The reaction 2 NO+2 H2 ⟶N2+2 H2 O is second order in [NO] and first order in [H2] . A threestep

step 1 This question describes a mechanism for the chemical reaction of 2 moles nitrogen monoxide with

The reaction 2 NO+2 H2 ⟶N2+2 H2 O is second order in [NO]...

The reaction $2 \mathrm{NO}+2 \mathrm{H}_{2} \longrightarrow \mathrm{N}_{2}+2 \mathrm{H}_{2} \mathrm{O}$ is second order in $[\mathrm{NO}]$ and first order in $\left[\mathrm{H}_{2}\right] .$ A threestep mechanism has been proposed. The first, fast step is the elementary process given as equation (14.25) on page 632. The third step, also fast, is $\mathrm{N}_{2} \mathrm{O}+$ $\mathrm{H}_{2} \longrightarrow \mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O} .$ Propose an entire three-step mechanism, and show that it conforms to the experimentally determined reaction order.

Key Concepts

Reaction Intermediates and Approximations

Reaction intermediates are species that are formed and consumed during a reaction mechanism but do not appear in the overall balanced equation. Since their concentrations are often very low, approximations such as the steady-state approximation (SSA) are employed to simplify the kinetic analysis. SSA assumes that the concentration of intermediates remains constant over the course of the reaction, which allows for the derivation of a rate law that correlates with experimental observations.

Consistency with Experimental Rate Law

A key aspect of validating a proposed reaction mechanism is ensuring that when combining the kinetics of each elementary step, the resulting overall rate law reflects the observed reaction order with respect to each reactant. This involves mathematical derivations using the rate laws of the elementary steps, identifying the RDS, and applying the steady-state approximation, which collectively ensure that the proposed mechanism is kinetically consistent with experimental data.

Elementary Reaction Steps

Elementary steps are single, indivisible processes that occur in a reaction mechanism. Each elementary reaction occurs with its own specific molecularity and rate law, which directly reflects the stoichiometry of the step. Understanding these individual steps is essential for constructing a mechanism that accurately reproduces the overall rate law observed in experiments.

Reaction Mechanism

A reaction mechanism is a detailed, stepwise description of the series of elementary processes that occur from reactants to products. It must account for the overall reaction stoichiometry and provide insight into the formation and consumption of transient species, known as intermediates. This mechanism helps in understanding how individual steps, each with their own rate laws, combine to produce the experimentally observed kinetics.

Rate-Determining Step

The rate-determining step (RDS) is the slowest elementary reaction in a mechanism and thus controls the overall reaction rate. The kinetics of the RDS usually dictate the experimentally observed reaction order, even though the overall reaction may involve several steps. Identifying the RDS is critical for linking the mechanism to the observed rate law.

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