Azeotropic Mixtures: Azeotrope, Types of Azeotropic Mixtures, Applications, Azeotropic Distillation, Practice Problems and Frequently asked questions (FAQs)

Each pure liquid has its unique vapour pressure and boiling point.But what about a mixture of two or more pure liquids? Will they boil as if the other is not there or will it have its own different boiling point?. If so will it be higher or lower than the individual components?

The liquid may boil at a boiling point of each pure liquid successively unaffected by others giving pure liquids in steps or may boil at a constant temperature giving a mixture with constant proportions of both of them. The boiling point can be either lower or higher than both of the component liquids.

For example, an ethanol aqueous solution is inseparable from distillation. Why so?

Let us know more about them as they are important in many industrial processes of purification.

Table of Contents

Azeotrope
Types of Azeotropic mixtures
Applications of azeotropes
Azeotropic distillation
Practice problems
Frequently asked questions

Azeotrope

An azeotrope is a mixture that boils at one temperature (constant boiling) which will be different from both of the pure liquids. The relative concentration of each component of an azeotrope mixture will be the same both in the liquid phase and the vapour phase so that separation into pure components will never be possible by distillation.This is in contrast to the ideal solution, where one component is usually more volatile than the other so that one prefers to boil over the other Separation into pure components can be easily obtained by simple distillation or fractional distillation depending on the boiling point difference.

An azeotropic mixture is a mixture of at least two different liquids. The mixture may have a higher boiling point than each component, or it may have a lower boiling point.When dealing with such mixtures, the components can usually be extracted from the solution by fractional distillation or essentially repeated stepwise distillation (hence the term "fractional"). The more volatile components tend to evaporate and are collected separately, while the least volatile components remain in the still and end up in two pure separate solutions.

Types of Azeotropic Mixtures

In any liquid, there will be interactions between the molecules. Considering two independent liquids X and Y, there will be X-X and Y-Y interactions in pure liquids. When they are mixed, there is a possibility of interaction between the two liquids as X-Y interactions. Depending upon the nature of the interaction of X-Y with respect to X-X and Y-Y, there can be three possibilities..

Depending on the nature of interactions between the pure liquids and between the liquid in a mixture, liquid mixtures can be classified into three categories.

Ideal solutions
Maximum boiling azeotropes
Minimum boiling azeotropes:

Ideal Solutions

When the X- Y interactions between the mixed liquids are not much different, they form what is called an ideal solution meaning they behave independently of each other boiling at their own individual boiling points.An ideal mixture obeys Raoult’s law by which the vapour pressure of any liquid in a mixture is a mole fraction of the pure solvent liquid. Organic compounds of the same homologous series are examples.

Maximum boiling azeotrope or negative azeotrope:

When the X-Y interactions are stronger than the individual X-X and Y-Y interactions, depending upon the strength of interactions, both molecules are prevented by escaping to form vapour resulting in higher boiling liquid. These liquids show a lesser or negative deviation of expected Raoult’s vapour pressure. In such a case, initially, the lower boiling point liquid shall boil to a constant concentration boiling liquid. This liquid will boil at a high boiling point and also will have the same mixed composition as that of the boiling liquid. As a result, the liquid mixture of that composition cannot be separated into pure liquid by distillation.

Azeotropic mixtures with higher boiling points than their individual components are azeotropic mixtures with maximum boiling points. Consider, for example, hydrochloric acid consisting of a weight concentration of approximately 20 percent and 79 percent of water.Water boils at 373 K and hydrochloric acid boils at about 188 K, while the azeotropic mixture boils at about 383 K. This is a boiling point higher than the boiling point of its constituents.

Minimum boiling azeotrope or positive azeotrope:

When the X-Y interactions are weaker than the individual X-X and Y-Y interactions, depending upon the strength of interactions, both molecules escape out favourably resulting in lower boiling liquid. These liquids show a larger or positive deviation of expected Raoult’s vapour pressure. In such a case, the liquid shall boil to a constant concentration of boiling liquid. This liquid will boil at a lower boiling point and also will have the same mixed composition as that of the boiling liquid. As a result, the liquid mixture of that composition cannot be separated into pure liquid by distillation.

An azeotropic mixture having a lower boiling point than its constituents is called a minimum boiling azeotropic mixture. For example, ethanol is about 95 per cent by weight and 4 percent is water. Water boils at 373 K and ethanol boils at about 351.5 K, while the azeotropic mixture boils at about 351.15 K, indicating a lower boiling point than its constituents.

Heterogeneous azeotropes:

If the components of the azeotrope are not completely miscible, the azeotrope is found within the miscibility gap. Therefore, phase splitting can occur for the smallest boiling azeotropic mixtures that deviate significantly from Raoult's law. This forms a non-uniform azeotropic mixture with a minimum boiling point that has an equilibrium gas phase with the two liquid phases. The components of these azeotropic mixtures are not completely miscible. These types of azeotropic mixtures are also called heteroazeotropic mixtures. For example, a water-chloroform azeotropic mixture. When shaken together and left to stand, two separate layers are formed. This mixture boils at 53.30oC, but the boiling point of water is 100oC and chloroform is 61.20oC.

Homogeneous azeotropes:

This type of azeotrope is called a homogeneous azeotrope when the components of the azeotrope are completely miscible with each other in all proportions. To create a homogeneous azeotropic mixture, for example, any amount of ethanol can be mixed with any amount of water.

Binary azeotrope:

A azeotropic mixture with two components is called a binary azeotropic mixture. For example, a mixture of halothane (66%) and diethyl ether (33%). In anaesthesia, this azeotropic mixture is frequently employed.

