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Distillation Curves: Vapour Pressure, Boiling Point, Distillation Curves, Azeotropes, Practice Problems & Frequently Asked Questions

Distillation Curves: Vapour Pressure, Boiling Point, Distillation Curves, Azeotropes, Practice Problems & Frequently Asked Questions

Let us start with water. You might have heated this water for so many purposes and seen vapours coming out of the hot water. Cooling this vapour condenses to water back. This is distillation.

Distillation is one of the oldest ways of water purification and is still used today, although not as a home treatment method. It is capable of effectively removing a wide range of contaminants from drinking water, including bacteria, and inorganic and organic chemicals.

To purify water, distillation uses evaporation. Steam is created by heating contaminated water. Large non-volatile organic molecules and inorganic substances do not evaporate with water and are left behind.

Distillation successfully removes inorganic substances like metals (lead), nitrate, and other unwanted particles like iron and hardness from a contaminated water source. Microorganisms such as bacteria and viruses are also killed during the boiling process. Water is distilled to eliminate oxygen and trace metals. As a result, some people claim that distilled water tastes flat.

The steam cools and condenses to make pure water.

This distillation is useful not only for water but for many liquids for separation, purification, etc.

On heating, the temperature of the liquid increases to a maximum of its boiling point. Then it remains there till all the liquid evaporates out.

But the distillation for separation is not so simple when it comes to the mixtures of solid in liquid or liquid in liquid.

Let us know the principles of this boiling and separation

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Table of Content:

  • Vapour Pressure
  • Boiling Point
  • Distillation
  • Distillation curves
  • Azeotropes
  • Practice problems
  • Frequently asked questions

Vapour Pressure:

The pressure exerted by a vapour in equilibrium with its condensed phase (solid or liquid) in a closed system at a specific temperature is referred to as vapour pressure, also known as vapour equilibrium pressure. This pressure exists in a thermodynamic equilibrium system. Equilibrium vapour pressure is a common way to measure how quickly a liquid is evaporating. This is related to the propensity of particles to escape from liquids (or solids). Volatile materials are referred to as those that have a high vapour pressure at room temperature. Keep in mind that vapour pressure refers to the force a vapour exerts on a liquid's surface.

Boiling Point:

The boiling point of a substance is the temperature at which the vapour pressure of the liquid equals the air pressure.

The rates at which liquid molecules are expelled from the surface into the gas phase and the average kinetic energy of a liquid both rise with temperature. The molecules of the liquid eventually accumulate sufficient kinetic energy to cause them to all vaporise. At this point, the liquid begins to boil and the vapour pressure reaches air pressure. The temperature at which this phenomenon occurs is known as the liquid's boiling point and remains the same till all the liquid gets converted into vapour.

Distillation Curves:

Distillation curves depict the variation of the vapour pressure of liquid with temperature. The vapour pressure of a pure liquid increases linearly with an increase in temperature till it reaches the boiling point.

But, the heating of a liquid mixture produces vapours of all its components. The concentration of the vapours shall depend on -

  • the individual boiling points
  • differences in the boiling points and
  • the interactions between the components.

So, change in the vapour pressure of the liquid mixture may or may not be linear with an increase in temperature.

Hence, a distillation of mixtures may (ideal mixtures) or may not (non-ideal mixtures) provide substantially pure samples.

Ideal Mixture Distillation Curve:

An ideal mixture contains almost similar components, so as not to have interactions between and shall have zero volume change and zero enthalpy of mixing. When the activity coefficients of the constituents are identical to one at all concentrations, temperatures, and pressure, a mixture is said to be ideal.

Raoult's law states that the vapour pressure of a substance is proportional to its molar composition in solution. Equation (1) depicts Raoult's law.

PA= PAo XA …. …. …. …. …. (1)

where PAo denotes the vapour pressure of a pure A sample and XA denotes the mole fraction of A in the liquid mixture.

According to Dalton's law of partial pressures, the total pressure in a closed system can be calculated by adding the partial pressures of each gaseous component. Equation(2) depicts Dalton's law.

Ptotal= PA+ PB ….. ….. …. …. …. (2)

Equation(3) shows the vapour produced by a miscible two-component system (compounds A + B) using a combination of Raoult's and Dalton's laws. This combination law demonstrates that the vapours produced by distillation are affected by the vapour pressure and quantity of each component (mole fraction).

