What is defined as the increase or decrease in concentration of one of the products per unit time?

The increase or decrease in concentration of one of the products per unit time refers specifically to how quickly or slowly a reaction occurs. This is termed the reaction rate. It measures the change in concentration of reactants or products over time and is a crucial concept in kinetic chemistry, as it provides an understanding of how fast the reaction is proceeding.

The reaction rate can be expressed numerically, often defined as the change in concentration (usually in molarity) divided by the change in time (usually in seconds). The greater the change in concentration over a smaller time frame, the higher the reaction rate, indicating a faster reaction. This concept is foundational in determining how various factors such as temperature, concentration, and catalysts affect the speed of chemical reactions.

In contrast, the equilibrium constant reflects the ratio of the concentrations of products to reactants at equilibrium and does not describe the rate of a reaction. Activation energy is the minimum energy required to initiate a reaction, and yield refers to the amount of product obtained from a reaction compared to the theoretical amount possible. None of these concepts pertain directly to the rate of a reaction itself, which is why the reaction rate is the appropriate answer for this question.