Rate Determining Step

Deducing the Rate Equation: If the mechanism is known, the rate equation can be written by looking at the reactants in the RDS. For a step $2A + B \rightarrow \text{products}$, the rate equation would be $\text{rate} = k[A]^2[B]$.

Predicting Mechanisms: If the experimental rate equation is $\text{rate} = k[X]$, it suggests that one molecule of $X$ is involved in the RDS. If a reactant $Y$ is present in the overall equation but absent from the rate equation, it must be involved in a fast step after the RDS.

Identifying Intermediates: Intermediates are species produced in one step and consumed in another. If an intermediate is formed in a fast step before the RDS and then reacts in the RDS, the reactants that formed that intermediate will appear in the rate equation.

Check the Orders: Always compare the experimental rate equation to the proposed RDS. If the rate equation is $\text{rate} = k[A]^2$, the RDS must involve two molecules of $A$ colliding, or a species derived from two molecules of $A$.

Zero-Order Reactants: If a reactant is zero-order, it is a major clue that this substance is involved only in steps after the rate-determining step. Do not include it in your proposed RDS mechanism.

Catalyst Identification: If you see a species in the rate equation that is not in the overall chemical equation, it is a catalyst. In a mechanism, look for a species that is a reactant in an early step and a product in a later step.

Stoichiometry Trap: A common mistake is assuming the overall balanced equation's coefficients are the orders of reaction. Remember that orders are determined experimentally and only reflect the stoichiometry of the RDS.

Ignoring Pre-RDS Steps: Students often forget that if the RDS involves an intermediate, the rate equation must be expressed in terms of the original reactants that formed that intermediate. This often leads to higher-order rate equations.

Speed vs. Equilibrium: Do not confuse a slow step with a reversible step. While the RDS is slow, it is the kinetic barrier, not the thermodynamic stability, that defines it.