The equilibrium constant
Reactions at equilibrium at a particular temperature are characterised by a number known as the equilibrium constant (K). The equilibrium constant for a particular reaction is related to concentrations of reactants and products present at equilibrium by an expression known as the reaction quotient.
The reaction quotient expression depends on the equation for the reaction. It has
The form of the expression is shown for the general chemical reaction below.
As their name implies, equilibrium constants are constant at a particular temperature; however the concentrations of the individual components in mixtures at equilibrium may differ widely.
The reaction quotient expression depends on the equation for the reaction. It has
concentrations of products multiplied in the numerator,
concentrations of reactants multiplied in the denominator
each concentration raised to a power equal to its coefficient in the balanced equation for the reaction.
concentrations of reactants multiplied in the denominator
each concentration raised to a power equal to its coefficient in the balanced equation for the reaction.
The form of the expression is shown for the general chemical reaction 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 (coefficients)
Because [A] means concentration of A in , the subscript c is sometimes omitted from Kc when the full reaction quotient expression is shown.
The number of different concentrations 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.
cC + dDA, B, C and D are all solutes or gases
a, b, c and d are the numbers required to balance the equation (coefficients)
Because [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 |
The number of different concentrations 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.As their name implies, equilibrium constants are constant at a particular temperature; however the concentrations of the individual components in mixtures at equilibrium may differ widely.
Each different combination of concentrations is referred to as an equilibrium composition.
Despite the differences in concentrations of individual components, these combine to give the same number for K when these are substituted into the reaction quotient expression.
Despite the differences in concentrations of individual components, these combine to give the same number for K when these are substituted into the reaction quotient expression.