Psolution= PAo XA+ PBo XB ….. ….. …. …. …. (3)

The vapour pressure of a compound reflects both the temperature of the solution and the boiling point of the compound. As the temperature rises, a greater proportion of molecules have enough energy to resist the intermolecular forces (IMFs) that keep them in the liquid phase. As a result, the vapour pressure of a compound always increases with temperature, though not linearly.

When two compounds are compared at the same temperature, the compound with the weaker IMF (the one with the lower boiling point) should enter the gas phase more easily. As a result, at any given temperature, a compound with a lower boiling point has a higher vapour pressure than a compound with a higher boiling point.

The vapour produced by a miscible two-component system (compounds A + B) is described by Equation (3). This mathematical equation can be used to generate phase diagrams, also known as distillation curves, that graphically correlate temperature to the molar composition of the liquid and vapour phases.

Psolution= PAo XA+ PBo XB

A generic distillation curve for a two-component system is shown in Figure (a). The x-axis represents molar composition, with the left side of the figure representing a pure compound Asample and the right side representing a pure compound B sample. A and B mixtures are represented between the two sides. The y-axis represents temperature, and the boiling points of each component are indicated ("bp A" and "bp B").

The tear-shaped appearance of the phase diagram is due to a variation in composition between the liquid and gas phases during the distillation of mixtures. During the boiling of the liquid, the tear-shaped zone represents conditions in which both liquid and gas coexist.

Consider distilling a mixture of 25 mol per cent A and 75 mol per cent B, which is characterised by the distillation curve in Figure (a). When the temperature reaches the bottom line of the teardrop (temperature x and point 'a in Figure b), the solution starts to boil. Since gas and liquid boils at identical temperature, the vaporisation process can be visualised as moving along the horizontal line in Figure (b) from point a to point b. The gas then compresses into the distillate after vaporisation, which is represented in Figure(b) as a vertical line running from point b to point c. The distillate in this instance would be 74 mol per cent A and 26 mol per cent B under ideal equilibrating circumstances. Because A has a lower boiling point and higher vapour pressure, it is "enriched" in A.

One vaporisation-condensation incident is represented by the leftward horizontal "movement" and the downward vertical "movement" in a distillation curve (see Figure b). This is referred to as a "theoretical plate" since it symbolises the straightforward distillation's ability to cleanse.

Non-Ideal Mixture Distillation curve: Azeotropes:

A non-ideal mixture is one that deviates from the ideal solution's norms, in which the interactions between molecules are the same (or extremely similar) to those between molecules of different components.

Most solutions containing more than one volatile component distil across a range of temperatures, whereas pure substances evaporate at a constant temperature. There is one exception to this rule: some combinations of particular compositions also distil at a constant temperature and the distillate also has a constant composition of components, such that pure liquids cannot be distilled from such mixtures. These combinations are known as "azeotropes," and they exhibit non-ideal behaviour and do not obey Raoult's law.

A blend of 95.6 percent ethanol and 4.4 percent water is one of the most well-known azeotropes. The maximum amount of ethanol that can be present in the distillate while distilling an ethanol and water mixture (for instance, when concentrating fermented grains to make hard liquor) is 95.6 percent. It is difficult to distil 100 percent pure ethanol while water is present in the distilling pot because the ethanol and water combine to form one pure material. Because it can be purified through distillation and is the least expensive, 95 per cent ethanol is typically utilised by chemists. It is also possible to buy "absolute ethanol" (> 99 per cent ethanol), but it is more costly since additional procedures (such as dry agents) are required to get rid of any leftover water prior to or after distillation.

When a solution deviates from Raoult's law, azeotropes form, which means that the vapour pressure produced by an azeotropic solution does not exactly correlate to its mole fraction. Deviations arise when the components are either attracted to or repelled by one another, i.e. when their intermolecular forces (IMFs) to themselves (e.g. A-A or B-B) deviate significantly from those of the other components (e.g. A-B). In other words, if the enthalpy of mixing is not zero, a solution will deviate from Raoult's law.

Azeotropic mixtures are characterised as having a lower boiling point than any of their constituents (referred to as a "minimum boiling azeotrope") or a greater boiling point than any of their constituents (referred to as a "high boiling azeotrope") (called a "maximum boiling azeotrope"). The 61 percent benzene,39 percent methanol azeotrope, for example, is a minimum boiling azeotrope because its boiling point is 58o C, which is lower than the boiling points of benzene (80o C) and methanol (65o C). Minimum boiling azeotropes occur far more frequently than maximum boiling azeotropes.

