Which equation expresses the instantaneous rate of reaction as a function of reactant concentrations?

• A. Activation energy
• B. Catalyst
• C. Rate Law
• D. Reaction coordinate diagram

Answer: (C)

The key idea is how the speed of a reaction depends on how much reactant is present. The instantaneous rate is described by the rate law, which gives the rate as a function of concentrations through a rate constant and reaction orders. For a general reaction, the rate can be written as rate = k [A]^m [B]^n, where m and n indicate how sensitive the rate is to each reactant’s concentration. This equation tells you the exact velocity of the reaction at a specific moment given the current concentrations, which is what is meant by the instantaneous rate. Activation energy, while important for how temperature affects the rate, sets the energy barrier and does not express rate as a function of concentrations. A catalyst changes the rate by providing an alternative pathway but does not alter the functional form of the rate law. A reaction coordinate diagram shows energy changes along the reaction path, not an equation linking rate to concentrations. So the rate law is the equation that expresses the instantaneous rate in terms of reactant concentrations.