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

Question

Absolute alcohol (100\% alcohol) is prepared by distilling rectified spirit over (a) \(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\) (b) \(\mathrm{CaCl}_{2}\) (c) \(\mathrm{Na}\) (d) \(\mathrm{Mg}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{CaCl}_{2} \) is used to prepare absolute alcohol.

Step 1: Understand the Problem

The problem asks us to identify the substance used in conjunction with distillation to prepare absolute alcohol from rectified spirit. Rectified spirit usually contains around 95% alcohol and 5% water.

Step 2: Recall Key Concept – Absolute Alcohol Preparation

Absolute alcohol can be prepared by removing water from rectified spirit. Dehydrating agents are typically used to remove the water content. Common dehydrating agents for alcohol preparation include calcium oxide (quicklime), calcium chloride, and magnesium.

Step 3: Evaluate Each Option

Let's consider each option:(a) \( \mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2} \) - Magnesium ethoxide is not used as a specific dehydrating agent in this context.(b) \( \mathrm{CaCl}_{2} \) - Calcium chloride is known for its dehydrating properties and is used in preparing absolute alcohol.(c) \( \mathrm{Na} \) - Sodium could potentially remove water but is more reactive.(d) \( \mathrm{Mg} \) - While magnesium can react with water, it isn't typically used to prepare absolute alcohol.

Step 4: Identify the Correct Answer

After evaluating the options, the most suitable substance used in making absolute alcohol by dehydration of rectified spirit is \( \mathrm{CaCl}_{2} \), due to its ability to efficiently remove water.

Key Concepts

Rectified Spirit

Rectified spirit is an important compound in the alcohol purification process. It is essentially a high concentration alcohol mixture, generally comprising about 95% ethanol and 5% water. This mixture is obtained through repeated distillations, which is why it is called 'rectified'. The name stems from the term 'rectification,' which stands for the process of refining a substance through distillation.
Using rectified spirit as a starting point is crucial when preparing absolute alcohol. Since it is already close to being pure ethanol, only a small amount of water needs to be removed to achieve the 100% ethanol goal. The fact that 5% of the solution is water means that simple distillation is insufficient to reach absolute purity. Other techniques, involving chemical agents, must come into play to eradicate the remaining water content.
Thus, the preparation of absolute alcohol involves choosing substances that can help remove this water efficiently without affecting the ethanol. This leads to the engagement of dehydrating agents.

Dehydrating Agents

Dehydrating agents are substances that facilitate the removal of water from another substance. They are pivotal in refining processes like the preparation of absolute alcohol. The functionality of these agents lies in their ability to bind with water molecules, thereby effectively reducing the water content in mixtures.
In the context of absolute alcohol preparation, different dehydrating agents can be employed:

• Calcium oxide (also known as quicklime) is frequently utilized for its strong water-absorbing properties.
• Calcium chloride is another widely used dehydrating agent, especially noted for its affordability and ease of handling.
• Other agents like magnesium have been utilized, though they aren't as common in this specific application.

The key is to select an appropriate dehydrating agent that not only removes water but also doesn't introduce any unwanted impurities into the alcohol mixture. Using these agents ensures that the production of absolute alcohol is both effective and efficient, resulting in a pure product.

Calcium Chloride

Calcium chloride ( CaCl_2 ) is an effective dehydrating agent that plays a crucial role in preparing absolute alcohol. Its hygroscopic properties allow it to absorb moisture from its surroundings and from mixtures, making it an excellent choice for removing water from rectified spirit.
It operates by forming hydrates when in contact with water molecules, effectively decreasing the water content and moving the rectified spirit closer to becoming absolute alcohol. This chemical affinity for water molecules stems from its ionic structure, which readily attracts and binds with water.
Beyond its use in alcohol preparation, calcium chloride is often applied in industrial settings for de-icing and dust control because it maintains similar moisture-binding properties.
Its use in refining alcohol is preferred due to its ability to perform at room temperature and without needing extensive energy input. With its efficiency and economic benefits, calcium chloride continues to be a reliable choice in the quest for absolute alcohol purity.