What are some common mistakes students make with dynamic equilibrium?
There is a persistent belief that equilibrium represents the cessation of chemical change, which of course is erroneous.
For the given reaction, there is a both a forward rate,
Depending on the equilibrium, the products or reactants may be favoured. As chemists, as physical scientists, we recognize that at equilibrium, the forward and reverse rates are EQUAL, i.e.:
Given this equality, we may write:
Now, clearly by definition,
So what's the lesson to learn? That equilibrium does NOT represent the cessation of chemical change, but rather equality of forward and reverse rates. Should the equilibrium be perturbed somehow (say be removing the products), the equilibrium will shift towards the PRODUCT side so as to re-establish the equilibrium. This is the origin of Le Chatelier's principle:
So the message? Equilibrium means equality of the forward and reverse rates.