Experiment 6 Simple and Fractional Distillation

Transcription

1 Experiment 6 Simple and Fractional Distillation

2 Vapor Pressure vs Temperature of Water Vapor Pressure vs Temperature of Water 25 Vapor Pressure vs Temperature of Water 25 Vapor Pressure (kpa) (kpa) Vapor Pressure (kpa) Temperature, C Temperature, Temperature, C C

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 ΔH vap is the energy necessary to vaporize a mole of a pure substance A vap B vap ΔH vap A ΔH vap B A liq B liq

5 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: P A obs = χ A P o A ; P B obs = χ B Po B where P A obs ; mixture; P B obs is the observed vapor pressure in the χ A; χ B is the mole fraction of A, B,... χ A = n A / (n A +n B ) χ B = n B / (n A +n B ) and P o A ; P o B 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 cyclohexanemethylcyclohexane boil? P T = P A obs +P B obs P T obs = χ A P o A + χ B P o B χ A = χ B =.5 1 Atm = 11 kpa

8 Pure C = Cyclohexane; Pure M = Methylcyclohexane T/K P C / kpa P M / kpa T/K P C / kpa P M /kpa

9 At what temperature will a 1:1 molar mixture of cyclohexanemethylcyclohexane boil? P T = P A obs +P B obs P T obs = χ A P o A + χ B P o B χ A = χ B =.5 1 Atm = 11 kpa T/K C /kpa M /kpa P T obs = = = 99.9 kpa 1 atm

10 As a 1:1 Mixture; C = Cyclohexane; M = Methylcyclohexane T/K P C / kpa P M / kpa T/K P C / kpa P M /kpa

11 What is the composition of the vapor at this temperature? T/K C /kpa M /kpa P C V C = n C RT ; P M V M = n M RT n C /n M = P C /P M = 128.5/71.1 = 1.8/1 At what temperature will this ratio of C/M distill? T/K C /kpa M /kpa =(1.8/1.8+1)* /(1.8+1)*66.9 = = 11

12 As a 1:1 Mixture; C = Cyclohexane; M = Methylcyclohexane T/K P C / kpa P M / kpa T/K P C / kpa P M /kpa

13 At what temperature will this ratio of C/M distill? T/K C /kpa M /kpa =(1.8/1.8+1) /(1.8+1) 66.9 = = 11 Summary First theoretical plate ratio 1.8/1; BP 362 K Second theoretical plate ~77.8/23.8 = 3.26; BP 36 K

14 An apparatus for fractional distillation 5 theoretical plates can be identified

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16 A look at a schematic diagram of a gas chromatograph:

17 nstrumental components arrier gas he carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, rgon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of etector which is used. The carrier gas system also contains a molecular sieve to remove ater and other impurities. ample injection port or optimum column efficiency, sample sizes should be small and should be introduced nto the column as a "plug" of vapor. The most common injection method is where a icrosyringe is used to inject sample through a rubber septum into a heated port at the ead of the column. SPME is an alternative method for introducing your sample. For acked columns, sample size ranges from tenths of a microliter up to 2 microliters. In reparative GC, sample sizes as much as a ml can be used in certain cercumstances. apillary columns, on the other hand, need much less sample, typically less than 1-3 ml. or capillary GC, split/splitless injection is used.

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19 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 1.5-1m 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 3 C, depending on the coating. Cyclodextrin is bonded in chiral columns. Capillary column

20 It is not unusual for a capillary column to have more an efficiency characterized by more than 1, theoretical plates.

21 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) Thermal conductivity (TCD) Electron capture (ECD) Nitrogenphosphorus Flame photometric (FPD) Photo-ionization (PID) Mass flow Concen tration Concen tration Mass flow Mass flow Concen tration Hydrogen and air Most organic cpds. Reference Universal 1 ng Make-up Halides, nitrates, nitriles, peroxides, anhydrides, organometallics 5 fg Hydrogen and air Nitrogen, phosphorus 1 pg Hydrogen and air possibly oxygen Make-up Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium Aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulphurs, some organometallics 1 pg 1 pg 2 pg Hall electrolytic conductivity Mass flow Hydrogen, oxygen Halide, nitrogen, nitrosamine, sulphur

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