Final answer:
The correct statement for a reaction at equilibrium is that the state of equilibrium can be expressed as a ratio, as described by the Law of Mass Action. Equilibrium is when the forward and reverse reaction rates are the same, leading to constant concentrations of substances. Thus, the correct answer is option d.
Step-by-step explanation:
The statement that applies to a reaction at equilibrium is The state of the equilibrium can be expressed as a ratio. This is because equilibrium is achieved when the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products over time. The Law of Mass Action allows us to express the state of equilibrium in the form of an equilibrium constant (K), which is the ratio of the concentration of products to the concentration of reactants, each raised to the power of their coefficients in the balanced chemical equation. The activation energy required to reach the transition state exists at a higher energy level than the reactants, and this energy is always positive, whether the reaction is exergonic (energy-releasing) or endergonic (energy-requiring).