Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is incorrect? (1) Catalyst undergoes permanent chemical change. (2) Particle size of solute in true solutions is \(10^{-3} \mathrm{~m}\). (3) Starch solution is a hydrosol. (4) Hydrolysis of liquid ester in the presence of mineral acid is an example of heterogeneous catalysis reactions. (a) 1,2 and 3 (b) 2 and 3 (c) 1,2 and 4 (d) 2,3 and 4

Step-by-Step Solution

Answer

The incorrect statements are 1, 2, and 4. Correct option is (c).

Step 1: Understanding Catalysts

Catalysts are substances that increase the rate of a chemical reaction without themselves undergoing any permanent chemical change. Hence, statement (1) 'Catalyst undergoes permanent chemical change' is incorrect.

Step 2: Understanding Particle Size in True Solutions

True solutions have solute particles that are typically less than 1 nanometer (our reference unit is in meters, so this is less than \(10^{-9} \mathrm{~m}\)), which means statement (2) 'Particle size of solute in true solutions is \(10^{-3} \mathrm{~m}\)' is incorrect.

Step 3: Understanding Hydrosols

A hydrosol is a type of colloidal solution where the dispersion medium is water. A common example is a starch solution. Thus, statement (3) 'Starch solution is a hydrosol' is correct.

Step 4: Classifying the Ester Hydrolysis Reaction

Hydrolysis of a liquid ester in the presence of a mineral acid is an example of homogeneous catalysis because both the reactants and the catalyst are in the same phase. Therefore, statement (4) 'Hydrolysis of liquid ester in the presence of mineral acid is an example of heterogeneous catalysis reactions' is incorrect.

Key Concepts

Catalysts

Catalysts are fascinating elements in chemistry that significantly change how reactions proceed. They have a crucial role in increasing the speed of chemical reactions, but interestingly, they do so without undergoing any permanent change themselves. This means that after the reaction concludes, the catalyst remains chemically unchanged and can be used again.
One of the most incredible aspects of catalysts is their ability to lower the activation energy needed for a reaction to occur. By doing this, they make it easier for reactants to convert into products, speeding up the process without being consumed or altered. Catalysts can be found in various forms, including metals, enzymes, and small organic molecules.
It's important to not confuse catalysts with the reactions they assist. They don't start reactions nor increase the product yield; they just make it happen faster.

True Solutions

True solutions are homogeneous mixtures of two or more substances. In these solutions, the solute is completely dissolved in the solvent, forming a clear solution. A defining characteristic of true solutions is the size of the particles involved.

• The solute particles in a true solution are extremely tiny, typically less than 1 nanometer or less than \(10^{-9}\) meters in size.
• This minuscule size allows them to be uniformly distributed throughout the solution without settling out or being filtered easily.

Due to this tiny size, true solutions do not scatter light, making them appear transparent. This differs significantly from colloids, where larger particle sizes can cause scattering.

Homogeneous Catalysis

Homogeneous catalysis is a process where the catalyst operates in the same phase as the reactants. This phase can be liquid, gas, or even a solid solution, but the key point is that the catalyst and the reactants are well mixed at the molecular level.
This type of catalysis has several advantages:

• It allows for a uniform reaction environment, which can lead to more predictable and controlled reaction rates.
• The catalyst can efficiently interact with the reactants since they are in the same phase, enhancing the overall reaction rate.

A classic example of homogeneous catalysis is the acid-catalyzed hydrolysis of an ester, where both the ester and the acid are in a liquid phase, making the reaction smooth and efficient.

Particle Size in Solutions

Particle size plays a crucial role in defining the properties and behaviors of solutions. In chemistry, understanding the distinctions between different types of mixtures largely depends on the size of the particles involved.
For true solutions, as mentioned earlier, the particle size is minuscule, typically less than \(10^{-9}\) meters. In contrast, colloidal solutions or colloids have larger particles, ranging from 1 nanometer to 1 micrometer.
The size impacts whether a mixture appears homogeneous or heterogeneous. True solutions with their tiny particles are typically homogeneous, while mixtures with larger particles scatter light and may appear cloudy, indicating a heterogeneous nature. It's fascinating how something as small as a particle's size can dramatically alter the properties of a mixture!