Question

Aniline is insoluble in water and possesses a vapour pressure of \(10-15 \mathrm{~mm} \mathrm{Hg}\) at \(373 \mathrm{~K}\). It can be conveniently purified by: (a) Simple distillation (b) Steam distillation (c) Crystallization (d) Sublimation

Short answer

Steam distillation (b) is suitable for purifying aniline.

Step-by-step solution

Understanding the Problem

Aniline is a solid with a certain vapor pressure at a given temperature. We want to know which method will purify aniline effectively given its vapor pressure range of 10-15 mmHg at 373 K.

Evaluating Distillation

Simple distillation purifies based on boiling point differences, but requires that the compound be miscible in its solvent. Aniline is insoluble in water, making simple distillation ineffective.

Considering Steam Distillation

Steam distillation is useful for temperature-sensitive compounds and enables the separation of insoluble liquids by co-distillation with water. Aniline's vapor pressure and insolubility make it suitable for purification through steam distillation because it allows vaporization at lower temperatures.

Assessing Crystallization

Crystallization is typically used for purifying solids that are soluble in solvents, followed by precipitation through cooling. Since aniline is not water-soluble at relevant conditions, crystallization is not applicable here.

Evaluating Sublimation

Sublimation works for compounds that can transition from solid to gas without becoming liquid, generally at stable low pressures. Aniline does not sublimate easily under expected conditions so this is not practical.

Choosing the Best Method

Given the options and aniline's properties (insolubility in water, specific vapor pressure), steam distillation efficiently purifies aniline by using water vapor to carry aniline vapor without needing it to dissolve.

Key concepts

Aniline Purification

Purifying aniline effectively depends on its inherent properties and understanding which method aligns best with these characteristics. Aniline is a chemical compound that is commonly used in the manufacture of dyes, pharmaceuticals, and plastics. To ensure its efficacy in these applications, it needs to be purified from other impurities that might compromise its performance.
One might consider several purification techniques such as:

• Simple distillation: Works for substances that dissolve in liquid solvents, which isn't suitable for aniline due to its water insolubility.
• Steam distillation: Preferred for aniline, particularly when it's imperative to keep temperatures lower.
• Crystallization and sublimation: Both require certain conditions that aniline doesn't meet effectively.

Choosing the correct purification technique not only affects the purity level but also the yield and integrity of the aniline, making steam distillation the top choice given its properties.

Vapor Pressure

Vapor pressure is a critical concept when discussing distillation processes, particularly for compounds like aniline. Vapor pressure indicates how much of a substance converts from liquid to vapor phase at a specific temperature.
In the case of aniline, it has a vapor pressure between 10-15 mm Hg at 373 K. This measurement tells us that aniline can readily evaporate at this temperature, aiding its purification through distillation.
Understanding vapor pressure helps us determine the temperatures needed for distillation and how efficiently the substance can be distilled without decomposing. For aniline, the vapor pressure supports using steam distillation as it can be vaporized without necessitating it to dissolve in water, which it can't.

Temperature-Sensitive Compounds

Temperature sensitivity is a significant factor when choosing a method for purifying chemicals like aniline. Some compounds decompose or react negatively at high temperatures, making methods like simple boiling unsuitable.
Aniline is somewhat sensitive to temperature, given that high temperatures might alter its chemical structure. steam distillation comes into play as an ideal method as it operates at temperatures lower than the compound's boiling point by utilizing steam to carry the aniline vapors.
This gentle heating process ensures that aniline remains intact while still allowing excess impurities to be removed. Therefore, steam distillation is ideal for retaining the chemical composition of such temperature-sensitive compounds.

Insolubility in Water

A compound's solubility directly impacts which purification methods are feasible. Aniline's insolubility in water plays a key role in determining the efficacy of different purification techniques.
This Insolubility means that simple distillation, which relies on the compound's miscibility in its liquid phase, won't work effectively for aniline. Instead, steam distillation benefits from this property as it allows aniline to be vaporized without requiring dissolution.
By co-distilling with steam, aniline is effectively separated, highlighting the benefits of steam distillation when dealing with insoluble substances. This process leverages the vapor pressure of aniline to enable its purification despite its lack of solubility.