What shows how the concentrations of species in the reaction depend on time?

• A. Endothermic
• B. Integrated rate law
• C. Reaction intermediate
• D. Differential rate law

Answer: (B)

The idea being tested is how concentrations change as time passes in a reaction. The tool for this is the integrated rate law, which comes from integrating the differential rate law with respect to time. It directly gives how concentrations like [A], [B], etc., evolve over time (for example, [A] = [A]0 e^{-kt} for a first-order step, with analogous forms for other orders). In contrast, the differential rate law tells how fast the reaction proceeds in terms of concentrations, not how those concentrations change over time. Endothermic describes heat flow and is about energetics, while a reaction intermediate is a species in the mechanism, not a time-dependent concentration relationship. So the integrated rate law is the correct choice because it explicitly shows concentration as a function of time.