Dynamic equilibrium — Chemistry, 14–17 years
Some reactions can go forwards and backwards. At equilibrium, both directions continue, but at equal rates, so the amounts stay steady unless conditions change.
Still-looking, still-moving
Imagine people leaving and entering a room at the same rate. The number inside stays constant, although movement has not stopped. A reversible chemical reaction can behave in the same way: forward and backward changes continue at equal rates. This is dynamic equilibrium.
Why does equilibrium matter?
In a closed container, reversible reactions do not usually finish by using up one side completely. Chemists needed to explain these steady mixtures and to improve the amount of a useful product. Equilibrium thinking shows how temperature, pressure or concentration can shift the balance.
Adding more reactant
Consider N₂ + 3H₂ ⇌ 2NH₃. At equilibrium, add nitrogen while keeping temperature and pressure fixed. The nitrogen concentration rises, so the forward reaction briefly becomes faster than the reverse reaction. More ammonia forms until the two rates become equal again; the final mixture has shifted towards ammonia.
Equal rates do not mean equal amounts
A common mistake is to think equilibrium means there are equal amounts of reactants and products. That seems natural because “balance” suggests equal sides. In fact, equilibrium means equal reaction rates; the amounts may be very different, depending on the reaction and its conditions.
Making fertiliser efficiently
The Haber process makes ammonia for fertilisers using a reversible reaction between nitrogen and hydrogen. Factories choose a pressure, temperature and catalyst that give a useful rate and yield, then remove ammonia so the balance favours making more. Energy use and cost limit how far this can be pushed.
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