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

The molecularity of a reaction may be the same or different as compared to order, i.e., option (d).

Step 1: Understanding Molecularity

Molecularity refers to the number of reactant molecules that participate in a single elementary step of a reaction. It is always an integer and is determined from the reaction mechanism. Unlike order, molecularity is not determined experimentally. It can be one (unimolecular), two (bimolecular), or three (termolecular) depending on the number of molecules involved in the step.

Step 2: Understanding Reaction Order

Reaction order is determined from the rate equation of the reaction and is obtained through experiments. It represents the sum of the powers of the concentration terms in the rate law. Reaction order can be a fraction, zero, or an integer, and it may not correspond directly to the stoichiometry of the overall or elementary reaction.

Step 3: Comparing Molecularity and Order

While molecularity is a theoretical concept derived from elementary steps, order is an empirical measurement. They can be equal if the rate equation corresponds directly to an elementary reaction. However, because reaction order is determined experimentally and can involve complex reaction mechanisms, it may not always match the molecularity.

Step 4: Identifying the Correct Option

The correct option is determined by understanding that although molecularity and order can be the same, especially in simple reactions, they can also differ because reaction order is not always the same as the stoichiometric coefficients of the elementary steps. Thus, molecularity may be the same as or different from the order of a reaction.

Key Concepts

Elementary reaction

An elementary reaction is the simplest form of a chemical reaction where reactants are transformed directly into products in a single step. This process involves a single molecular event, unlike more complex reactions that might consist of multiple steps forming an overall reaction mechanism. In elementary reactions, molecularity is a key factor.

• Molecularity: This term describes the number of reactant molecules involved in an elementary step. It is always a whole number, such as:
  • Unimolecular - involving one molecule.
  • Bimolecular - involving two molecules.
  • Termolecular - involving three molecules.

The molecularity of a reaction offers insight into the reaction mechanism but differs from the reaction order, which is derived from the rate law.

Reaction order

The reaction order is a concept derived from the rate equation, which represents the relationship between the concentration of reactants and the rate of the reaction. Unlike molecularity, the reaction order is determined empirically through experiments. This involves observing how changing the concentration of reactants affects the reaction rate.

• Rate Equation: The rate equation for a reaction takes the form: \[ ext{Rate} = k [A]^m [B]^n \] Where \(k\) is the rate constant; \([A]\) and \([B]\) are the concentrations of reactants; and \(m\) and \(n\) are the reaction orders with respect to each reactant.
• The overall reaction order is the sum \(m+n\), and it can be an integer or a fraction. It may not necessarily match the stoichiometry of the overall chemical equation or its elementary steps.

Changes in concentration often affect the reaction rate, which helps establish the reaction order essential for predicting how the reaction proceeds under different conditions.

Rate equation

The rate equation is a mathematical representation of the relationship between the concentration of reactants and the rate of the chemical reaction. It is vital for understanding how various factors impact the speed of a reaction.

• Components of the Rate Equation: The main components include the rate of the reaction and the concentrations of the reactants, each raised to a power (known as the order with respect to that reactant).
• This equation is derived through experimental data, allowing chemists to understand the effect of changing concentrations on the reaction rate.
• From the rate equation, the overall order of reaction provides critical insight into kinetic behavior and helps in predicting outcomes under varied conditions.

A deeper understanding of the rate equation can clarify why molecularity and reaction order may or may not align. While the molecularity of each elementary step is a direct count of molecules involved, the rate equation's order might reflect additional complexity in the reaction mechanism.