AP Chemistry — Unit 7: Equilibrium
Dynamic equilibrium
Equilibrium is not the reaction stopping — it is the forward and reverse reactions running at the same rate, so the amounts stop changing while the conversions keep going. The box below starts as pure colourless N₂O₄, which splits into brown NO₂. Nothing in the simulation pushes the mixture toward an answer: each particle is converted at random, at a rate set by the rate laws. Equilibrium is simply where those two rates happen to balance.
What to take away: compare Q (what the mixture is right now) with K (where it settles). Q wobbles around K instead of landing on it — with a countable number of particles that fluctuation is real, and it is what "dynamic" in dynamic equilibrium means. Q < K means the forward reaction is still winning; Q > K means the reverse is; Q = K is equilibrium. Adding N₂O₄, removing NO₂, or changing the volume all move Q, and the mixture drifts back until Q equals K again — that is Le Châtelier's principle. Temperature is the exception: it changes K itself. Because the forward reaction here is endothermic, heating raises K and drives the mixture browner, while cooling lowers K and drives it back toward colourless N₂O₄.