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

(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

(kPa

)

0

50

100

150

200

250

What is and what do the following measure?

vapor pressureThe 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.

ΔHvap is the energy necessary to vaporize a mole of a pure substance

Aliq Bliq

Avap Bvap

ΔHvapB

ΔHvapA

Ιf 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 = χAPoA; PB obs = χBPo

B

where PA obs; PB obs is the observed vapor pressure in the mixture;

χA; χB is the mole fraction of A, B, ... χA = nA/ (nA +nB)

χB = 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 asone theoretical plate

At what temperature will a 1:1 molar mixture of cyclohexane-methylcyclohexane boil?

PT = PA obs + PB obs

PT obs = χAPoA + χBPo

B

χA = χB = 0.5

1 Atm = 101 kPa

T/K PC/ kPa PM/ kPa T/K PC/ kPa PM /kPa300 14.1 6.7 354 101.3 55.4305 17.6 8.5 360 121.3 66.9310 21.7 10.6 362 128.5 71.1315 26.5 13.2 365 139.9 77.9320 32.2 16.2 370 160.5 90.2325 38.8 19.8 373 174 101.3330 46.5 24 380 208.8 119.3335 55.3 28.9 385 236.7 136.4340 65.4 34.6 390 267.3 155.3345 77 41.2 395 300.9 176.2350 90 48.7 400 337.5 199.1

Pure C = Cyclohexane; Pure M = Methylcyclohexane

At what temperature will a 1:1 molar mixture of cyclohexane-methylcyclohexane boil?

PT = PA obs + PB obs

PT obs = χAPoA + χBPo

B

χA = χB = 0.5

1 Atm = 101 kPa

T/K C /kPa M /kPa362 128.5 71.1

PT obs = 0.5∗128.5 +0.5∗71.1 = 64.3 + 35.6 = 99.9 kPa ≈ 1 atm

T/K PC/ kPa PM/ kPa T/K PC/ kPa PM /kPa300 14.1 6.7 354 101.3 55.4305 17.6 8.5 360 121.3 66.9310 21.7 10.6 362 128.5 71.1315 26.5 13.2 365 139.9 77.9320 32.2 16.2 370 160.5 90.2325 38.8 19.8 373 174 101.3330 46.5 24 380 208.8 119.3335 55.3 28.9 385 236.7 136.4340 65.4 34.6 390 267.3 155.3345 77 41.2 395 300.9 176.2350 90 48.7 400 337.5 199.1

As a 1:1 Mixture; C = Cyclohexane; M = Methylcyclohexane

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

At what temperature will this ratio of C/M distill?T/K C /kPa M /kPa360 121.3 66.9

=(1.8/1.8+1)*121.3 + 1/(1.8+1)*66.9 = 77.8+23.8 = 101

T/K PC/ kPa PM/ kPa T/K PC/ kPa PM /kPa300 14.1 6.7 354 101.3 55.4305 17.6 8.5 360 121.3 66.9310 21.7 10.6 362 128.5 71.1315 26.5 13.2 365 139.9 77.9320 32.2 16.2 370 160.5 90.2325 38.8 19.8 373 174 101.3330 46.5 24 380 208.8 119.3335 55.3 28.9 385 236.7 136.4340 65.4 34.6 390 267.3 155.3345 77 41.2 395 300.9 176.2350 90 48.7 400 337.5 199.1

As a 1:1 Mixture; C = Cyclohexane; M = Methylcyclohexane

At what temperature will this ratio of C/M distill?T/K C /kPa M /kPa360 121.3 66.9=(1.8/1.8+1)∗121.3 + 1/(1.8+1)∗66.9 = 77.8+23.8 = 101

Summary

First theoretical plate ratio 1.8/1; BP 362 K

Second theoretical plate ~77.8/23.8 = 3.26; BP 360 K

An apparatus for fractional distillation

5 theoretical plates can be identified

A look at a schematic diagram of a gas chromatograph:

Instrumental componentsCarrier gasThe 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 portFor 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 detectorresponds 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-up Halides, 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) Concen

trationMake-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