Answer
Equilibrium constant (K) is the constant value at a given temperature that describes the fraction of product concentrations and reactants concentrations, exclusively at equilibrium.
Reaction quotient is calculated in the same way, but it can be calculated at any point of reaction, not exclusively at equilibrium. Therefore, it does not have a constant value.
When Q = K, that means that the reaction is at equilibrium.
When Q < K, the system has too much reactant, and should consume it in order to reach equilibrium.
When Q > K, the system has too much products, and the rate of the reverse reaction should be greater in order to reach equilibrium.
Work Step by Step
Figure 12-8 (page 503) explains well the relation between Q and K.
Q is defined on the same page.
K was defined earlier on section 12-2, page 492
