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Experiment 2

Experiment 2. DISTILLATION AND GAS CHROMATOGRAPHY OF ALKANES. Objectives. To e xplore the relationship between molecular structure, intermolecular forces and boiling points of various types of compounds. To learn techniques of simple distillation and gas chromatography (GC).

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Experiment 2

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  1. Experiment 2 DISTILLATION AND GAS CHROMATOGRAPHY OF ALKANES

  2. Objectives • To explore the relationship between molecular structure, intermolecular forces and boiling points of various types of compounds. • To learn techniques of simple distillation and gas chromatography (GC). • To use these techniques to separate and identify compounds in an unknown mixture.

  3. Before coming to lab… • It is *highly recommended* that you watch the following video prior to coming to lab. • http://www.youtube.com/watch?v=3JlIPnyrZMw

  4. Structure, IMF, and Boiling Point • Alkanes = consist of ONLY carbon and hydrogen. • Straight chain alkanes = greater surface area contact = greater LDF between molecules = HIGHER BP! • Branched alkanes = less surface area contact = less LDF b/n molecules = LOWER BP! Notice more contact between molecules here!

  5. Table 2.1

  6. Distillation • Distillation = separates/purifies liquids. • Heat mixture of two or more liquids in a flask. • More volatile liquid vaporizes 1st, vapor passes into a water-cooled condensing column, it can reverts back into a liquid (condenses) on the cool glass, then trickles into a collection flask. • With further heating the less volatile liquids vaporize and distill at higher temperatures.

  7. Distillation • Involves vaporization, condensation, and collection of the condensate (distillate). • The temperature of the distillate is measured by a thermometer placed in the path of the vapor. Collect four 5 mL fractions in small test tubes!

  8. Simple Distillation • Simple distillation is most effective when the mixture contains only onevolatile component. • More than one….most effective if the boiling points of the components differ by at least 70 ºC.

  9. Chromatography • Chromatography = collective term for a family of laboratory techniques for the separation of mixtures. • Chromatography involves passing a mixture dissolved in a mobile phase (which moves) through a stationary phase (doesn’t move). • Different levels of interaction b/n compounds and mobile vs. stationary phases separates the analyte to be measured from other molecules in the mixture and allows it to be isolated.

  10. Gas Chromatography (GC) • In GAS chromatography, the MOBILE PHASE = He(g). • Gas chromatography is only used to separate volatile compounds in lab.

  11. STATIONARY PHASE = a nonpolar packing material. When analyzing compounds of similar polarity, the compounds will elute in order of volatility. Lower BP = low retention time (Rt) Gas Chromatography Higher boiling component Lower boiling component

  12. EXAMPLE GC RESULTS Original Sample Mixture Original Sample • The individual fractions collected are analyzed by GC during the course of the distillation. Based on peak size, it is evident that as the distillation proceeds, the later fractions contain mainly the higher boiling component of the mixture. Pentane ( GC Sample Solvent) pentane Alkane #1 hexane Alkane #2 Unknown X Fraction 2 Notice Fraction 1 contains more of Alkane #1 (lo BP) than Alkane #2 (hi BP) Fraction 1 Notice Fraction 5 contains ONLY of Alkane #2 (hi BP), and NO Alkane #1 (lo BP) Fraction 5 Fraction 5

  13. Product Analysis(Quantification of Components…Adjusted Area Percent) • Most often you will dissolve your compound or mixture in a low boiling solvent for GC analysis. • The relative areas of the components of interest must therefore be adjusted, to exclude the large % area of the solvent peak. • Refer to page 21 for further explanation. Adjusted area % = area % of peak of interest X 100 sum of area % of ALL peaks of interest* *omit area % of solvent peak

  14. OVERVIEW • Select and obtain 25 mL of an unknown. • Set up and perform a simple distillation. • Collect (4)-5mL fractions in small test tubes. • Prepare GC samples of Fraction #1 and Fraction #4. Submit for analysis. • Pick up GC results when ready and record standard retention times from STANDARD CHROMATOGRAM. • Identify alkanes in unknown by comparing sample retention times to standard retention times. • Quantify alkanes at beginning and end of distillation by calculating Adjusted Area Percent.

  15. Experimental Procedure(Simple Distillation) • Add the unknown mixture and 3 boiling chips to a round bottom flask. • Set up a simple distillation apparatus. • Begin water flow through condenser and apply heat to the flask. • Collect liquid distillate in small test tubes as it leaves the condenser. • Keep track of the temperature range for each fraction as it is collected. • Collect (4) 5mL fractions, leaving the final 5mL in the boiling flask. • Prepare GC sample for analysis.

  16. Product Analysis(GC Analysis) • Analyze your FIRSTand FOURTH fractions by GC. • SAMPLE PREP: • Transfer 5 drops of your sample into an auto sampler vial using a plastic pipet. • Add 1 mL of GC solvent (pentane) from the solvent dispenser in balance room. • Be sure to record your GC vial slot # in your laboratory notebook. • You will receive an email to your UNCW account when GC results are ready for pick-up!

  17. Table 2.3 • You cannot complete this table until you obtain your GC results from the board outside of Dobo 203. • The standard chromatogram will be provided on the front of the envelope containing the chromatograms. • Use only the standard run for your lab section!

  18. SAFETY CONCERNS • All compounds used in today’s experiment are extremely flammable. Wear safety goggles at all times and use extreme caution!

  19. WASTE MANAGEMENT • Pour all liquid waste into the bottle labeled “LIQUID ORGANIC WASTE”. • Do NOT pour any waste down the drain!

  20. CLEANING • Disassemble distillation apparatus in your lab hood. • In your lab hood, rinse all glassware with wash acetone ONLY directly into a beaker from your drawer (DO NOT REMOVE DISTILLATION APPARATUS FROM YOUR HOOD AT ANY TIME.) • Return distillation glassware to case in your hood. • Return all other glassware to your lab drawer.

  21. LABORATORY NOTEBOOK(Pre-lab) • OBJECTIVE (Must clearly state…) • What is the goal of the experiment? • What technique will you used to accomplish it? • How will you determine if it worked? • TABLE OF PHYSICAL DATA (Complete the following table using MSDS sheets from a site on WWW Links ONLY. Wikipedia is unacceptable) • REFERENCE TO PROCEDURE (Must include…) • full title • Edition • authors • page numbers where actual procedure can be found

  22. LABORATORY NOTEBOOK(In-lab) • DATA/CALCULATIONS • Record which unknown was used • Record distillation range of each fraction collected • Record GC vial slot number for fraction #1 • Record GC vial slot number for fraction #4 • Record GC sample solvent used • Show one EXAMPLE of an adjusted area percent calculation • EXPERIMENTAL PROCEDURE • In paragraph form, briefly describe the procedure that you actually followed during the lab. • Paragraph must be written in PAST TENSE, PASSIVE VOICE. • Include any volumes of chemicals used during the experiment • Include any mistakes, accidents, or observations if necessary.

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