Which of the following best defines the term "reaction rate"?

The term "reaction rate" is best defined as the change in concentration of reactants over time. This definition encapsulates the fundamental aspect of reaction rates, which focuses on how quickly the concentration of the substances involved in the reaction changes as the reaction proceeds.

In essence, it quantifies the speed of a chemical reaction, providing insight into how fast reactants are converted into products. Measuring this change in concentration over a specified time frame allows chemists to assess the efficiency and dynamics of the reaction.

While other choices address important aspects of chemical reactions, they do not capture the full essence of reaction rates. For example, describing the speed at which reactants are consumed does touch upon a related concept, but it doesn't explicitly consider the time factor or changes in concentration. Similarly, the total amount of product formed pertains to the outcome of the reaction rather than the rate at which it occurs, and the time taken for the reaction to reach completion does not necessarily reflect the reaction rate during the process itself. Thus, focusing on the change in concentration of reactants over time offers the most accurate and complete definition of reaction rate.