What is the difference between a global reaction rate expression and one based on elementary reactions?

A global reaction rate expression is an overall equation that represents the rate of a chemical reaction as a function of the concentration of the reactants, products, and other factors such as temperature and pressure. This expression is usually determined experimentally, and it does not provide any detailed information about the individual steps involved in the reaction.

An elementary reaction rate expression, on the other hand, is an equation that represents the rate of an individual reaction step, known as an elementary reaction. Each elementary reaction has its own rate expression, which describes the relationship between the rate of that step and the concentrations of the species involved in that step. Elementary rate expressions are typically derived from the reaction mechanism and the elementary steps involved in the reaction.

The main differences between a global reaction rate expression and one based on elementary reactions are: 1. Global rate expressions provide an overall description of the reaction rate, while elementary rate expressions describe the rates of individual reaction steps within the reaction mechanism. 2. Global rate expressions are determined experimentally, while elementary rate expressions are derived from the reaction mechanism and the elementary steps. 3. Global rate expressions do not provide detailed information about the reaction mechanism and individual reaction steps, while elementary rate expressions provide insights into the reaction mechanism and individual steps involved. By understanding the differences between global and elementary reaction rate expressions, it becomes clear that they serve different purposes in the study of chemical kinetics. A global reaction rate expression is useful for predicting the overall reaction rate under various conditions, while elementary reaction rate expressions help chemists to understand the detailed mechanism and steps of a reaction, allowing them to optimize reactions and design more efficient catalytic systems.