Demonstrate an understanding of the theory, principles and practice of separation of liquid mixtures by distillation techniques

b        saturated vapour pressure

Saturated vapour pressure is the pressure existing at a given temperature in a closed vessel containing a liquid and the vapour from that liquid after equilibrium conditions have been reached. It is dependent only on temperature.

c        partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume.

d        total pressure.

Also known as the stagnation pressure.  This is the pressure at which a liquid will come to rest.

2. state and explain the effect of changes in temperature on the vapour pressure of a liquid.

The effects of a change in temperature on the vapour pressure of a liquid follow Le Chatelier’s Principle of Equilibrium, “If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.”

By using this, it is possible to say that if a liquid is in a sealed container and is heated to a certain temperature, some of the liquid will turn to a vapour.  If the temperature is then increased further, then more of the liquid will become a vapour, increasing the vapour pressure that is present.

3.  relate vapour pressures to the boiling points of liquids

a pure liquids

Pure liquids produce a vapour when the pressure at the surface of the vapour equals the pressure of the surrounding atmosphere.  If the pressure of the surrounding atmosphere is lower than at sea level, then the boiling point will be lower.  If the pressure is greater than that of sea level, the boiling point will be raised.

b miscible liquid mixtures

When two fully miscible liquids are in a mixture and heated, the vapour pressure will not have the same composition as the liquid.  Instead, the resulting vapour mainly consists of the more volatile of the two liquids, the one with the lower boiling point.  

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c immiscible liquid mixtures.

In the case when two immiscible liquids are sealed in a container, the vapour pressure that is measured is only the pressure of the liquid that is floating on top of the other.  If the two liquids are agitated, the vapour pressure and the boiling point of the liquid mixture will be lower than that of both the two liquids.

5. state and explain Dalton’s law of partial pressures.

Dalton's Law of Partial Pressure:

The pressure of a mixture of gases is equal to the sum of the pressures of all of the constituent gases alone.

Mathematically, this can be represented as:

PressureTotal = Pressure1 + Pressure2 ... Pressuren 

This basically says that the total pressure is the same as the sum of each individual component pressure.

6. state and explain how Raoult’s law can be used to calculate the total vapour pressure of ideal binary liquid mixtures.

Raoult’s Law states that: The vapour pressure of a solution of a non-volatile solute is equal to the vapour pressure of the pure solvent at that temperature multiplied by its mole fraction.

It can be used to calculate the total vapour pressure of an ideal binary liquid mixture as using the mathematical equation, derived from the number of different liquids involved and their concentrations.

7. distinguish between ideal and non-ideal liquid mixtures.

An ideal mixture contains two liquids which are both entirely miscible within each other.  When these are boiled, the boiling point is located approximately half way between the boiling points of both substances involved.  Any vapour produced will contain the same concentration of each component as the liquid does.

This is the opposite in a non-ideal mixture.  If neither of the liquids involved are miscible with each other, then they will form two layers in the container.  If a vapour is produced, then the vapour will only consist of the liquid component situated at the top of the container.  This is always the liquid with the lowest density and the lowest boiling point, hence why it will be found at the surface and also produce a vapour earlier.

8. explain the effects of boiling point differences on the separation of miscible liquids.

When separating miscible liquids, very accurate equipment is required.  This is because the boiling temperatures lower when two miscible liquids are mixed.  This can cause problems when separating them.  The most effective way is to constantly heat them up, while undergoing a reflux as they will completely separate over time, both condensing at different levels.

9. list common distillation techniques and explain the principles behind each one

a        simple batch

Simple batch distillation is where one container is filled with a liquid, it is heated up and is drawn off at different levels depending on its boiling points.  This is used when there is a specific product needing to be made once.

b        flash

Flash distillation occurs when liquid is heated in a low pressure vessel, effectively making the boiling point and flash point of each component lower than at atmospheric pressure.

c        continuous fractionation

Continuous fractionation is the normal method employed by a refinery.  This is where there is a continuous feed of a liquid into a vessel at atmospheric pressure.  The vessel can contain various methods of extraction including having trays which catch any liquid that forms and runs it out of the vessel.

d        batch fractionation

Batch fractionation is the same as simple batch distillation.

e        azeotropic

An azeotrope is a liquid mixture that distils without change in composition.  Azeotropic distillation incorporates the use of an entrainer, or solvent, to help facilitate the distillation process.  This is to lower the boiling point of the azeotrope that is contained in the solution so that full distillation can occur.  If this entrainer were not present, then the maximum level of distillation that would be possible would be up to approximately 96% which is not pure enough for certain processes.

f        steam

Steam distillation is used when temperature sensitive materials are involved in the distillation process.  The steam is used to lower the boiling points of the compounds involved in the process due to temperature sensitive, organic compounds decomposing and breaking down at high, sustained temperatures.  If the temperatures are still too high, this process can also be combined with vacuum distillation.

g        vacuum

Vacuum distillation lowers the pressure in the top of the tower, thus reducing the vapour pressure and lowering the boiling point of the fluids contained.  This makes evaporation of the components with a very high boiling point easier, producing more saleable product.

h        pressure.

Pressure distillation is another term for vacuum distillation.