IB Chemistry R2.3 Notes

Now you can see how the equilibrium constant is the ratio of rate expressions. As a result, the formula only applies when the system is at equilibrium. If it has not reached this point yet, it is still in the reaction and the same formula describes the reaction quotient (Q). So in the reaction:

aW + bX ⇌ cY + dZ

The formula for this is:

$Q = \frac{[Y]^{c}[Z]^{d}}{[W]^{a}[X]^{b}}$

The reaction quotient describes where the reaction lies with respect to the equilibrium.

1. If Q < K_c, it means the reaction still has more reactants and is undergoing a forwards reaction to produce more products and reach equilibrium.
2. If Q > K_c, it means the reaction still has more products and is undergoing a reverse reaction to produce more reactants and reach equilibrium.

Lastly, if Q = K_c, it means equilibrium has been reached.