NOTES: A'2 Separation by Distillation

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NOTES A.2 Separation by Distillation
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• Substances can often be separated by taking
  advantage of their different physical properties
  remember the Foul Water Lab?

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Different liquids may be separated by

• gt 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
• gt BOILING POINT

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

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• ? the separation of
• liquid substances
• according to their
• differing boiling
• points is called
• DISTILLATION.

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? 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

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Distillation Example
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• 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?
• , , ,

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• 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

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NOTES A.3 Petroleum Refining
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• 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.

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• ? 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

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Steps in Oil Refining

• 1) the crude oil is heated to about 400C in a
  furnace then pumped into a distilling column
  (fractionating toweroften more then 100 ft
  tall!)

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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

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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.)

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NOTES A.4 A Look At Petroleums Molecules
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• in the refining process, some fractions never
  condense to the liquid state they remain gases!

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Gaseous Fraction

• ? the gaseous fraction includes
• ? compounds with LOW boiling points (less than
  40C)
• ? small hydrocarbons (1 - 4 C atoms)
• why do they have such low boiling points?

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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

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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)

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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.

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