For statements (I) and (II), state for each if they are true or false. (I) At equilibrium, a system is always static. (II) At equilibrium, the concentrations of the reactants and the products are always equal. If both are true, state if (II) is a correct explanation for (I).
Chemical equilibrium is the state with a rate of the forward reaction is equal to the rate of the reverse reaction. Consider the reaction A plus B giving AB. The reverse reaction would be AB decomposing to give A plus B. The graph shows how the rate of the forward reaction decreases fast initially and then more slowly and then remains constant from time 𝑥. The graph also shows how the reverse reaction has no product at time zero and then the reaction commences and increases in rate until a constant rate occurs at time 𝑥.
After time 𝑥, both the forward and reverse reactions continue to react at a reaction rate of 𝑦. The system is not static with A plus B continuing to react to form AB at a rate of 𝑦 and the reverse reaction continues to occur with AB decomposing to give A plus B at a rate of 𝑦. There is constant change in the system. And it’s not static; new products are forming, and reactants are reforming. We call this a dynamic equilibrium because there is movement in the system, although the rates of the forward and reverse reactions are the same and are constant.
So statement (I), at equilibrium, a system is always static, is false. In a static equilibrium, the rates of both reactions are zero. Usually, when we refer to a chemical equilibrium, we are referring to a dynamic equilibrium.
Let’s look at statement (II). The graphs show two different scenarios for the reaction of A plus B giving AB where the reaction is reversible. In the first graph, equilibrium is reached at time 𝑥 and then the concentrations of A, B, and AB remain constant. The same thing occurs after time 𝑥 in graph two. However, the concentration of AB product in graph one at a concentration of 𝑃 is higher than the equilibrium concentration of the reactants at concentration 𝑄. In graph two, the concentration of the products at point 𝑃 is much lower than the concentration of the reactants, point 𝑄, at equilibrium.
So, statement (II), at equilibrium, the concentrations of the reactants and products are always equal, is false. Because statements (I) and (II) are both false, we do not need to consider the last statement.