What's the difference between elementary and composite reactions?

Elementary reactions occur in a single step, while composite reactions involve multiple sequential steps.

Elementary reactions are the simplest type of chemical reactions. They occur in a single step, with no intermediate products formed. This means that the reactants are directly converted into the products. The rate of an elementary reaction depends on the concentration of the reactants. For example, the reaction between hydrogen and iodine to form hydrogen iodide is an elementary reaction. It occurs in a single step, with hydrogen and iodine molecules colliding to form hydrogen iodide.

On the other hand, composite reactions, also known as complex reactions, involve multiple sequential steps. Each step is an elementary reaction itself, and the overall reaction is the sum of these steps. Composite reactions often involve the formation of intermediate products, which are formed in one step and consumed in another. The rate of a composite reaction is determined by the slowest step, known as the rate-determining step. An example of a composite reaction is the reaction between nitrogen dioxide and carbon monoxide to form nitrogen monoxide and carbon dioxide. This reaction occurs in two steps, with nitrogen dioxide molecules colliding to form nitrogen tetroxide and oxygen in the first step, and nitrogen tetroxide reacting with carbon monoxide to form nitrogen monoxide and carbon dioxide in the second step.

In summary, the key difference between elementary and composite reactions lies in the number of steps involved. Elementary reactions occur in a single step, while composite reactions involve multiple steps. Understanding this difference is crucial for predicting the rate of a reaction and the mechanism by which it occurs.