What defines the rate of reaction?

The rate of reaction is defined as the change in concentration of reactants or products over a specific period of time. This definition emphasizes that the rate can be quantified by measuring how quickly either the reactants are consumed or the products are formed in a chemical reaction. By concentrating on the change in concentration, it becomes possible to evaluate the speed of the reaction under various conditions, facilitating comparisons across different reactions or the same reaction under different circumstances.

The total pressure of the system does influence reaction rates, especially in gaseous reactions or when dealing with equilibrium, but it does not define the rate itself. Change in temperature over time may affect the overall rate but is not a direct measure of how fast a reaction occurs—it is a condition influencing the rate rather than a definition of it. The number of moles remaining at equilibrium indicates the state of the reaction at a specific point but does not provide insight into the dynamics of the reaction, particularly during the initial phases when reacting species are changing rapidly. Thus, focusing on concentration changes per unit time provides a clear and direct method for defining the reaction rate.