Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Glacial acetic acid is obtained by (a) treating vinegar with dehydrating agents (b) crystallizing, separating and melting acetic acid (c) chemically separating acetic acid (d) distilling vinegar

Step-by-Step Solution

Answer

The correct method is (a) treating vinegar with dehydrating agents.

Step 1: Understand the Definitions

First, we need to understand what 'glacial acetic acid' and 'dehydrating agent' mean. Glacial acetic acid is a concentrated form of acetic acid, which means it should not contain any water molecules, typically found at concentrations above 99%. A dehydrating agent is a substance used to remove water from another substance.

Step 2: Assess the Options

To find the correct method of obtaining glacial acetic acid, let's evaluate the given options. (a) 'Treating vinegar with dehydrating agents' - This process could remove water from acetic acid, potentially leading to 'glacial' form. (b) 'Crystallizing, separating and melting acetic acid' - This suggests a physical separation technique which may not achieve glacial purity without additional steps. (c) 'Chemically separating acetic acid' - This seems vague and does not specify removal of water. (d) 'Distilling vinegar' - Distillation may increase concentration, but generally doesn't remove all water without further treatments.

Step 3: Identify the Correct Method

The key factor in obtaining 'glacial' acetic acid is removing water efficiently. Option (a) 'treating vinegar with dehydrating agents' explicitly mentions using substances to remove water from acetic acid, which aligns with our goal. Crystalization and melting (b), chemical separation (c), and distillation (d) alone are less specific about completely removing water.

Key Concepts

Dehydrating Agents

When we talk about dehydrating agents, we're referring to substances specifically used to remove water from other substances. These agents are vital in different chemical processes because they ensure the concentration of a substance by extracting water molecules.
Some common dehydrating agents include:

• Calcium chloride (CaCl₂)
• Sulfuric acid (H₂SO₄)
• Phosphorus pentoxide (P₂O₅)

They work by either absorbing water themselves or by facilitating reactions that eliminate water. In the context of making glacial acetic acid, using a dehydrating agent is an effective method to ensure water molecules are removed, ensuring a high-purity form. This process is particularly essential because glacial acetic acid requires a concentration of over 99%, free of any water contamination.

Physical Separation Techniques

Physical separation techniques involve processes that do not alter the chemical composition of substances but separate them based on their physical properties. Common techniques include filtration, crystallization, and centrifugation.
When thinking about crystallization, it's a process where a solid forms from a solvent, due to changes in temperature or concentration. With acetic acid, one method is to crystallize and then separate out pure acetic acid crystals.
However, while physical separation can isolate acetic acid up to a point, achieving a purity level for glacial acetic acid (over 99%) is tricky without additional steps like dehydration. Therefore, physical methods alone might not be sufficient in reaching the desired purity for forming glacial acetic acid directly.

Distillation Process

Distillation is a popular method used to purify liquids by heating them to create vapor and then cooling the vapor back into a liquid. It's a process based on differences in boiling points between substances.
In the context of vinegar, distillation can be used to concentrate its acetic acid component as vinegar typically contains about 5-8% acetic acid. By heating the mixture, you separate the acetic acid from water and other components.
However, one limitation of standard distillation is that it may not effectively remove all water, especially when aiming for glacial acetic acid purity. Additional treatments like adding dehydrating agents might be necessary to ensure the complete removal of water molecules.

Water Removal Methods

Water removal methods are critical in achieving highly concentrated solutions like glacial acetic acid. Removing water is often a crucial step to obtain the desired concentration and purity.
These methods include:

• Using dehydrating agents, which are substances that absorb or react with water, as discussed.
• Utilizing heating processes such as evaporation or drying, where water is removed through vaporization.
• Employing special separation processes, like membrane filtration, that specifically target water molecules.

Each water removal method has its own set of advantages and limitations. Specifically for glacial acetic acid, the use of a dehydrating agent is a reliable approach, since it efficiently eliminates water to achieve a purity level of more than 99%. This step is crucial for creating glacial acetic acid free from water, hence achieving the 'glacial' designation that signifies its highly concentrated state.