Ternary azeotropes:

A azeotrope with three components is called a ternary azeotrope. For example, an azeotropic mixture of Propanone, methyl alcohol and chloroform.

Applications of azeotropes:

Some applications of azeotropes include:-

1. It is primarily used as a standard in a variety of fields, including the testing of chromatographs, detectors, and columns.

2. It can also be used to separate the constituents of azeotropic mixtures. One example is the difficulty in separating the components of acetic acid and water. It is well known that ethyl acetate reacts with water to form azeotropes. It is found to boil at 70.4oC. Thus, by adding small amounts of ethyl acetate to this acetic acid and water mixture, it would be possible to distil away the azeotropes formed, i.e., ethyl acetate and water, leaving almost pure acetic acid in the distillation flask. Acetic acid has a boiling point of 118.1oC, so it is left behind in the distillation flask.

Azeotropic distillation:

An azeotropic mixture is a mixture of two or more liquids that cannot be separated by simple distillation because the vapour produced by the boiling azeotropic mixture contains about the same proportion of the liquid itself. Therefore, azeotropic distillation is a special type of distillation that uses a special technique to decompose the azeotropic mixture.

The most common method of destroying an azeotropic mixture involves the addition of a material separation agent.

Entrainer:

An entertainer is a chemical that can also interact with the components of mixture or azeotrope so as to break the existing interaction between the components of a mixture. The volatility of the components is now changed to break the existing azeotropic interactions and allow the individual components to boil at their boiling points to give pure components. For example benzene acts as an entrainer in the water- alcohol azeotrope.

Addition of entrainer becomes useful when the deviation from Raoult’s Law is higher resulting in either minimum or maximum boiling azeotropes.

Distillation of water and ethanol:

It is known that water and ethanol form an azeotropic mixture.and cannot be separated by ordinary distillation But by adding entrainers that can interact with water and alcohol separately and influence the volatility of both water and alcohol, the azeotrope can be broken facilitating the separation by distillation. Nonpolar chemicals like Toluene, cyclohexane, are used. Carcinogenic benzene and other flammable chemicals like ether, acetone are avoided.

Practice problems:

Q.1. The system that forms maximum boiling point azeotrope is

Acetone-chloroform (C) ethanol-acetone
N-hexane-n-heptane (D) carbon disulphide-acetone

Answer: (A)

Solution: Due to strong intermolecular interactions between acetone and chloroform lesser number of molecules vaporise resulting in low vapour pressure and high boiling point.

Q.2. When nitric acid and water are combined in the ratios of 68% to 32%, respectively, they form an azeotrope with a maximum boiling point. It means

(A) A-B interactions are stronger than A-A and B-B interactions
(B) A-B interactions are weaker than A-A and B-B interactions
(C) vapour pressure of the solution is more than pure components
(D) vapour pressure of a solution is less since only one component vaporises.

Answer: (A)

Solution: Since A-B interactions are stronger than A-A and B-B interactions the solution shows a negative deviation from Raoult’s law,.i.e. Less vapour pressure and hence higher boiling point.

Q.3. A relative volatility close to unity could indicate the presence of

(A) Separation point (C) Deep point

(B) Pinch point (D) Extraction point

Answer: (B)

Solution: A relative volatility close to unity could indicate the presence of a pinch point. It is technically feasible to separate components that form a pinch point.

Q.4. It is possible to create a minimum boiling point azeotrope by adding

(A) Entrainer
(B) water
(C) Any chemical
(D) Toluene

Answer: (A)

Solution: An entrainer is an agent that when added to close boiling or azeotropic mixtures increases the differences in boiling points so as to separate the components by distillation. These agents are sefic to the components of the mixtures and cannot be universalized. Thus the correct option is A.

Frequently asked questions:

1. What is the difference between ideal solution and azeotropes?
Answer: An ideal solution is a uniform mixture of components whose physical properties are linked to pure components. These solutions are supported by Raoult's law, which states that the interaction between solute and solvent molecules is the same as for those molecules alone. Examples of ideal solutions are benzene and toluene. The azeotropic mixture does not fit this idea because the component ratio of the non-vaporized solution at boiling is the same as the component ratio of the vaporised solution. Therefore, an azeotropic mixture can be defined as a solution in which the vapour has the same composition as its liquid. As you can imagine, this kind of substance is very difficult to distil. In fact, pure ethanol is essentially non-existent, so the azeotropic mixture, which is the most concentrated form of ethanol, is about 95.6% by weight ethanol.

2. Why is the distillation process ineffective for separating azeotropes?
Answer: In the case of distillation, separation is performed using the difference in boiling point between the components. An azeotropic mixture is a constant boiling mixture, meaning that it boils at a constant temperature. For example, an ethanol-water mixture with a composition of 95.63% ethanol and 4.37% water forms an azeotropic mixture and continues to boil at a constant temperature of 78.2oC. Therefore, it is impossible to achieve an ethanol concentration of 95.63% or higher with ordinary distillation. To overcome this, azeotropic distillation can be used.

3. What distinguishes azeotropic distillation from extractive distillation?
Answer: The main difference between extraction distillation and azeotropic distillation is the process of separating the mixture. Extractive distillation requires the use of a specific separation solvent for each mixture. It should not tend to form an azeotropic mixture. Thus, extraction distillation can be regarded as a relatively simple method compared to azeotropic distillation.

4. How to break an azeotrope?
Answer: Azeotropic mixtures can be mixed with entrainers which interacts with the components of the azeotropic mixtures and alters the interaction between them. This way ot breaks the azeotropic mixture and makes way for the separation of the components by distillation