DISTILLATION

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DISTILLATION

• ORG I LAB

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Vapor Pressure

• For a liquid, at any temperature, some molecules
  are evaporating from the surface.
• As the temperature goes up, the number of
  molecules evaporating in a given time increases.
• If the liquid is in an open container, the
  molecules escape into the atmosphere.
• In a closed container, some of the vapor
  molecules strike the walls of the container and
  return to the liquid.
• Soon a state of equilibrium is reached in which
  over any time period, the number of molecules
  evaporating number of molecules condensing back
  to the liquid.
• The pressure of the vapor at this point is known
  as its vapor pressure.

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THE BOILING POINT

• The vapor pressure of a pure liquid rises
  steadily as the temperature is increased .
• The Boiling Point of a liquid is the temperature
  at which vapor pressure is equal to the external
  pressure.
• The normal boiling point of a liquid is the
  temperature at which its vapor pressure is 760
  torr, normal atmospheric pressure at sea level

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Distillation

• In a Distillation a liquid is heated to its
  boiling point, the vapors expand out of the
  container and are then cooled below the boiling
  point so that they recondense as a liquid
• An apparatus for a simple distillation is shown
  to the right.
• During a simple distillation, the liquid is
  vaporized and condensed one time.
• The thermometer measures the temperature at which
  the liquid is currently boiling.

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THE TEMP/TIME RELATIONSHIP

• The temperature remains constant throughout the
  distillation of a pure liquid. At the boiling
  point, the liquid and vapor are in
  equilibrium...if the composition of each phase
  remains constant, the temperature will remain
  constant
• A mixture of two volatile liquids begins to
  distill at a temperature slightly higher than the
  boiling point of the lower boiling liquid, the
  temperature rises steadily over time ending at a
  temperature slightly below that of the higher
  boiling component.
• The composition of the liquid and vapor in
  equilibrium changes constantly over time also.
  At the beginning the vapor contains mostly A, at
  the end mostly B.

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Raoults Law

• For a mixture of two miscible liquids (A and B),
  the total vapor pressure is the sum of the
  individual vapor pressures (Daltons Law)
• Ptotal PA PB
• The vapor pressures of the individual components
  is given by Raoults Law.
• PA XAPA And PB XB PB
• where
• PA is the vapor pressure of pure A and PB is
  the vapor pressure of pure B
• and
• XAand XB are the mole fractions of A and B in the
  liquid
• where
• XA nA/(nA nB) and XB nB/(nA nB)
• where
• nA and nB are the number of moles of each
  component in the liquid.

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Daltons Law

• According to Daltons Law of Partial Pressures
• PA XAPt PB XBPt
• where
• PA and PB are the partial pressures of the two
  gases
• and
• Pt is the total pressure (Pt PA PB)
• and
• XA and XB are the mole fractions of A an B
  respectively in the vapor.
• and
• XA nA/(nA nB) , XB nB/(nA nB)
• where
• nA and nB are the number of moles of each
  component in the vapor.

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Vapor Enrichment

• Combining Raoults Law and Daltons law, we can
  obtain the following relationship
• If A is more volatile than B, BPA lt BPB and PA gt
  PB
• then
• XA/XB gt XA/XB
• The ratio of A/B in the vapor is greater than the
  ratio of A/B in the liquid. The vapor is
  enriched in the more volatile (lower boiling)
  component relative to the liquid.
• During the distillation, since the vapor always
  contains more A than B, the fraction of B in the
  liquid increases continuously causing the boiling
  point of the solution to increase

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Distillation

• This figure represents the LIQUID-VAPOR
  COMPOSITION DIAGRAM FOR A TWO COMPONENT MIXTURE
• When a mixture AB of a is heated, the total vapor
  pressure (composed of the contributions of PA and
  PB) will rise until it is equal to the external
  vapor pressure.
• The temperature at which this occurs for various
  compositions of the liquid is show in the lower
  curve.

• In this example, the liquid having composition
  W boils at temperature t.
• The vapor in equilibrium with the liquid has
  composition y.
• The vapor condenses to give liquid of
  composition Z.
• After the first drop of liquid distills, the
  fraction of B in the liquid increases slightly,
  increasing the boiling point of the solution
• The composition of both the liquid and vapor
  changes continuously.

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Fractional Distillation

• Fractional distillation is only useful for
  separating compounds which have very different
  boiling points.
• Fractional distillation can be used to separate
  liquids with similar boiling points.
• On its way up the fractionating column, the vapor
  condenses and revaporized many times.
• At each stage of condensation/ revaporization,
  the vapor is further enriched in the lower
  boiling component.

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Fractional Distillation

• AB at composition of 5 A boils at temperature L1
  and the vapors with composition V1 enter the
  column at that temperature. The vapor will
  condense to a liquid with composition V1. The
  condensate L2 has a lower boiling point (because
  it has more of the lower boiling liquid A) and
  will thus vaporize at a lower temperature (warmed
  up by coming in contact with the additional
  vapors from below) to give vapors of composition
  V2. These vapors will condense somewhat farther
  up the column to give a condensate L3. If the
  column is long enough or contains sufficient
  surface area that many successive
  vaporization-condensation steps (theoretical
  paltes) can occur, the distillate that comes over
  the top is nearly pure A. Distillation yielding
  pure A continues until all of A is removed, after
  which the temperature at the thermometer rises to
  the boiling point of B.

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Fractional Distillation Set-up
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Proper Thermometer Depth