Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Higher order \((>3)\) reactions are rare due to : [Main 2015] (a) shifting of equilibrium towards reactants due to elastic collisions (b) loss of active species on collision (c) low probability of simultaneous collision of all the reacting species (d) increase in entropy and activation energy as more molecules are involved

Step-by-Step Solution

Answer

Option (c): Low probability of simultaneous collision of all the reacting species.

Step 1: Understanding Reaction Order

The order of a chemical reaction is defined as the sum of the powers of the concentration terms in the rate equation. A higher order reaction refers to reactions that have an order greater than 3, implying multiple reactants are involved in a single step.

Step 2: Evaluating Simultaneous Collision

For a chemical reaction to occur, molecules must collide with enough energy and proper orientation. As the order of the reaction exceeds 3, the complexity increases because it's less likely for so many molecules to collide simultaneously with the precise energy and orientation needed for a reaction.

Step 3: Analyzing Options

Among the given options, (a) refers to equilibrium issues that are not directly about reaction order, (b) concerns the loss of species which is a general issue but not exclusive to high order, (c) directly addresses the simultaneous collision challenge faced by high-order reactions, and (d) involves thermodynamic aspects which can be relevant but are not as specific to why high order reactions are rare.

Step 4: Selecting the Correct Option

Based on the explanation, option (c) 'low probability of simultaneous collision of all the reacting species' correctly identifies the main reason why higher order reactions are rare. Simultaneous collisions of more than three molecules with the correct energy and orientation are statistically improbable.

Key Concepts

Higher Order Reactions

Higher-order reactions, which are reactions with an order greater than three, are quite rare in chemical processes. This is mainly because they involve the interaction of multiple reactant molecules in a single mechanistic step. When a reaction is of higher order, it includes more complex interactions among molecules. This implies that at least four or more molecules must come together simultaneously. Higher-order reactions are less common because:

• They require all involved reactants to collide precisely at once, which is highly unlikely.
• Such reactions are sensitive to conditions like concentration and temperature, which can affect reaction rate drastically.
• The likelihood of a successful interaction decreases significantly as more reactants are involved.

These characteristics make higher order reactions less frequent compared to lower-order ones like first or second-order reactions.

Simultaneous Collision

Simultaneous collisions are improbable in chemical kinetics, especially as the number of participating molecules increases. For a reaction to occur, molecules must collide with each other with sufficient energy and proper orientation. As the complexity of a reaction grows, with a high number of molecules involved, the chances of these simultaneous conditions being met decreases. In higher-order reactions, more than three molecules need to meet and collide effectively in a single step, making these reactions statistically rare. This situation leads to:

• A low probability of multiple successful collisions occurring at once.
• The need for precise alignment of reactants, which adds to the difficulty.
• An increase in the amount of energy required to initiate the reaction.

Thus, the simultaneous collision requirement largely explains why reactions of higher order are not typically observed.

Chemical Kinetics

Chemical kinetics is the study of the speed or rate at which chemical reactions occur and the factors affecting them. Understanding chemical kinetics is crucial because it helps explain how different variables, such as temperature and concentration, influence the rate of chemical reactions. In the context of higher-order reactions, chemical kinetics provides insight into:

• How reaction mechanisms proceed with multiple reactants.
• The impact of various factors on the rate of complex reactions.
• The mathematical expressions, like rate equations, that describe the kinetics of these reactions.

By exploring these elements, students can comprehend why certain reactions, especially higher-order ones, are inefficient under typical conditions.

Rate Equation

The rate equation is a fundamental concept in chemical kinetics that quantitatively describes the relationship between the rate of a reaction and the concentrations of its reactants. For a given reaction, the rate equation has the general form:\[ rate = k [A]^m [B]^n ... \]Here, \([A]\) and \([B]\) are concentrations of the reactants, and \(m\) and \(n\) are the reaction orders with respect to each reactant. The total reaction order is the sum of \(m, n,\) etc.In higher-order reactions, the rate equation becomes more complex because:

• It involves a larger number of concentration terms.
• The reaction orders add up to more than three, reflecting the extensive interaction of reactants.
• Understanding and measuring all necessary parameters becomes challenging.

These complexities in crafting and interpreting rate equations highlight the rarity and intricacy of higher-order reactions.