PHARCAL - EX15 (DISTILLATION)

Vapor Pressure

is a measure of its tendency to change into gaseous or vapor state

The higher the vapor pressure of a substance is

the more this tends to be in the vapor state

Volatile substances

Substances whose molecules are held together by weak intermolecular attractive forces exhibit this characteristic.

Boiling point

the temperature at which the vapor pressure equals atmospheric pressure

At this temperature, thermal agitation can overcome the attractive forces between the molecules of the liquid

resulting in vapors seen to form bubbles which rise rapidly through the liquid and escape through the gaseous state.

Application of heat increases the kinetic energy of particles in the liquid state.

This kinetic energy of particles results in their escape from the liquid state to the gaseous (vapor) state; thereby, increasing the vapor pressure.

Boiling point

constant for identical molecules

Distillation

the basis for the method of purification of organic compounds

Distillation

the process of vaporizing a substance, condensing the vapor, and collecting the condensate in another container.

Distillation technique

useful for separating a mixture when the components have different boiling points

Distillation

It is the principal method of purifying a liquid.

Types of distillation

Simple distillation

Vacuum distillation

Fractional distillation

Steam distillation

Simple distillation

a large temperature difference between boiling points of the components to be purified is required to obtain an efficient separation.

temperature difference between boiling points 30 to 50 degrees celsius

This makes the technique of simple distillation inefficient for the separation of components

grossly impure materials

result from simple distillation

Simple distillation

considered as a means of purification if carried out slowly enough so that superheating of the vapor is avoided.

Superheating

leads to decreased efficiency of separation and to erroneous temperature readings.

Irregular ebullition or Bumping

superheating results

irregular ebullition can be prevented

few chips of porous plate (porcelain chips or commercial microporous boiling chips) or capillary tubes are added to the reaction system in the flask

Vacuum distillation

distillation under reduced pressure

vacuum distillation

employed in the purification of organic compounds which partially or completely decompose before their normal boiling points are reached.

Vacuum distillation

carried out at a temperature lower than the normal boiling point as a result of the reduction of pressure in the system.

rotary evaporator (Rotavap)

is an equipment that is commonly used in the laboratory. This is used in the concentration of plant extracts in volatile solvents such as ethanol and ethyl acetate, both of which do not easily evaporate at room temperature and atmospheric pressure.

Fractional distillation

successive distillations are carried out in an automatic way without collecting or transferring the immediate fractions.

Hempel column

It is a simple glass tube filled to within a few cm of the side arm with a suitable packing (³/₁₆ in. by ³/₁₆in. glass or porcelain rings or crushed carborundum-6 mesh)

Vigreux column

bubble-cap columns, and the modified Hempel column

Steam distillation

the organic compound purified co-distills with water. This is effected by passing steam through a hot mixture of the compound and water. If a mixture of two immiscible liquids is distilled, the boiling point will be the temperature at which the sum of the individual vapor pressures is equal to that of the atmosphere.

Dalton’s law of partial pressures

“When two or more gases or vapors which do not react chemically with one another are mixed at constant temperature, the sum of their individual vapor pressures is equal to the total pressure exerted by the system.”

Steam distillation

The boiling point of the mixture to be steam distilled will be lower than the boiling point of even the more volatile component. Since of the components is water, steam distillation at atmospheric pressure will result in the separation of the higher boiling point component at a temperature below 100⁰C