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Why does the balanced chemical equation not always predict a reaction's rate law?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

A balanced chemical equation does not always predict a reaction's rate law because the overall equation often represents the net result of a complex, multi-step process rather than a single elementary step.12

While a balanced equation accounts for the stoichiometry of the reactants and products, the actual rate of a reaction is determined by its reaction mechanism—the specific sequence of individual elementary steps by which the transformation occurs. In many cases, these reactions proceed through a series of steps where one step is significantly slower than the others. This "rate-determining step" dictates the overall speed of the reaction, and the rate law is derived from the concentrations of the species involved in that specific step (or steps leading up to it), rather than the stoichiometric coefficients of the overall balanced equation.312

For example, in the reaction $H_{2} + 2 ICl \to I_{2} + 2 HCl$ , the stoichiometry might suggest a specific dependency, but experimental evidence shows the rate depends on the concentrations of $H_{2}$ and $ICl$ to the first power, reflecting a stepwise mechanism rather than a single-step collision.2