Minimum boiling azeotropes develop when the intermolecular forces between the components are greater between themselves (A-A/B-B) than between each other (A-B). When two components do not have an affinity for one another, such as when one component can form a hydrogen bond while the other cannot, this typically occurs. In these situations, the components significantly aggregate in solution, reducing the entropy of the solution and increasing the likelihood that it will turn into a gas. Because of this, these solutions boil at a lower temperature than predicted by Raoult's law because of their greater vapour pressure. Due to solution attractions that cause lower vapour pressures and hence greater boiling temperatures than anticipated, maximum boiling azeotropes have the opposite effect of what is expected.

The figure displays a distillation curve for an ethanol-water mixture. In Figure(c), the intersection of the two tear droplets forms the least boiling azeotrope. This graph explains why it is impossible to distil a mixture of ethanol and water to produce a distillate that contains more than 95.6 per cent ethanol.

Consider distilling an ethanol/water mixture with the ingredients in Figure (d). Following each evaporation-condensation process (a to b to c, then c to d to e in Figure d), the ethanol concentration rises. On the other side, the material is always steered toward the azeotropic composition, which is the lowest point on the curve.

Only when there is no water left in the system may concentrations higher than 95.6 per cent of ethanol be distilled (at which point the phase diagram in Figure is no longer applicable). It should be clear why water must be carefully removed using drying agents prior to distillation since it forms minimum-boiling azeotropes with many organic compounds. Failure to remove trace water results in a moist distillate.

Practice Problems:

Q1. Consider a sugar solution [Sugar + Water]; during vaporisation, only water evaporates because sugar is_

(A) Volatile
(B) Non-Volatile
(C) Cryogenic
(D) None of the mentioned

Answer: (B)

Solution: The Non-volatile compound has no boiling point. As a result, the sugar has no vapour pressure.

Q2. Two liquids A and B form a binary solution at 25 o C and the total vapour pressure was found to be 300 torr . The mole fraction of A in the solution 0.7 and in vapour phase is 0.35. Calculate the vapour pressure of the pure component of A?

(A) 300 torr
(B) 1300 torr
(C) 150 torr
(D) 200 torr

Answer: (C)

Solution: Given YA= 0.35   YB= 1-0.35 = 0.65

XA= 0.70   XB= 1-0.70 = 0.30

We know that, YA= PAP= XAPAoP

PAo= PYAXA=3000.350.70=150torr

Q3. Two liquids X and Y boil at 110o C and 130o C respectively. Which has higher vapour pressure at 50o C?

(A) X
(B) Y
(C) Both X and Y
(D) None of the above

Answer: (A)

Solution: Lower the boiling point more volatile it is and has high vapour pressure. Hence, liquid X will have higher vapour pressure at 50o C

Q4. When liquid A is added to liquid B, vapour pressure decreases while, when liquid C is added to the same B vapour pressure increases. Arrange A, B, C in increasing order of their boiling points.

(A) C < B < A
(B) B < C < A
(C) A< B < C
(D) A < C < B

Answer: (A)

Solution: When the compounds are compared at the same temperature, the compound with the weaker IMF (the one with the lower boiling point) should enter the gas phase more easily. As a result, at any given temperature, a compound with a lower boiling point has a higher vapour pressure than a compound with a higher boiling point.

The order of boiling points are C < B < A

Frequently asked questions:

Q1. What is the importance of the distillation curve?
Answer:
One of the most essential and useful parameters that are measured for complex fluid mixes is the distillation (or boiling) curve. The distillation curve is a graphical representation of a fluid mixture's boiling temperature plotted versus the volume fraction distilled.

Q2. Why do impurities enhance boiling points?
Answer:
Impurities in the solution enhance the boiling point. This is because impurities diminish the water molecules available for vaporisation during boiling. The heat required to vaporise the same amount of impure solution is more than the heat required to vaporise a pure solution.

Q3. Why is there a rise in temperature when chloroform and acetone are mixed?
Answer:
The bonds between chloroform molecules and acetone molecules are dipole-dipole interactions, but as the chloroform and acetone molecules mix, they begin to create hydrogen bonds, which are stronger bonds that result in the release of energy. This causes an increase in temperature.

Q4. The benzene-toluene binary system is closer to the ideal solution than the hexane-acetic acid binary system. Why?
Answer:
Intermolecular forces (IMFs) of benzene and toluene are comparable while the same is very different for hexane and acetic acid.

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