Experiment 6 Simple and Fractional Distillation

1. Experiment 6 Simple and Fractional Distillation

3. 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.

4. DHvap is the energy necessary to vaporize a mole of a pure substance Avap Bvap DHvapA DHvapB Aliq Bliq

5. If 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 = BPoB 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; PoB are the vapor pressures of pure A and B

6. A diagram of a simple distillation apparatus liq vapor liq This process is referred to as one theoretical plate

7. At what temperature will a 1:1 molar mixture of cyclohexane-methylcyclohexane boil? PT = PA obs +PB obs PT obs = APoA + BPoB A = B = 0.5 1 Atm = 101 kPa

8. Vapor Pressures of Pure Cyclohexane (C); Methylcyclohexane (M)

9. C = Cyclohexane; M = Methylcyclohexane

10. At what temperature will a 1:1 molar mixture of cyclohexane-methylcyclohexane boil? PT = PA obs +PB obs PT obs = APoA + BPoB A = B = 0.5 1 Atm = 101 kPa T/K C /kPa M /kPa 362 128.5 71.1 PT obs = 0.5*128.5 +0.5*71.1 = 64.3 + 35.6 = 99.9 kPa ≈ 1 atm

11. What is the composition of the vapor at this temperature? T/K C /kPa M /kPa 362 128.5 71.1 PCVC = nCRT ; PMVM = nMRT nC/nM = PC/PM = 128.5/71.1 = 1.8/1

12. An apparatus for fractional distillation 5 theoretical plates can be identified

14. A look at a schematic diagram of a gas chromatograph:

15. Instrumental components Carrier 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.

17. Columns There 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. Capillary column 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.

18. It is not unusual for a capillary column to have more an efficiency characterized by more than 100,000 theoretical plates.

19. Detectors There 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-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) 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