Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct statements: (a) The order of an elementary reaction is equal to its molecularity (b) The order of a reaction can be zero (c) For second order reaction, order of reaction \(=2 \times\) molecularity. (d) The order of inversion of cane sugar is 2 .

Step-by-Step Solution

Answer

Statements (a) and (b) are correct; (c) and (d) are incorrect.

Step 1: Understand Elementary Reaction and Molecularity

For an elementary reaction, the order of reaction is indeed equal to its molecularity because the stoichiometry of the reaction determines how many molecules or atoms interact in a single step, which defines both the order and molecularity.

Step 2: Order Can Be Zero

In chemical kinetics, a reaction order can indeed be zero. This means that the rate of reaction is independent of the concentration of the reactant(s).

Step 3: Evaluate Statement about Second Order Reaction

For a second order reaction, the order is 2 by definition. However, the molecularity cannot be multiplied by 2 to get the order because molecularity is applicable only for elementary reactions, not composite reactions.

Step 4: Cane Sugar Inversion

The inversion of cane sugar refers to the hydrolysis reaction where the order is actually 1, not 2. It's a first-order reaction.

Key Concepts

Elementary Reactions

Elementary reactions are the simplest types of chemical reactions. They occur in a single step and involve a direct interaction between reactants.
When scientists talk about the order of an elementary reaction, they mean how the concentration of reactants affects the rate of the reaction. Because these reactions are straightforward, the order is equal to the molecularity of the reaction.

• Molecularity refers to the number of molecules coming together to react in a single step.
• For example, if two molecules collide to produce a reaction, the molecularity is 2, and so is the order of the reaction.

Understanding elementary reactions helps us get to the heart of more complex reactions. If we know how individual steps work, we can piece together the bigger picture.

Molecularity

Molecularity is an important concept in understanding chemical reactions. It's all about how many reactant molecules participate in forming the products.
Molecularity can be:

• Unimolecular, where only one reactant molecule is involved (e.g., decomposition).
• Bimolecular, involving two molecules, like in many simple reaction collisions.
• Termolecular, which are rare and involve three reactant molecules.

A key thing to note is that molecularity is only relevant for elementary reactions, while order can refer to overall reactions that comprise several elementary steps. Remember, molecularity is a concept based purely on the stoichiometry of these elementary steps, while reaction order is determined by experimental observation.

Zero Order Reactions

Zero order reactions are quite unique in chemical kinetics. For these reactions, the rate is independent of the concentration of the reactant.
This means even if you change how much reactant you have, the speed of the reaction stays constant.

• The rate equation for a zero order reaction can be expressed as: \ \ rate = k \ \, where \( k \) is the rate constant.
• An example could be a reaction occurring on a surface, where a certain amount of catalyst is available, and increasing or decreasing the concentration of reactant doesn’t affect speed.

Zero order reactions exemplify how varying mechanisms in reactions work and emphasize that not all reactions behave the same way when we tweak concentrations.

Cane Sugar Inversion

The inversion of cane sugar is a classic example studied in chemistry, especially when learning about reaction rates.
This process involves converting sucrose (cane sugar) into simpler sugars known as glucose and fructose through hydrolysis.

• The reaction is catalyzed by acids and is a first-order reaction, despite the common misconception that it is of order 2.
• This means the reaction rate depends linearly on the concentration of sucrose only.

Understanding the inversion of cane sugar helps us see how catalysis and reaction order provide insights into the complexities of biological and chemical processes.