Final answer:
A reversible reaction is at equilibrium when the forward and reverse reaction rates are equal, indicating a dynamic equilibrium with constant reactant and product concentrations despite ongoing reactions in both directions.
Step-by-step explanation:
If a reaction is reversible, it can be said to have reached equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction.
This means we are in a state of dynamic equilibrium, where the concentrations of the reactants and products remain constant over time because they are consumed and re-formed at the same rate.
Before the equilibrium is reached, this would be considered part of the pre-equilibrium phase where the rate of the reverse reaction is generally slower than the rate of the forward reaction.
So, the correct answer to the question is (a) When the forward reaction rate equals the reverse reaction rate. At equilibrium, we observe that the reaction is in a dynamic state where no observable changes in concentration occur, even though reactions continue to happen in both directions.
A chemical equation of a reaction at equilibrium is often denoted using a double-headed arrow, indicating the reversible nature of the process.