to their coefficient in the balanced equation.
The number of different concentrations appearing in the numerator and in the denominator depends on the number of different reactants and products.
For aA
bB + cC, there will be two concentrations in the numerator and one in the denominator.!!!Note that some substances appearing in the reaction equation do NOT appear in the reaction quotient.
Solids and pure liquids do not appear because, provided some is present, the amount of these 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 because they are present in large excess and their concentration does not change as a consequence of reaction.
Solvents, like H2O for reactions in aqueous solution, do not appear because they are present in large excess and their concentration does not change as a consequence of reaction.
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 of concentrations is referred to as an equilibrium composition.
Despite the differences in concentrations of the individual components, when these are substituted into the reaction quotient expression, they combine to give the same number for K.
Despite the differences in concentrations of the individual components, when these are substituted into the reaction quotient expression, they combine to give the same number for K.