The equilibrium constant K and the reaction quotient Q
One way in which the equilibrium constant K can be calculated is to determine all equilibrium concentrations and substitute these into an expression known as the reaction quotient. The form of the expression depends on the equation for the reaction.
| Q = | [C]c[D]d |
| [A]a[B]b | |
| reaction quotient | |
cC + dDand A, B, C and D are substances that are all dissolved (solutes) or are all gases
where a, b, c and d are coefficients in the balanced equation
and [A] = concentration of A in
where a, b, c and d are coefficients in the balanced equation
and [A] = concentration of A in
For a particular reaction the reaction quotient Q can be calculated using concentrations of A, B, C and D present at any time during the course of the reaction. If the system has reached equilibrium, the calculated value of Q equals K, the equilibrium constant for the reaction.
While K is a constant at a particular temperature, each of [A], [B], [C] and [D] can differ for different equilibrium mixtures; however they combine as shown above to give the same number K. Thus the equilibrium constant is a constant NOT the equilibrium composition.
Note that products appear in the numerator of the reaction quotient expression used to calculate K. Therefore if a reaction is
product-favoured (products in higher concentration than reactants at equilibrium) K is large
reactant-favoured (products in lower concentration than reactants at equilibrium) K is small
reactant-favoured (products in lower concentration than reactants at equilibrium) K is small