Are reactant and product concentrations equal at equilibrium?
Short answer: Constant, not equal. A large K means the mixture is product-heavy.
The trap
The wrong idea
“At equilibrium, reactant and product concentrations are equal.”
It sounds reasonable, and a lot of JEE/NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
For a reaction with K = 1000 at equilibrium, products compared to reactants are:
- Equal
- Much more
- Much less
Show the answer
B. Much more
Constant, not equal. A large K means the mixture is product-heavy.
The click
At equilibrium the concentrations are constant, not equal. Constant means they stop changing with time. Their actual values depend on the reaction and are fixed by the equilibrium constant K at that temperature.
For a reaction aA + bB ⇌ cC + dD, K_c = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ), using the concentrations at equilibrium. If K = 1000, the product term is a thousand times the reactant term, so the equilibrium mixture is strongly product-heavy (for a simple A ⇌ B, [B]/[A] = 1000). If K is tiny, say 10⁻¹⁰, hardly any product forms.
A common rough guide: K > 10³ means products predominate; K < 10⁻³ means reactants predominate; in between, appreciable amounts of both are present. Only when K is close to 1 (for simple reactions with equal coefficients) are the amounts comparable.
The confusion comes from mixing up equal rates with equal amounts. At equilibrium the forward and backward rates are equal, but each rate also depends on its own rate constant, and those differ, so equal rates can occur at very unequal concentrations. Also, the same equilibrium can be reached from many different starting mixtures: the individual concentrations differ from case to case, but the ratio in the K expression is the same at a given temperature.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.