When the difference between the boiling point of two or more than two liquids is more than 25k?

PURPOSE

Fractional distillation is a technique used to separate miscible liquids that have boiling point difference of less than 25 ºC. 

SIMPLE vs. FRACTIONAL DISTILLATION

The only difference in apparatus between simple and fractional distillation methods is the use of a fractionating column used during fractional distillation.  The fractionating column is placed directly above the boiling flask as shown below.  The fractionating column may be packed with a variety of materials (a glass helices packed column is used in Chem0330). 

COLUMN PACKING

Each type of packing has a known HETP (height equivalent to a theoretical plate) that is a measure of efficiency.  The lower the HETP (measured in cm) the more 'theoretical plates' contained within a column.  A theoretical plate is simply the distance it takes within the column for one complete vaporization-condensation cycle (or simple distillation) to occur.  Because vaporization and condensation are equilibrium processes, the most important factor in carrying out a good fractional distillation experiment is heating slowly.  This allows for equilibrium to be established throughout the column and the liquids contained in the mixture to separate.  The best fractional distillation experiments have a high reflux ratio; liquid continually condenses back into the boiling flask

APPARATUS

DATA

The same type of information can be collected for fractional distillation as was for simple distillation.  However, fractional distillation is usually carried out to separate two miscible liquids.  As the hot vapors pass through the column, they become successively more rich in the lower boiling component.  This vapor is condensed and the 'fraction' collected.  The temperature can be recorded to determine the compound's identity.  This leaves the high boiling point component in the boiling flask.  When enough energy is added to the boiling flask, the high boiling point component will boil, ascend the column as a gas, and condense as a second fraction.  In this way, the two components can be isolated and their respective boiling points recorded for purposes of identification.

The identity of the each unknown can be determined.  In this example, the low boiling point component is methanol (bp = 65.0 ºC) and the high boiling point component is isopropyl acetate (bp = 88.0 ºC).  If 50 mL of the unknown mixture was distilled, then the mixture would have contained approximately 20 mL of methanol and 30 mL of isopropyl acetate.  The decrease in temperature at 20 mL is a result of having completely distilled the methanol.  The isopropyl acetate does not have enough energy to boil; its vapor has not reached the thermometer bulb and so the temperature the thermometer reads falls until this component reaches the bulb.  If this occurs, increasing the setting on the variac will provide the necessary energy (in the form of heat) to distill the second component. 

Question 33 Is Matter Around Us Pure ? - Exercise

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Answer:

Fractional column is the most important part of fractional distillation apparatus. It is provided with glass beads in it. This column helps to obstruct the upward movement of the vapours of the two liquids. The vapours of high boiling liquid get condensed earlier at a lower level. Latent heat released helps to take the vapours of low boiling liquid to a height in the fractionating column.

The advantages are as given below

1. This method can separate the liquids with a boiling point difference about or less than 25 K,
2. During the process, both evaporation and condensation take place simultaneously.
3. A mixture (like petroleum) can also be separated by fractional distillation process which contains several components.

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To separate the components of a mixture using the following techniques:

• Separating funnel
• Chromatography
• Centrifugation
• Simple distillation
• Fractional distillation

The Theory

Most materials in our surroundings are mixtures of two or more components. Mixtures are either homogeneous or heterogeneous. Homogeneous mixtures are uniform in composition, but heterogeneous mixtures are not uniform in composition.

Air is a homogeneous mixture and oil in water is a heterogeneous mixture. Homogeneous and heterogeneous mixtures can be separated into their components by several physical methods. The choice of separation techniques is based on the type of mixture and difference in the chemical properties of the constituents of a mixture.

What are types of separation techniques?

Various types of separation processes are: 

• Crystallization
• Filtration
• Decantation
• Sublimation
• Evaporation
• Simple distillation
• Fractional distillation
• Chromatography
• Centrifugation
• Separating funnel
• Magnetic separation
• Precipitation

Let’s discuss some of the separation techniques

Using a separating funnel:

A separating funnel is used for the separation of components of a mixture between two immiscible liquid phases. One phase is the aqueous phase and the other phase is an organic solvent. This separation is based on the differences in the densities of the liquids. The liquid having more density forms the lower layer and the liquid having less density forms the upper layer.

Applications:  

• To separate a mixture of oil and water.
• To separate a mixture of kerosene oil and water.

