Topic 7.10 Reaction Quotient and Le Chatelier's Principle: explain the direction of an equilibrium shift quantitatively by comparing the reaction quotient Q with K after a disturbance.

What this topic is asking

The College Board (Topic 7.10) wants you to explain the direction of an equilibrium shift quantitatively by comparing the reaction quotient Q with K after a disturbance. This gives a rigorous basis for Le Chatelier's principle: a shift happens because the disturbance moves $Q$ away from $K$, and the system responds to restore $Q = K$.

A disturbance changes Q, not K

This is the key reframing. Le Chatelier's qualitative rules all reduce to: the disturbance moves $Q$ off $K$. Adding product raises $Q$; adding reactant lowers $Q$; compressing a gas changes the concentrations and so changes $Q$. None of these touch $K$, so the system must react to bring $Q$ back to $K$.

Restoring Q equals K

So the direction of the shift is read directly from the comparison: $Q < K$ means shift right (toward products); $Q > K$ means shift left (toward reactants). This is the same rule as in Topic 7.3, now applied to a system that was at equilibrium and then disturbed.

Why this is better than the qualitative rule

The Q-versus-K approach explains the mechanism behind Le Chatelier's principle and removes ambiguity. For example, if you add a species that appears on both sides, or change conditions in a way where the qualitative rule is unclear, computing $Q$ and comparing it with $K$ always gives the right direction. It also makes clear why temperature is special: temperature is the only change that alters $K$ itself, so it shifts equilibrium by moving the target, not just $Q$.

Try this

Q1. A system at equilibrium has product added so that $Q$ becomes greater than $K$. State the direction of the shift. [1 point]

• Cue. Reverse (toward reactants), to lower $Q$ back to $K$.

Q2. Explain why changing the volume of a gas equilibrium can shift it without changing $K$. [2 points]

• Cue. Changing the volume changes the concentrations and so changes $Q$, but $K$ depends only on temperature, so the system shifts to restore $Q = K$.