6.0 Reaction Quotient ($Q$) and prediction of direction of reaction

Reaction quotient is determined in the same way ass equilibrium constant at any stage of reaction. If it is determined by concentration of species then ${Q_c}$ and if by partial pressure then ${Q_p}$.

For a general reaction

$$aA + bB \rightleftharpoons cC + dD$$

$${Q_c} = \frac{{{{\left[ C \right]}^c}{{\left[ D \right]}^d}}}{{{{\left[ A \right]}^a}{{\left[ B \right]}^b}}}$$

$${Q_p} = \frac{{{{\left[ {\frac{{{P_C}}}{{RT}}} \right]}^c}{{\left[ {\frac{{{P_D}}}{{RT}}} \right]}^d}}}{{{{\left[ {\frac{{{P_A}}}{{RT}}} \right]}^a}{{\left[ {\frac{{{P_B}}}{{RT}}} \right]}^b}}}$$

Three cases arise :-

1. If $Q = K$, reaction is at equilibrium.

2. If $Q < K$, $Q$ will tend to increase. As a result reaction will proceed in forward direction.

3. If $Q > K$, $Q$ will tend to decrease, so as to become equal to $K$. As a result, the reaction will proceed in backward direction.