Table \(12.2\) illustrates how the average rate of a reaction decreases with time. Why does the average rate decrease with time? How does the instantaneous rate of a reaction depend on time? Why are initial rates used by convention?

The average rate of a chemical reaction is the change in concentration of a reactant or product per unit time. It is usually expressed in units of moles per liter per second (M/s). The average rate can decrease with time because reactants are being used up in the reaction, which leads to a decrease in the concentration of reactants, and consequently, a decrease in the rate.

The instantaneous rate of a reaction is the rate at a specific point in time. It is the slope of a tangent line to the curve representing the concentration of a reactant or product as a function of time. The instantaneous rate depends on the time, as it represents the reaction rate at any given moment during the reaction. However, it is important to note that for many reactions, the instantaneous rate will decrease as time progresses due to diminishing reactant concentrations.

Initial rates are used by convention for several reasons: 1. Easier to Measure: It is often easier to measure the reaction rate at the beginning of the reaction before the system becomes too complex due to the formation of products and side reactions. 2. Simplification: By focusing on the initial rate, we can assume that only the reactants are present in the reaction mixture, which simplifies the mathematical treatment and analysis of the data. 3. Rate Laws: Initial rates are essential for determining the reaction order and rate constant by using the rate law. The rate law establishes the relation between reactant concentrations and the reaction rate, which is crucial for understanding the reaction mechanism and predicting the reaction behavior. Overall, understanding the concepts of average and instantaneous rates of reaction and the significance of initial rates allows us to better analyze and predict chemical reaction behavior.