Problem 25 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Pure water boils at \(373 \mathrm{~K}\) and nitric acid at \(359 \mathrm{~K}\). The azeotropic mixture of water and nitric acid boils at \(393.5 \mathrm{~K}\). On distillation of the azeotropic mixture, (a) pure nitric acid will distil over first. (b) pure water will distil over first. (c) one of them will distil over with small amount of the other. (d) both of them will distil over in the same composition as they are in the mixture.

Step-by-Step Solution

Answer

When the azeotropic mixture of water and nitric acid is distilled, neither pure water nor pure nitric acid distils over first; both of them distil over in the same composition as they are in the mixture.

Step 1: Understand the concept of an azeotrope

An azeotrope is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This is because the mixture has a constant boiling point and vapor composition. When an azeotropic mixture is boiled, the vapor has the same composition as the liquid.

Step 2: Analyze the boiling points

Pure water boils at 373 K, and pure nitric acid boils at 359 K. However, the azeotropic mixture of water and nitric acid boils at a higher temperature of 393.5 K.

Step 3: Determine which component distils over first

Because an azeotropic mixture has a constant boiling point and vapor composition, during distillation, the mixture does not separate into its components. Thus, neither pure water nor pure nitric acid will distil over first.

Step 4: Identify the correct answer based on azeotrope characteristics

When an azeotrope is boiled, the components will distil over in the same composition as they are in the mixture, because the vapor composition is the same as the liquid. Therefore, 'both of them will distil over in the same composition as they are in the mixture' is the correct answer.

Key Concepts

Understanding Distillation

Distillation is a widely used separation technique based on the differences in boiling points of substances. It involves heating a liquid mixture to create vapor and then cooling this vapor to get a purified liquid. The process exploits the fact that different components have different boiling points; hence they vaporize at different temperatures.

However, distillation is not always straightforward. Take, for example, an azeotropic mixture, which is a special type of mixture that presents a challenge to this separation method. When dealing with such mixtures, traditional distillation techniques don't apply because of their unique properties, which we will discuss further in the upcoming sections.

Significance of Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas phase. In a pure substance, this process occurs at a consistent temperature under given pressure conditions.

In mixtures, the boiling point can indicate the presence of different substances based on the temperatures where transitions occur. For single components, like pure water boiling at 373 K or nitric acid at 359 K, this principle is used in distillation to separate them from mixtures. However, in the presence of certain mixtures like azeotropes, the boiling points can behave anomalously, as seen with the water and nitric acid azeotropic mixture boiling at 393.5 K.

Azeotrope Characteristics

An azeotrope is a unique blend of substances that behaves as a single entity during the boiling process. This mixture has two defining characteristics: a consistent boiling point and consistent vapor composition. Unlike other mixtures, where different substances boil off at different temperatures, an azeotrope's components boil together.

These characteristics mean that when an azeotrope is heated, the resulting vapor is identical, in proportion, to the azeotrope liquid mixture. This principle explains why an azeotrope such as the mixture of water and nitric acid doesn't separate into its substances upon distillation — because the boiling point is unique to the combination and doesn't change during the process.

Vapor Composition of Azeotropes

Unlike typical liquid mixtures, an azeotrope maintains the same vapor composition as the liquid from which it vaporizes. It means that the proportion of individual components in the vapor phase is exactly the same as in the liquid phase.

This constant vapor composition is a direct consequence of the azeotropic point — the specific concentration at which an azeotrope exhibits its unique boiling point. Due to this constant ratio, it's impossible to separate the components by standard distillation. Each time you attempt to distill an azeotropic mixture like water and nitric acid, they will vaporize and condense back into the same original proportions, reflecting the consistent nature of the azeotrope.