Reversible reaction: A reaction that is occurring in both directions.
Dynamic equilibrium: The condition that exists in a sealed container when the rate of the forward and backward reactions in a reversible reaction mixture are equal
Example: The reaction between hydrogen gas and nitrogen gas to make ammonia:
- When only Nitrogen and Hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of Hydrogen and Nitrogen are at their highest.
- As the reaction proceeds, the concentrations of Hydrogen and Nitrogen gradually decreases, so the rate of forward reaction will decrease. However, the concentration of Ammonia is gradually increasing and so the rate of the backward reaction will increase (Ammonia will decompose to reform hydrogen and nitrogen).
- Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal:
3H2 (g) + N2 (g) ⇌ 2NH3 (g)
When the rates of forward and backward reactions become equal