How do rate expressions differ for parallel reactions?

Rate expressions for parallel reactions differ because each reaction has its own unique rate equation and rate constant.

In a system where multiple reactions, known as parallel or competitive reactions, occur simultaneously, each reaction proceeds at its own rate. This is because each reaction has its own unique rate equation, which is determined by the reaction mechanism and the order of the reaction. The rate equation shows the relationship between the rate of the reaction and the concentrations of the reactants. It typically has the form rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of the reactants, and m and n are the orders of the reaction with respect to A and B.

The rate constant, k, is another factor that differentiates the rate expressions for parallel reactions. The rate constant is a proportionality constant in the rate equation that is specific to a particular reaction at a given temperature. It reflects the likelihood of a reaction occurring when the reactants collide. Different reactions have different rate constants because they have different activation energies and reaction mechanisms.

In parallel reactions, it's also important to consider the concept of reaction selectivity, which refers to the preference for one reaction pathway over another. The selectivity of a reaction can be influenced by factors such as the relative rates of the parallel reactions, the concentrations of the reactants, and the reaction conditions. The rate expressions for the parallel reactions can be used to predict the selectivity of the reaction under different conditions.

In summary, the rate expressions for parallel reactions differ in terms of their rate equations and rate constants. These differences reflect the unique characteristics of each reaction, including its reaction mechanism, order, and activation energy. Understanding these differences is crucial for predicting the behaviour of systems involving parallel reactions.