Question

Predicting Outcomes The balanced equation for a rapid homogeneous reaction between two gases is as follows: \(4 \mathrm{A}+\mathrm{B} \longrightarrow 2 \mathrm{C}+2 \mathrm{D}\) . Because the simultaneous collision of four molecules of one reactant with one molecule of the other reactant is extremely improbable, what would you predict about the nature of the reaction mechanism for this reaction system?

Short answer

The reaction mechanism likely involves multiple elementary steps rather than a single high-order step, due to the improbability of simultaneous collisions of all reactant molecules.

Step-by-step solution

Understand the Given Reaction

The reaction is given as: \[4 \mathrm{A} + \mathrm{B} \rightarrow 2 \mathrm{C} + 2 \mathrm{D}\] Recognize that the balanced equation indicates that four molecules of reactant A react with one molecule of reactant B to form two molecules each of products C and D.

Analyze Collision Probability

Consider the likelihood of all four molecules of A colliding simultaneously with one molecule of B. Such a high-order collision is very improbable due to the low probability of simultaneous collisions.

Infer Reaction Mechanism

Since high-order collisions are improbable, the reaction likely proceeds through a series of simpler, more probable steps. This implies the reaction mechanism involves multiple elementary steps, each with fewer molecules colliding.

Propose Possible Mechanism

A more plausible mechanism might involve intermediate formations where, for example, two A molecules react with B first, followed by the reaction of the resulting intermediate with the remaining A molecules to finally produce C and D. Each elementary step would involve fewer particles interacting.

Describe the Likely Pathway

Summarize the pathway: the reaction likely occurs through consecutive elementary steps rather than a single step involving all five reactant molecules. This multi-step process lowers the complexity and increases the likelihood of each individual step.

Key concepts

Chemical Reactions

A chemical reaction involves the transformation of one or more substances into new substances. In the reaction given \[4 \mathrm{A} + \mathrm{B} \rightarrow 2 \mathrm{C} + 2 \mathrm{D}\], four molecules of 'A' combine with one molecule of 'B' to produce two molecules each of 'C' and 'D'. Reactions often involve breaking bonds in the reactants and forming new ones in the products.
For instance, reactant 'A' might break a bond, allowing it to form a new bond with reactant 'B'. This transformation process is fundamental to understanding chemistry and its myriad applications.
Identifying the products and reactants is the first step in predicting the behavior of a reaction.

Reaction Mechanism

A reaction mechanism outlines the step-by-step sequence of elementary reactions by which overall chemical change occurs. For the reaction \[4 \mathrm{A} + \mathrm{B} \rightarrow 2 \mathrm{C} + 2 \mathrm{D}\], a direct collision involving all five molecules is highly unlikely due to the improbability of such a high-order collision.
Instead, the reaction mechanism shows how a reaction proceeds through a series of simpler steps. These steps are called 'elementary steps', and each involves fewer molecules interacting at a time.
Understanding the reaction mechanism helps chemists to control and optimize reactions in various applications.

Collision Theory

Collision theory explains how and why reactions occur at the molecular level. According to this theory, for a reaction to occur:

• Molecules must collide with enough energy to overcome the activation energy barrier
• The colliding molecules must be in the proper orientation

In the context of the reaction \[4 \mathrm{A} + \mathrm{B} \rightarrow 2 \mathrm{C} + 2 \mathrm{D}\], the simultaneous collision of four 'A' molecules with one 'B' molecule is highly improbable.
Therefore, the reaction is more likely to proceed through a series of collisions involving fewer molecules (elementary steps), making the process more manageable and increasing the likelihood of the reaction occurring.

Elementary Steps

Elementary steps are the individual steps that make up a reaction mechanism. Each step involves a small number of molecules colliding and reacting.
In the example, the improbable high-order collision suggests the reaction happens in smaller steps. One possible sequence could involve two 'A' molecules first reacting with 'B' to form an intermediate. This intermediate could then react with additional 'A' molecules to produce the final products, 'C' and 'D'.
By breaking down the reaction into elementary steps, chemists can better understand and manipulate the reaction process.