Problem 92 - Chemistry The Science in Context

Question

The reaction between \(\mathrm{NO}\) and \(\mathrm{H}_{2}\) is second order in \(\mathrm{NO}\). Does this mean that the reaction could occur in just one step?

Step-by-Step Solution

Answer

Answer: It seems unlikely that the reaction could occur in just one step, as the molecularity of the reaction would need to be 3, whereas the reaction is second order in NO. However, experimental evidence should be considered to confirm the reaction mechanism.

Step 1: Analyze the given reaction

The given reaction is between NO and H2. We need to determine the molecularity of the reaction to see if the reaction could occur in just one step.

Step 2: Examine the order of the reaction

The reaction is second order in NO. This means that the rate of the reaction is directly proportional to the square of the concentration of NO. Mathematically, it can be written as: Rate = k[NO]^2

Step 3: Investigate the molecularity of the reaction

Molecularity is the number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction. Molecularity determines the number of steps a reaction could take to proceed. If the reaction occurs in one step, it means the molecularity and the order of the reaction are the same. For this reaction, if it occurs in one step, then we should expect two NO molecules to collide with each other and a H2 molecule. On the other hand, if it requires multiple steps, it means that one or more intermediate reactions are taking place to achieve the overall second order in NO.

Step 4: Compare reaction order and molecularity

The reaction is second order in NO. This tells us that in a single step, the reaction could involve the collision of two NO molecules and a H2 molecule. Since there are three reacting species here, the molecularity of the reaction would be 3, not 2. Therefore, based on this understanding, it seems unlikely that the reaction could occur in just one step. However, this conclusion should be corroborated by experimental evidence, which can give us more insight into the reaction mechanism (if intermediates exist) and provide definitive proof if the reaction indeed occurs in more than one step.

Key Concepts

Chemical Kinetics

Chemical kinetics is the study of the speed or rate at which chemical reactions occur. It is a critical field of chemistry because it helps us understand the factors that affect reaction rates, such as temperature, concentration, and the presence of catalysts.

When you think about chemical reactions, imagine a dance of atoms and molecules, moving and colliding. Just like in a busy dance hall, the speed at which dancers meet can depend on how many are on the floor or how fast the music is. Similarly, in chemical kinetics, we examine how the concentration of reactants and the conditions they are in influence the pace at which they react to form products. A fundamental understanding of kinetics can even help us control these reactions, making them faster or slower according to our needs.

Reaction Mechanisms

The process by which reactants are converted to products is known as the reaction mechanism. It provides a step-by-step description of each stage in a reaction, highlighting how bonds break and form during the course of the reaction.

Think of it as a recipe, detailing each ingredient and instruction to transform your raw ingredients into a delicious meal. In chemical reactions, the 'recipe' could be simple, with everything happening in one quick step, or complex, involving multiple intermediate reactions before reaching the final product. The mechanism gives us a clue about the number of intermediates and transition states in a reaction, which is crucial for understanding how it happens.

Molecularity of Reaction

In chemical kinetics, molecularity refers to the number of reacting molecular entities involved in an elementary step of a reaction mechanism. It is always an integer and applies only to elementary reactions, which are reactions that occur in a single step.

For example, a bimolecular reaction involves two reacting entities, while a termolecular reaction involves three. A basic understanding of molecularity helps us predict how reactants might come together. It is important to note that the molecularity of a reaction strictly refers to its single steps and not the overall order of a complex reaction, which can involve multiple sequential steps.

Rate Laws

Rate laws, or rate equations, are mathematical expressions that describe the relationship between the rate of a reaction and the concentration of reactants. They often take the form of a simple equation: Rate = k[Reactant]^n, where 'k' is the rate constant, '[Reactant]' is the concentration of reactant, and 'n' is the reaction order with respect to that reactant.

This equation is like a magic formula that allows chemists to predict how quickly a reaction will occur under different conditions. The value of 'n' can tell us if the reaction rate is influenced by the concentration of the reactants (n > 0), unaffected (n=0), or inhibited by the reactants (n < 0). Rate laws are vital in designing chemical processes and ensuring safety in laboratories and industry.