How is the rate of reaction defined with respect to product concentration?

• A. Change in reactant concentration over time
• B. Rate equal to product concentration divided by time
• C. Change in product concentration over time
• D. Rate is constant

Answer: (C)

The rate of reaction with respect to product concentration is the rate at which the product builds up over time—the rate of formation of the product. This is the time derivative of the product’s concentration, written as d[P]/dt, and it is positive as the product forms. This makes it the best choice because it directly measures how quickly the product concentration changes.

For example, if the product concentration increases from 0.10 M to 0.12 M in 2 seconds, the rate is (0.12 − 0.10)/2 = 0.01 M/s. The other statements describe different ideas: focusing on the reactant would be a rate of consumption for the reactant, not the product; simply taking [P]/t gives an average value that ignores how the rate changes over time; and saying the rate is constant is generally incorrect because rate often depends on concentrations and can vary during a reaction.