Distillation

Question 1

Why should you not clean your glassware with water immediately before conducting
your distillation?

Answer 1

Water forms azeotrophic mixtures with organic compounds

The water could vaporize when heated and disrupt the purity of your compound

Question 2

What two factors determine the vapor composition above a liquid mixture at any given temperature?

Answer 2

First factor - external pressure
Second factor - sample purity

Question 3

what is vapor pressure

Answer 3

it is the pressure the gaseous form of a liquid in equilibrium above the surface of the liquid compound

Question 4

what is the boiling point

Answer 4

it is the temperature where the vapor pressure equals the external pressure acting on the surface of the liquid (760 torr)

Question 5

how do you calculate the vapor fraction of compound A in a mix of B and A

Answer 5

Xa= Pa/Ptotal

Question 6

how do you calculate the tempertaure of boiling?

Answer 6

when Ptotal = 760 mmHg as seen by Ptotal=NaPA + NbPb

Question 7

what temperatures do pure liquids boil

Answer 7

1-2°C

Question 8

what is the minimum diffference in boiling points needed to cleanly separate a mixture of two liquids by simple distillation

Answer 8

100°C

Question 9

what is boiling point a function of

Answer 9

external pressure and purity

Question 10

what is the vapor phase more enriched in

Answer 10

it is enriched in the lower boiling point component

Question 11

as the vapor gets richer in ______ the pot gets richer in ______

Answer 11

A, lower BP

B, higher BP

Question 12

First drops of distillate most pure sample of _____; as
distillation continues the distillate will contain less ____ and
more ______

Answer 12

First drops of distillate most pure sample of A; as
distillation continues the distillate will contain less A and more B

Question 13

why is fractional distillation preferred

Answer 13

it allows multiple vaporization cycles within fractionating column

Question 14

what determines the efficiency of fractional distillations

Answer 14

the number of theoretrical plates, each plate represents one theoretical plate

Question 15

why must you heat is slowly?

Answer 15

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)

Question 16

for a fractional distillation, what does the lower thermometer read

Answer 16

a boiling point in between A and B

Question 17

what does the upper thermometer read

Answer 17

the boiling point of compound A

Question 18

what happens to the upper thermometer as A is fully distilled

Answer 18

it increases to B

Question 19

what is chromatography

Answer 19

separation of compounds based on their distribution between mobile phase and stationary phase

Question 20

what determines the retention time in gas chromatography generally

Answer 20

lower boiling compounds elute faster and have a smaller retention time

Question 21

what determines retention times overall

Answer 21

– boiling point of compound
– column length
– carrier gas flow rate (easy to modify)
– oven temperature (easy to modify)

Question 22

When ddistilling a compound A and nonvolatile B, what happens to the boiling point

Answer 22

it is elevated

Question 23

what determines the efficiency of a fractional distillation

Answer 23

Efficiency determined by number of
theoretical plates; each theoretical
plate represent one vaporization-
condensation cycle

Question 24

what is HEPT

Answer 24

height of fractionating column
equivalent to one theoretical plate

Question 25

in a fractionating column, what will the pot BP be between

Answer 25

A and B

Question 26

Thermal Conductivity Detector (TCD

Answer 26

Monitors changes in heat lost from
hot filament to carrier gas. When
sample elutes, sample will
conduct heat different than pure
He, so we can measure a signal

Question 27

what is an azeotrope

Answer 27

mixture that distills at constant temperature and composition

Question 28

whats a high boiling azeotrope

Answer 28

it requires hgih temperatures, Ptotal is less than Pa + PB, strong imfs

Question 29

what is a low boiling azeotrope

Answer 29

it requires low temps, Ptotal is more than PA + PB, weak imfs