Experiment 6
Simple and Fractional Distillation
Vapor Pressure vs Temperature of Water
Temperature, °C
0 20 40 60 80 100 120 140
Vap
or P
ress
ure
(kPa
)
0
50
100
150
200
250 Vapor Pressure vs Temperature of Water
Temperature, °C
0 20 40 60 80 100 120 140
Vap
or P
ress
ure
(kP
a)
0
50
100
150
200
250
Vapor Pressure vs Temperature of Water
Temperature, °C
0 20 40 60 80 100 120 140
Vap
or P
ress
ure
(kP
a)
0
50
100
150
200
250
What is and what do the following measure?
vapor pressure The vapor pressure of a pure substance is the pressure exerted by the substance against the external pressure which is usually atmospheric pressure. Vapor pressure is a measure of the tendency of a condensed substance to escape the condensed phase.
boiling point: When the vapor pressure of a liquid substance reaches the external pressure, the substance is observed to boil.
normal boiling point: When the external pressure is atmospheric pressure, the temperature at which a pure substance boils is called the normal boiling point.
Hvap is the energy necessary to vaporize a mole of a pure substance
Aliq Bliq
Avap Bvap
HvapB
HvapA
f mixing liq A and liq B and mixing gas A and gas B results in no heat of mixing in both the liquid and gas phase, the system is considered to be ideal Then:
PA obs = APoA; PB obs = BPo
B
where PA obs; PB obs is the observed vapor pressure in the mixture;
A; B is the mole fraction of A, B, ... A = nA/ (nA +nB)
B = nB/ (nA +nB)
and PoA; Po
B are the vapor pressures of pure A and B
A diagram of a simple distillation apparatus
liq vapor liqThis process is referred to as one theoretical plate
At what temperature will a 1:1 molar mixture of cyclohexane-methylcyclohexane boil?
PT = PA obs + PB obs
PT obs = APoA + BPo
B
A = B = 0.5
1 Atm = 101 kPa
T/K C/ kPa M/ kPa T/K C/ kPa M /kPa
300 14.1 6.7 354 101.3 55.4
305 17.6 8.5 360 121.3 66.9
310 21.7 10.6 362 128.5 71.1
315 26.5 13.2 365 139.9 77.9
320 32.2 16.2 370 160.5 90.2
325 38.8 19.8 373 174 101.3
330 46.5 24 380 208.8 119.3
335 55.3 28.9 385 236.7 136.4
340 65.4 34.6 390 267.3 155.3
345 77 41.2 395 300.9 176.2
350 90 48.7 400 337.5 199.1
Vapor Pressures of Pure Cyclohexane (C); Methylcyclohexane (M)
T/K C/ kPa M/ kPa T/K C/ kPa M /kPa
300 14.1 6.7 354 101.3 55.4
305 17.6 8.5 360 121.3 66.9
310 21.7 10.6 362 128.5 71.1
315 26.5 13.2 365 139.9 77.9
320 32.2 16.2 370 160.5 90.2
325 38.8 19.8 373 174 101.3
330 46.5 24 380 208.8 119.3
335 55.3 28.9 385 236.7 136.4
340 65.4 34.6 390 267.3 155.3
345 77 41.2 395 300.9 176.2
350 90 48.7 400 337.5 199.1
C = Cyclohexane; M = Methylcyclohexane
At what temperature will a 1:1 molar mixture of cyclohexane-methylcyclohexane boil?
PT = PA obs + PB obs
PT obs = APoA + BPo
B
A = B = 0.5
1 Atm = 101 kPa
T/K C /kPa M /kPa362 128.5 71.1
PT obskPa ≈ 1 atm
What is the composition of the vapor at this temperature?
T/K C /kPa M /kPa362 128.5 71.1
PCVC = nCRT ; PMVM = nMRT
nC/nM = PC/PM =
128.5/71.1 = 1.8/1
An apparatus for fractional distillation
5 theoretical plates can be identified
A look at a schematic diagram of a gas chromatograph:
Instrumental componentsCarrier gas The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.
Sample injection port For optimum column efficiency, sample sizes should be small and should be introduced onto the column as a "plug" of vapor. The most common injection method is where a microsyringe is used to inject sample through a rubber septum into a heated port at the head of the column. SPME is an alternative method for introducing your sample. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. In preparative GC, sample sizes as much as a mL can be used in certain cercumstances. Capillary columns, on the other hand, need much less sample, typically less than 10-3 mL. For capillary GC, split/splitless injection is used.
ColumnsThere are two general types of column, packed and capillary. Packed columns contain a finely divided, inert, solid support material (commonly diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 6mm. Liquid coatings vary depending on polarity. Most are non-volatile waxes or organic liquids.
Fused silica open tubular columns have much thinner walls than the glass capillary columns, and are given strength by the polyimide coating. These columns are flexible and can be wound into coils. They have the advantages of physical strength, flexibility and low reactivity. Temperature range of use varies from room temperature to about 300 ° C, depending on the coating. Cyclodextrin is bonded in chiral columns.
Capillary column
It is not unusual for a capillary column to have more an efficiency characterized by more than 100,000 theoretical plates.
DetectorsThere are many detectors which can be used in gas chromatography. Different detectors will give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. The FID is mostly commonly used.
Flame ionization (FID)
Mass flow
Hydrogen and air Most organic cpds.100 pg
107
Thermal conductivity (TCD)
Concentration
Reference Universal 1 ng
107
Electron capture (ECD)
Concentration
Make-upHalides, nitrates, nitriles, peroxides, anhydrides, organometallics
50 fg
105
Nitrogen-phosphorus
Mass flow
Hydrogen and air Nitrogen, phosphorus 10 pg
106
Flame photometric (FPD)
Mass flow
Hydrogen and air possibly oxygen
Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium
100 pg
103
Photo-ionization (PID)
Concentration
Make-up Aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulphurs, some organometallics
2 pg
107
Hall electrolytic conductivity
Mass flow
Hydrogen, oxygen Halide, nitrogen, nitrosamine, sulphur
