The mechanism for the gas-phase reaction of nitrogen dioxide...

The mechanism for the gas-phase reaction of nitrogen dioxide with carbon monoxide to form nitric oxide and carbon dioxide is thought to be $$\mathrm{NO}_{2}+\mathrm{NO}_{2} \longrightarrow \mathrm{NO}_{3}+\mathrm{NO} \quad \text { Slow }$$ $$\mathrm{NO}_{3}+\mathrm{CO} \longrightarrow \mathrm{NO}_{2}+\mathrm{CO}_{2} \quad \text { Fast }$$ Write the rate law expected for this mechanism. What is the overall balanced equation for the reaction?

Key Concepts

Overall Reaction Balancing

Overall reaction balancing involves summing the individual steps of the reaction mechanism while canceling any intermediates that appear on both the reactant and product sides. This gives the net stoichiometric equation that represents the complete chemical transformation, independent of the intermediate species that may form during the process.

Rate Law

The rate law expresses the dependence of the reaction rate on the concentration of its reactants. For elementary steps, the rate law follows directly from the stoichiometry. Thus, if the slow (rate-determining) step involves a bimolecular collision between two molecules of a species, the rate is proportional to the square of that species’ concentration.

Rate-Determining Step

The rate-determining step is the slowest elementary step in a multi-step mechanism and controls the overall reaction rate. Its rate law, based on the molecularity of the slow step, directly gives the experimentally observed rate law for the process. In constructing rate laws, we often assume the overall rate is equal to the rate of this slow step.

Elementary Reactions

Elementary reactions are single-step processes with no intermediates where the rate law can be written directly from the stoichiometry of the step. In a multi?step mechanism, each elementary step has its own molecularity and consequently its own rate expression, which is key to building the overall kinetic description of the reaction.

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