Distillation
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Why should you not clean your glassware with water immediately before conducting your distillation?
Water forms azeotrophic mixtures with organic compounds The water could vaporize when heated and disrupt the purity of your compound
What two factors determine the vapor composition above a liquid mixture at any given temperature?
First factor - external pressure Second factor - sample purity
what is vapor pressure
it is the pressure the gaseous form of a liquid in equilibrium above the surface of the liquid compound
what is the boiling point
it is the temperature where the vapor pressure equals the external pressure acting on the surface of the liquid (760 torr)
how do you calculate the vapor fraction of compound A in a mix of B and A
Xa= Pa/Ptotal
how do you calculate the tempertaure of boiling?
when Ptotal = 760 mmHg as seen by Ptotal=NaPA + NbPb
what temperatures do pure liquids boil
1-2°C
what is the minimum diffference in boiling points needed to cleanly separate a mixture of two liquids by simple distillation
100°C
what is boiling point a function of
external pressure and purity
what is the vapor phase more enriched in
it is enriched in the lower boiling point component
as the vapor gets richer in ______ the pot gets richer in ______
A, lower BP B, higher BP
First drops of distillate most pure sample of _____; as distillation continues the distillate will contain less ____ and more ______
First drops of distillate most pure sample of A; as distillation continues the distillate will contain less A and more B
why is fractional distillation preferred
it allows multiple vaporization cycles within fractionating column
what determines the efficiency of fractional distillations
the number of theoretrical plates, each plate represents one theoretical plate
why must you heat is slowly?
want upper thermometer to read BP of pure A must heat slow enough to allow vapor-liquid equilibration within fractionating column allows time for less volatile B to condense to liquid and more volatile A to vaporize (allows separation)
for a fractional distillation, what does the lower thermometer read
a boiling point in between A and B
what does the upper thermometer read
the boiling point of compound A
what happens to the upper thermometer as A is fully distilled
it increases to B
what is chromatography
separation of compounds based on their distribution between mobile phase and stationary phase
what determines the retention time in gas chromatography generally
lower boiling compounds elute faster and have a smaller retention time
what determines retention times overall
– boiling point of compound – column length – carrier gas flow rate (easy to modify) – oven temperature (easy to modify)
When ddistilling a compound A and nonvolatile B, what happens to the boiling point
it is elevated
what determines the efficiency of a fractional distillation
Efficiency determined by number of theoretical plates; each theoretical plate represent one vaporization- condensation cycle
what is HEPT
height of fractionating column equivalent to one theoretical plate