Final answer:
The rate law for complex reactions cannot be determined directly from the balanced equation. The rate law for elementary reactions can be derived from their balanced equations. The overall rate law for complex reactions can only be determined through experimental data.
Step-by-step explanation:
The rate law for complex reactions cannot be determined directly from the balanced chemical equation for the overall reaction. This is because the overall reaction is a representation of the chemical change that takes place, whereas elementary reactions are explicit representations of the bond-breaking and bond-making events that occur. The rate law for an elementary reaction can be derived directly from its balanced chemical equation. For example, for a unimolecular elementary reaction, the rate law is rate = k[A].
In contrast, the rate law for a complex reaction, which is a combination of multiple elementary reactions, can only be determined through experimental data. This is because the rate of a complex reaction depends on the rate of each elementary step and their respective concentrations. By studying the experimental data, the overall rate law for the complex reaction can be determined.
Therefore, when calculating the rate law for complex reactions, we consider the elementary reactions to be in equilibrium because their rate laws can be derived directly from their balanced chemical equations, whereas the overall reaction rate law cannot be determined solely from the balanced equation.