Chromatography:

Chromatography is a separation technique used to separate the different components in a liquid mixture. It was introduced by a Russian Scientist Michael Tswett. Chromatography involves the sample being dissolved in a particular solvent called mobile phase. The mobile phase may be a gas or liquid. The mobile phase is then passed through another phase called stationary phase. The stationary phase may be a solid packed in a glass plate or a piece of chromatography paper.

The various components of the mixture travel at different speeds, causing them to separate. There are different types of chromatographic techniques such as column chromatography, TLC, paper chromatography, and gas chromatography.

Paper chromatography is one of the important chromatographic methods. Paper chromatography uses paper as the stationary phase and a liquid solvent as the mobile phase. In paper chromatography, the sample is placed on a spot on the paper and the paper is carefully dipped into a solvent. The solvent rises up the paper due to capillary action and the components of the mixture rise up at different rates and thus are separated from one another.

Applications:  

• To separate colors in a dye.
• To separate pigments from natural colors.
• To separate drugs from blood.

Centrifugation:

Sometimes the solid particles in a liquid are very small and can pass through a filter paper. For such particles, the filtration technique cannot be used for separation. Such mixtures are separated by centrifugation. So, centrifugation is the process of separation of insoluble materials from a liquid where normal filtration does not work well. The centrifugation is based on the size, shape, and density of the particles, viscosity of the medium, and the speed of rotation. The principle is that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.

The apparatus used for centrifugation is called a centrifuge. The centrifuge consists of a centrifuge tube holder called rotor. The rotor holds balanced centrifugal tubes of equal amounts of the solid-liquid mixture. On rapid rotation of the rotor, the centrifuge tubes rotate horizontally and due to the centrifugal force, the denser insoluble particles separate from the liquid. When the rotation stops, the solid particles end up at the bottom of the centrifuge tube with liquid at the top.

Applications: 

• Used in diagnostic laboratories for blood and urine tests.
• Used in dairies and home to separate butter from cream.
• Used in washing machines to squeeze water from wet clothes.

Simple distillation:

Simple distillation is a method used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.

The distillation process involves heating a liquid to its boiling points, and transferring the vapors into the cold portion of the apparatus, then condensing the vapors and collecting the condensed liquid in a container. In this process, when the temperature of a liquid rises, the vapor pressure of the liquid increases. When the vapor pressure of the liquid and the atmospheric pressure reach the same level, the liquid passes into its vapor state. The vapors pass over the heated portion of the apparatus until they come into contact with the cold surface of the water-cooled condenser. When the vapor cools, it condenses and passes down the condenser and is collected into a receiver through the vacuum adapter.

 Applications:  

• Separation of acetone and water.
• Distillation of alcohol.

Fractional distillation:

Fractional distillation is used for the separation of a mixture of two or more miscible liquids for which the difference in boiling points is less than 25K. The apparatus for fractional distillation is similar to that of simple distillation, except that a fractionating column is fitted in between the distillation flask and the condenser.

A simple fractionating column is a tube packed with glass beads. The beads provide surface for the vapors to cool and condense repeatedly. When vapors of a mixture are passed through the fractionating column, because of the repeated condensation and evaporation, the vapors of the liquid with the lower boiling point first pass out of the fractionating column, condense and are collected in the receiver flask. The other liquid, with a slightly higher boiling point, can be collected in similar fashion in another receiver flask.

Applications:

• Separation of different fractions from petroleum products.
• Separation of a mixture of methanol and ethanol.

Learning outcomes

1. Student understands the following terms: solvent extraction, chromatography, RF , centrifugation, simple  distillation, fractional distillation, etc.
2. Student acquires skills to perform experiments using the following techniques in the chemistry lab:
   • Solvent extraction using separating funnel
   • Paper Chromatography
   • Centrifuge machine
   • Simple distillation & fractional distillation
3. Student can identify the components in the mixture based on the knowledge of Rf value, boiling point, density, etc.
4. Student is able to select & design the  above techniques based on the  chemical and physical properties  of the components in the mixture.
5. Students  acquire the skill  to arrange the requirements for each technique through the animations, simulators and videos.
6. Students will be able to select and perform suitable separation techniques based on the available information about the nature of the components in the mixture.

Let’s try out these separation techniques……

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