Concept:Chemical equilibrium is a dynamic state in which the forward and backward reaction rates are equal, so the concentrations of reactants and products remain constant over time.
Explanation:In a reversible reaction, the forward reaction converts reactants into products, while the backward reaction converts products back into reactants.
At equilibrium, both reactions continue to occur, but at exactly the same speed, so there is no net change in the amounts of reactants or products.
Equilibrium does not mean that the amounts of reactants and products are equal — only that their concentrations stay constant.
It also does not mean the reaction has gone to completion, because both reactants and products are still present in the mixture.
Reactions do not cease at equilibrium; the process is dynamic, with the forward and backward changes balancing each other.
Hence, the defining condition of a reaction at equilibrium is that the forward and backward reaction rates become equal.
Answer:C. The rates of the forward and backward reactions are equal.