Which of the following statements is true of a reversible reaction at dynamic
equilibrium? (A) The concentration of reactants is decreasing. (B) The concentration of products is increasing. (C) The rate of the backward reaction is less than the rate of the forward
reaction. (D) The rate of the forward reaction is less than the rate of the backward
reaction. Or (E) the rate of the forward reaction is equal to the rate of the backward
In this question, we want to know what is true when describing a reversible reaction
at dynamic equilibrium.
A reversible reaction is a reaction that proceeds in both directions. Let’s look at a general equation where A and B react reversibly to form C and D. We can see two half arrows indicating that the chemical reaction is reversible. In the forward reaction, A and B react to produce C and D. In the backward reaction, C and D react to produce A and B.
In a reversible reaction, there is a point at which the concentrations of the
reactants and the products no longer change. This does not mean the reaction has stopped but that the reaction has reached
equilibrium. This is what we mean by dynamic equilibrium. A dynamic equilibrium is an equilibrium between forward and backward reactions that
occur at the same nonzero rate. Both reactions continue to occur at the same rate, with both products and reactants
If we look at our answer choices, we know that answer choices (A) and (B) must be
incorrect, since at dynamic equilibrium the concentrations of both reactants and
products have stopped changing. We can also eliminate answer choices (C) and (D), since dynamic equilibrium is
reached when the rates of both the forward and backward reaction are equal.
Therefore, the statement that is true of a reversible reaction at dynamic equilibrium
is answer choice (E). The rate of the forward reaction is equal to the rate of the backward reaction.