Question

The oxidation of \(\mathrm{NO}\) to \(\mathrm{NO}_{2}\), one of the reactions in the production of \(\mathrm{smog}\), appears to involve carbon monoxide. A possible mechanism is $$ \begin{aligned} \mathrm{CO}+\cdot \mathrm{OH} & \longrightarrow \mathrm{CO}_{2}+\mathrm{H}^{\cdot} \\ \mathrm{H} \cdot+\mathrm{O}_{2} & \longrightarrow \mathrm{HOO} \\ \mathrm{HOO} \cdot+\mathrm{NO} \longrightarrow & \mathrm{OH}+\mathrm{NO}_{2} \end{aligned} $$

Short answer

The mechanism involves creating and then consuming the hydroxyl radical, OH, and the hydroperoxyl radical, HOO, suggesting that CO acts as a catalyst.

Step-by-step solution

Identify the Reaction Steps

Understand that the mechanism is composed of a sequence of elementary steps. Each step shows how molecules and radicals (species with unpaired electrons) interact to proceed to the next step.

Identify Intermediates and Catalysts

Determine which species are intermediates (species that are produced and then consumed, without appearing in the overall reaction). Also, check if there are catalysts – substances that speed up a reaction without being consumed.

Write the Overall Balanced Reaction

Add up all the steps of the mechanism ensuring that intermediates do not appear in the final balanced equation. The catalyst may appear in the beginning and end but does not change through the reaction.

Verify the Mechanism with the Experimentally Determined Rate Law

Ensure the proposed mechanism makes sense with the kinetics of the reaction. This often requires comparing the rate law predicted by the mechanism with one determined experimentally. This step is theoretical and would require experimental data, which has not been provided in this exercise.

Key concepts

Elementary Steps

The term 'elementary steps' refers to the individual stages in a reaction mechanism that, as a whole, describe the sequence of events at the molecular level leading to the final product of a chemical reaction. It's crucial to recognize that an overall chemical equation often summarizes a complex set of moves between reactants that occur in a series of elementary steps. Each of these steps involves the direct transformation of one set of reactant molecules into an immediate set of product molecules. An analogy would be to think of it as a dance routine, where each move (elementary step) combines to form an entire performance (the full reaction).

Understanding elementary steps is fundamental in dissecting how a reaction progresses and identifying how molecules interact and rearrange during a chemical change. A key point here is that each elementary step should obey the laws of conservation of mass and charge just as the overall balanced equation does.

Reaction Intermediates

Imagine being a detective piecing together clues to solve a mystery. 'Reaction intermediates' are like the hidden clues in the story of a chemical reaction. These are the species that are formed in one step of a mechanism and consumed in a subsequent step. They're crucial to the progression of the reaction, but you won't find them in the initial reactants or the final products, similar to a spy who leaves no trace behind.

Understanding the role of intermediates helps chemists manipulate reactions to synthesize new compounds or improve yields. They can be quite reactive and short-lived, hence they rarely accumulate during the reaction. It's important to correctly identify these intermediates when analyzing a reaction mechanism, as they often provide insight into the pathway and dynamics of a chemical reaction.

Rate Law

If you've ever followed a recipe, you've probably noticed that some ingredients impact the flavor more than others. Similarly, in chemistry, the 'rate law' is a mathematical equation that expresses how the rate of a chemical reaction changes in response to the concentration of reactants. It indicates which reactants are the 'key ingredients' that have the most influence on how fast the reaction happens.

In the rate law, the reaction rate is proportional to the product of the reactants' concentrations, each raised to a power. These powers, or 'reaction orders', are essential clues to the reaction mechanism. They often reveal which elementary steps control the speed of the overall reaction – the so-called rate-determining step. It's paramount for students to grasp that the rate law is not derived from the overall balanced equation, but from the mechanism and, especially, the slowest step of that mechanism.

Chemical Kinetics

The process of cooking illustrates 'chemical kinetics' quite fittingly. When you adjust the stove's heat, you change the rate at which your meal cooks. Similarly, chemical kinetics is the field of study that focuses on the speed (or rate) of a chemical reaction and the factors that affect this speed. Within this domain, scientists investigate how different conditions such as temperature, pressure, concentration, and the presence of a catalyst can change the way chemical reactions proceed over time.

Understanding chemical kinetics is essential for controlling processes such as the synthesis of chemicals, the degradation of pollutants, or even baking a cake to perfection. The kinetics of a reaction provide essential information about its mechanism, which in turn can help improve industrial processes, develop new technologies, and understand natural phenomena on a molecular level. This branch of chemistry is not only fascinating but also incredibly practical, impacting numerous fields from material science to biochemistry.