What is the rate of a reaction?

• A. The speed at which reactants are converted into products.
• B. The total energy released by the reaction.
• C. The number of reactant molecules involved.
• D. The volume of products formed per unit time.

Answer: (A)

Rate of reaction is the speed at which reactants are converted into products. It’s measured by how quickly the concentration of reactants falls or the concentration of products rises over time, commonly expressed in units like mol/L per second. For gases, you can also describe it as the volume of gas formed per unit time, but the fundamental idea remains—the faster the reactants are turned into products, the higher the rate.

Energy released describes how much energy changes during the reaction, not how fast that change happens. The number of reactant molecules involved is just the amount, not the speed of transformation. Expressing a rate as volume per time is a valid way to quantify rate for gases, but it is a description of the measurement, not the definition of rate itself.