Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

The molecularity of a reaction is (a) always two (b) same as its order (c) different than the other (d) may be same or different as compared to order

Step-by-Step Solution

Answer

(d) may be same or different as compared to order.

Step 1: Understanding Molecularity

Molecularity refers to the number of molecules that participate in a single step of a reaction mechanism. It can be unimolecular, bimolecular, or termolecular, depending on if one, two, or three molecules are involved in the reaction step, respectively.

Step 2: Understanding Reaction Order

The order of a reaction is the sum of the powers to which all reactant concentrations are raised in the rate law expression. Reaction order is determined experimentally and can be zero, a whole number, or even a fraction.

Step 3: Comparing Molecularity and Order

While molecularity is always a small integer (1, 2, or 3), the order of a reaction is not restricted to these values. Molecularity applies only to individual elementary reactions. Reaction order can be the same or different from the molecularity, depending on the reaction mechanism.

Step 4: Analyzing Options

Evaluate each option: (a) "always two" is incorrect as molecularity can be 1, 2, or 3. (b) "same as its order" is incorrect; they might but do not have to match. (c) "different than the other" is incorrect; they might be the same. (d) "may be same or different as compared to order" is correct given the variability in comparison between molecularity and reaction order.

Key Concepts

Molecularity

Molecularity is a term used in chemistry to describe the number of molecules or atoms that come together in a single elementary reaction step. Think of molecularity as a snapshot of a particular instant when reactants collide and react. It is only applicable to elementary reactions, not complex reactions. The possible values for molecularity are quite limited, as it can only be a small whole number:

• **Unimolecular** reactions involve a single molecule undergoing a transformation. An example of this is the decomposition of a single molecule into two products.
• **Bimolecular** reactions involve the interaction of two molecules. Common examples include simple chemical reactions where two reactant molecules collide to form a product.
• **Termolecular** reactions, though rare, involve three molecules colliding simultaneously to cause a reaction. This scenario is less common due to the low likelihood of three molecules meeting at the same time.

Molecularity is straightforward since it can only be an integer: 1 (unimolecular), 2 (bimolecular), or 3 (termolecular). It describes the actual event in terms of reactants participating directly and cannot be fractional or zero.

Reaction Order

Unlike molecularity, which pertains to a single elementary step, reaction order provides an overview of the entire reaction mechanism. It is the sum of the exponents to which each concentration term is raised in the rate law expression of the reaction. Here are some key points about reaction order:

• It is determined experimentally, based on how changes in reactant concentrations affect the reaction rate.
• The order can be zero, a whole number, or a fraction, reflecting different dependencies of the rate on reactant concentrations.
• The reaction order is not limited to the number of molecules involved and could involve more complex dependencies than molecularity.

Reaction order can be used to predict how different concentrations affect reaction speed. For example, a first-order reaction means the rate is directly proportional to the concentration of one reactant. Understanding the reaction order helps in optimizing reaction conditions and predicting reaction behavior.

Reaction Mechanism

A reaction mechanism is a detailed step-by-step description of how a reaction occurs at the molecular level. It breaks down the overall reaction into a series of elementary steps, each characterized by its molecularity. The complete mechanism provides insight into the pathways and intermediates that lead to the final products.
A few aspects to consider about reaction mechanisms include:

• Each step in the mechanism has its own molecularity, and the sequence of steps governs the overall rate and order of the reaction.
• The slowest step, known as the rate-determining step, often dictates the reaction's rate law and thus the reaction order.
• Understanding the mechanism helps in identifying catalysts or inhibitors, as well as proposing alterations to improve reaction efficiency.

In summary, while molecularity refers to individual reaction steps, reaction mechanism presents the broader picture of how those steps collaborate to facilitate the transformation from reactants to products. Knowing the mechanism allows chemists to manipulate conditions to favor desired outcomes, making it a powerful tool in chemical research and industry.