2.3.5 (HL)—Reaction quotient (Q)

Q uses concentrations at any time, not necessarily equilibrium. Q < K means the forward direction is needed; Q > K means the reverse direction is needed; Q = K means equilibrium.

Calculate Q with the same expression as K but using the current concentrations. If Q = 0.20 and K = 5.0, too little product is present relative to equilibrium, so the forward direction lowers the mismatch. Recalculate after composition changes; Q is a snapshot, not a new constant.

Worked $Q$ example: for $\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$, $Q=[\ce{NH3}]^2/([\ce{N2}][\ce{H2}]^3)$. If every current concentration is $0.50\,\mathrm{mol\,dm^{-3}}$, $Q=(0.50)^2/[(0.50)(0.50)^3]=4.0$. At 475 K, $K=0.59$, so $Q>K$: the mixture contains too much product relative to equilibrium and the reverse direction is favoured until $Q=K$.