Which statement is true about rate laws for non-elementary steps?

• A. They always match stoichiometric coefficients
• B. They can differ from stoichiometric coefficients
• C. They are undefined
• D. They must be integers

Answer: (B)

Think about how a rate law is determined. For an elementary step, the rate depends directly on the concentrations of the reacting species raised to powers equal to how many molecules collide in that step (the molecularity). But a non-elementary step is a lumped representation of several fast and slow elementary steps. The overall rate is then controlled by the actual mechanism, including any fast equilibria and intermediates, not by the simple coefficients you see in the balanced equation for the overall step.

Because the rate law hinges on the detailed sequence of elementary events, it need not mirror the stoichiometric coefficients of the overall equation. The observed reaction order with respect to each reactant can be different from those coefficients, and you can even get fractional or unusual orders depending on the mechanism and any equilibria or steady-state considerations involved.

A classic illustration is when a fast pre-equilibrium forms an intermediate, and a subsequent slower step governs the rate. The rate law ends up in terms of the intermediate, which, through the equilibrium relationship, becomes a function of the initial reactants. This can yield a rate law like rate = k [A][B] rather than something that directly matches the overall stoichiometry, demonstrating that non-elementary steps can produce rate laws that differ from stoichiometric coefficients.