8. Q versus K

a A(aq) + b B(aq) c C(aq) + d D(aq)
a A(g) + b B(g) c C(g) + d D(g)

If we want to know what happens when we make a mixture of A and/or B and/or C and/or D, with specific concentrations (in case of homogeneous equilibria in solutions) or partial pressures (in case of homogeneous gas equilibria) we have to calculate Q.

There are three possibilities.

• Q = K_c or Q = K_p

In this (very sporadic) case nothing happens upon mixing the components: the concentrations/partial pressures are such that there is equilibrium.

• Q < K_c or Q < K_p

In this case Q is too small (smaller then Q_eq) to have equilibrium. So Q has to increase: there should be more C and D and less A and B. The forward reaction occurs until equilibrium is reached.

a A + b B c C + d D

• Q > K_c or Q > K_p

In this case Q is too large (larger then Q_eq) to have equilibrium. So Q has to decrease: there should be less C and D and more A and B. The reverse reaction occurs until equilibrium is reached.

c C + d D a A + b B

Summary

Q	Reaction
= Kc or Kp	no reaction
< Kc or Kp	forward reaction
> Kc or Kp	reverse reaction