At equilibrium in a reversible reaction, the amounts of reactants and products do not change because the forward and reverse reactions occur at equal rates. This dynamic state is known as chemical equilibrium.
The principle behind this behavior is known as the law of mass action, which states that the rate of a chemical reaction is directly proportional to the concentrations (or pressures) of the reactants. In a reversible reaction, both the forward and reverse reactions follow this principle.
Initially, when the reaction starts, the forward reaction dominates as there is an excess of reactants. As the reaction progresses, the concentrations of the reactants decrease, and the concentrations of the products increase. This leads to an increase in the rate of the reverse reaction.
Eventually, a point is reached where the rates of the forward and reverse reactions become equal. At this stage, the system has reached equilibrium. At equilibrium, the concentrations (or pressures) of the reactants and products remain constant over time because the rates of the forward and reverse reactions are balanced.
It's important to note that although the amounts of reactants and products do not change at equilibrium, it does not mean that the reactions have stopped. The forward and reverse reactions continue to occur, but they occur at the same rate, resulting in no net change in the concentrations of reactants and products.