This topic accounts for approximately 6% of your exam marks.
stable
Low
Stable6%
Haber process conditions and Le Chatelier's principle regularly examined.
What equilibrium looks like
Place a reversible reaction in a sealed container with no escape route for any chemical — a
At first only the forward reaction occurs (no products to react in the reverse direction yet); its rate is highest
As the products build up, the reverse reaction starts and speeds up
The forward rate falls (reactants becoming scarcer) and the reverse rate rises (products becoming more abundant)
Eventually the two rates become equal: the system is at
Two defining features
The rate of the equals the rate of the
The concentrations of reactants and products stay constant from this point onward (provided temperature and pressure do not change)
Why "dynamic", not "static"
At equilibrium the reactions have not stopped — both still happen at full speed
Each forward conversion of a reactant into a product is matched by a reverse conversion of a product back into a reactant
The chemicals interchange constantly; only the overall amounts stay fixed
Closed-system requirement
Equilibrium can only be reached when nothing can leak out (or leak in)
Open systems lose products as they form (a gas escaping into the air, for instance), so the reverse reaction never has enough material to keep up and equilibrium is never reached
Exam tip
Define dynamic equilibrium
Stating what dynamic equilibrium means is a regular 2-marker, so you need to know: the forward and reverse reactions happen at the same rate, and the concentrations of reactants and products stay constant. "Constant" means unchanging over time — not that the two concentrations are equal to each other.