Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Potassium stearate is obtained by the saponification of an oil or fat. It has the formula \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}-\mathrm{COO}^{-} \mathrm{K}^{+} .\)The molecule has a lyopho- bic end \(\left(\mathrm{CH}_{3}\right)\) and a lyophilic and \(\mathrm{COO}^{-} \mathrm{K}^{+}\). Potassium stearate is an example of (1) lyophobic colloid (2) associated colloid or micelle (3) multimolecular colloid (4) macromolecular colloid (a) 2 only (b) 1 and 2 (c) 3 and 4 (d) 4 only

Step-by-Step Solution

Answer

The correct answer is (a) 2 only: associated colloid or micelle.

Step 1: Understand Lyophilic and Lyophobic Parts

In the molecule \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}-\mathrm{COO}^{-} \mathrm{K}^{+} \), the \( \mathrm{CH}_{3} \) end is lyophobic, meaning it repels water. Meanwhile, the \( \mathrm{COO}^{-} \mathrm{K}^{+} \) end is lyophilic, meaning it attracts water.

Step 2: Examine Characteristics of Colloids

Lyophobic colloids do not like water and often require stabilizers. Associated colloids, or micelles, are formed by amphiphilic molecules that have both lyophilic and lyophobic ends.

Step 3: Identify the Type of Colloid

Potassium stearate fits the description of an associated colloid (micelle) because it self-assembles in water due to its amphiphilic nature, with the lyophilic end interacting with water and the lyophobic end avoiding it.

Step 4: Determine the Correct Answer

The question asks for the types of colloids potassium stearate can be an example of. Given the molecule's amphiphilic nature forming micelles, it is classified as an (2) associated colloid or micelle. Thus, the correct answer is option (a) 2 only.

Key Concepts

Saponification

Saponification is an essential reaction in soap-making, involving the conversion of fats or oils into soap and alcohol by treating them with an alkali. To picture this, think of heating butter or oil and adding a substance similar to baking soda. What happens next is saponification. In technical terms, this process involves the hydrolysis of an ester, where triglycerides (the core components of fats and oils) react with a strong base like sodium or potassium hydroxide.

This reaction yields glycerol and soap. For instance, when potassium stearate is formed from saponification, the oil or fat reacts with potassium hydroxide. The fatty acids in the oil are converted to soap (potassium stearate), while the glycerol remains as a byproduct. This is why both soap and alcohol are obtained.

• The fat hydrolyzes into fatty acids and glycerol.
• The fatty acids react with the base to form soap.
• Pictured as: Fat + Alkali → Soap + Glycerol.

Through saponification, the amphiphilic properties of molecules like potassium stearate come alive, setting the stage for micelle formation in aqueous solutions.

Amphiphilic Molecules

Amphiphilic molecules are fascinating compounds loved in chemistry discussions because of their unique dual affinity nature. One part of the molecule is hydrophobic (water-repelling), while the other is hydrophilic (water-attracting). This structure allows amphiphilic molecules to interact with different environments, making them versatile.

Taking the example of potassium stearate, its long hydrocarbon tail (\(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}\) ) is the hydrophobic part, helping it evade the watery environment, while the head part (\(\mathrm{COO}^{-} \mathrm{K}^{+}\) ) is hydrophilic. This allows it to interact with water, making it an extraordinary mediator between oil and water.

• Hydrophobic end: Typically a long carbon chain.
• Hydrophilic end: A polar or ionic charged head.
• Promotes formation of associations like micelles in water.

Such molecules are essential in the creation of emulsions, cleaning agents, and are fundamental in biological membranes owing to their adaptable nature.

Micelle Formation

Micelle formation is a simple yet remarkable self-assembly process occurring when amphiphilic molecules like potassium stearate are placed in water. Imagine the molecules as tiny spheres, with their hydrophilic heads facing the aquatic environment and hydrophobic tails tucked away inside, safe from water.

When these molecules reach a particular concentration, known as the critical micelle concentration (CMC), they spontaneously organize into spherical structures called micelles. The inside shelters the hydrophobic sections away from water, effectively reducing the energy of the system.

• Hydrophilic heads face outward, interacting with water.
• Hydrophobic tails are hidden inside the micelle.
• Allows the cleaning action of soaps, capturing oily residues.

This mechanism is incredibly useful in daily life, from cleaning actions in detergents removing grease stains to biomedical applications where they can target and deliver drugs with precision.