Does a chemical reaction stop at equilibrium?
Short answer: Equilibrium is dynamic: both directions continue at equal rates, so concentrations stay constant.
The trap
The wrong idea
“At equilibrium, the reaction has stopped.”
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
At chemical equilibrium:
- Both reactions stop
- Forward and backward rates are equal
- Only the forward reaction runs
Show the answer
B. Forward and backward rates are equal
Equilibrium is dynamic: both directions continue at equal rates, so concentrations stay constant.
The click
No. Chemical equilibrium is dynamic: molecules keep reacting in both directions, and the forward and backward reactions simply run at the same rate. Because products form exactly as fast as they are used up, all the concentrations stay constant, and to the eye nothing seems to be happening.
How it gets there: at the start only reactants are present, so the forward rate is high and the backward rate is zero. As products build up, the forward rate falls (fewer reactant molecules) and the backward rate rises (more product molecules). When the two rates become equal, the concentrations stop changing. That state is equilibrium.
The evidence comes from isotope labelling. If some deuterium (D₂) is added to an equilibrium mixture of N₂, H₂ and NH₃, deuterium soon turns up in the ammonia (as NH₂D, NHD₂ and ND₃) even though the overall amounts of each substance stay the same. That can only happen if the forward and backward reactions are still running.
A good picture is a crowded escalator with people going up and coming down at the same rate: the number of people on each floor stays the same, but people are moving all the time. Equilibrium also needs a closed system; in an open container a gaseous product can escape, and the reaction may then go to completion instead.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.