Reaction rate is defined as

• A. The change in concentration of a reactant or product over time.
• B. The final concentration of product.
• C. The rate of energy release.
• D. The number of collisions per second.

Answer: (A)

Reaction rate tells you how fast a reaction proceeds, quantified as the change in concentration of a reactant or product per unit time. This definition captures both how quickly reactants are consumed and how quickly products are formed, using time as the measure of speed. For a general reaction aA → bB, the instantaneous rate can be written as rate = - (1/a) d[A]/dt = (1/b) d[B]/dt, so the same rate is obtained whether you track the decrease in A or the increase in B (normalized by their stoichiometric factors). The final amount of product, or the total energy released, do not describe how fast the system changes, and the number of collisions per second is related to factors that influence rate but does not itself define it.