Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the most suitable method for removing the traces of water from ethanol? (a) Heating with sodium metal (b) Hassing dry HCl gas through it (c) Distilling it (d) Reacting with \(\mathrm{Mg}\)

Step-by-Step Solution

Answer

Distillation (Option c) is the most suitable method for reducing water in ethanol.

Step 1: Understanding the Problem

The objective is to find the most suitable method for removing water from ethanol. This process is referred to as drying or dehydrating ethanol. We need to consider which of the given methods is best for this purpose based on chemical properties and practical effectiveness.

Step 2: Evaluate Heating with Sodium

Sodium metal reacts with water to form sodium hydroxide and hydrogen gas, effectively removing water by reacting with it. The reaction is vigorous, as sodium reacts violently with water, even in small amounts like those found in wet ethanol. But using sodium is hazardous and not the most practical laboratory method.

Step 3: Evaluate Passing Dry HCl Gas

Passing dry HCl gas through ethanol is not suitable for removing water. HCl is a gas that can dissolve in water, forming hydrochloric acid, but it does not remove water. Instead, it could lead to the presence of HCl in the solution or form aqueous HCl, which doesn't achieve the intended purpose.

Step 4: Evaluate Distillation

Distillation involves heating the ethanol and collecting its vapors to separate it from water. Ethanol boils at 78.37 °C, whereas water boils at 100 °C, so fractional distillation can be used to remove water. However, ethanol and water form an azeotrope, which limits pure ethanol separation by simple distillation, but it can reduce water concentration.

Step 5: Evaluate Reacting with Mg

Magnesium (Mg) does not have a known reaction that would effectively remove water from ethanol. It might introduce impurities into the ethanol rather than remove water. Thus, this method is not suitable.

Step 6: Conclusion

Among the options, distillation (Option c) is the most suitable method for reducing water in ethanol, although it may not fully achieve complete dehydration due to azeotrope formation.

Key Concepts

Distillation of ethanol

Distillation is a widely used technique to separate mixtures based on differences in boiling points. In the case of ethanol, this method is particularly useful because ethanol and water have different boiling points.
Ethanol boils at approximately 78.37 °C, while water boils at 100 °C. During distillation, the mixture is heated to the boiling point of ethanol. Ethanol vaporizes first, leaving behind most of the water.

In the laboratory, fractional distillation is commonly used due to its effectiveness in separating ethanol from mixed components. This process involves the use of a fractionating column, which helps create multiple condensation and evaporation steps, enhancing purification.

• Advantages: Economical and widely applicable.
• Limitations: Does not completely separate ethanol from water due to azeotrope formation.

Distillation is an essential method to reduce water content in ethanol, making it a preferred choice despite its limitations.

Azeotrope

An azeotrope is a mixture of two or more liquids that has a constant boiling point and composition throughout the distillation process.
In the case of ethanol and water, they form an azeotrope at about 95% ethanol and 5% water, boiling at approximately 78.1 °C.

This means that during distillation, you cannot easily achieve 100% pure ethanol because the azeotropic mixture distills unchanged. This is a challenge in the distillation process as it limits the purity achievable with conventional methods.

• Characteristics: Constant boiling mixture, cannot be separated by simple distillation.
• Solutions: Use of azeotropic distillation or drying agents to break the azeotropic composition.

Understanding azeotropes is crucial for designing efficient processes for purifying chemicals like ethanol.

Removal of water from solvents

Removing water from solvents like ethanol is an important process in chemical industries and laboratories.
Various methods are employed besides distillation:

• Drying Agents: Substances like calcium oxide or molecular sieves can absorb water, helping achieve dry ethanol.
• Chemical Reactions: Involves reactions with substances that can form compounds with water, like sodium, though these methods are often hazardous.

Each technique has its applications and limitations, often influenced by the required purity and practical considerations like safety and cost.
Efficient water removal ensures quality in chemical processes and product outcomes, making understanding these methods essential.

Chemical properties of ethanol

Ethanol is a versatile alcohol with several notable chemical properties that influence its behavior in various applications.
It is a polar solvent, capable of dissolving a wide range of substances, making it essential in industries from pharmaceuticals to perfumes.

• Boiling Point: Ethanol has a relatively low boiling point of 78.37 °C, making it volatile.
• Hydrogen Bonding: Due to the presence of hydroxyl groups, ethanol forms hydrogen bonds, affecting its interactions with other substances.

Ethanol's miscibility with water is a significant property that is leveraged in many solutions and mixtures; however, it also makes drying or dehydration particularly challenging. Understanding these properties allows for better application and manipulation of ethanol in scientific and industrial processes.