NOTES: A.2 – Separation by

Distillation

*Substances can often be separated by taking advantage of their different physical properties… remember the “Foul Water Lab”?

Different liquids may be separated by:

> DENSITY – less dense

liquids will float on

top (oil on top of water)

**this only works if the 2

substances are insoluble in

each other

> BOILING POINT**

**the boiling point of a liquid is the temperature at which the liquid boils or VAPORIZES (becomes a gas)

● the separation of

liquid substances

according to their

differing boiling

points is called

DISTILLATION.

DISTILLATION:

● a liquid mixture is heated

● the substance with the LOWER boiling point will vaporize first and leave the distillation flask

● it is then passed through a condenser where it is converted back to a liquid

● eventually, the second substance begins to boil

● each condensed liquid is called a DISTILLATE and can be collected separately

Distillation Example:

Possible Components of a Distillation Mixture

Substance Formula Boiling Point (°C)

2-propanol

(rub. alcohol)

C3H7OH 82.4°C

Acetone C3H6O 56.5°C

Water H2O 100°C

Cyclohexane C6H12 80.7°C

If were given a mixture containing the 4 liquids shown in the table above, in what order would you collect them using a distillation apparatus?

, , ,

If were given a mixture containing the 4 liquids shown in the table above, in what order would you collect them using a distillation apparatus?

acetone, cyclohexane, 2-propanol, water

NOTES: A.3 – Petroleum Refining

**the techniques of distillation are used in the refining of petroleum, however, crude oil is a complex mixture of many compounds, so the techniques must be applied in a large-scale oil refinery.

● instead of separating crude oil into pure substances (single distillates), the refining process produces several mixtures, called FRACTIONS.

● this process is known as FRACTIONAL DISTILLATION

● the compounds in each fraction have a particular range of boiling points and specific uses

Steps in Oil Refining:

1) the crude oil is heated to about 400°C in a furnace then pumped into a distilling column (fractionating tower…often more then 100 ft tall!);

Steps in Oil Refining:

2) the component substances of the crude oil vaporize – as they rise up the column, they recondense into liquids as they cool;

Steps in Oil Refining:

3) trays are arranged at appropriate heights to collect the various fractions:

-the smaller, lighter molecules have lower boiling points and condense high in the column;

-the larger, heavier molecules condense lower in the column (at higher temps.)

NOTES: A.4 – A Look At Petroleum’s

Molecules

*in the refining process, some fractions never condense to the liquid state – they remain gases!

Gaseous Fraction:

● the gaseous fraction includes:

compounds with LOW boiling points (less than 40°C)

small hydrocarbons (1 - 4 C atoms)

*why do they have such low boiling points?

INTERMOLECULAR FORCES

● INTERMOLECULAR FORCES = forces of attraction between molecules

as a result of weak intermolecular forces (they are only slightly attracted to each other), these small hydrocarbon molecules readily separate from each other and rise through the distillation column as gases

Liquid Fractions:

● the liquid fractions of petroleum (i.e. gasoline, kerosene, and heavier oils) include:

compounds with intermediate boiling

points

medium hydrocarbons (5 - 20 C atoms)

Solid Fraction (Residue)

● the thick, sticky

residue fraction

(never vaporizes;

stays at the bottom) includes:

the largest hydrocarbons with the strongest intermolecular forces (why they never vaporize!)

these compounds are solid at room temp.