SAMPLE QUESTIONS FOR THE FINAL EXAM FOR CHEM 2050 1) List the steps that need to be taken to properly perform a recrystallization. 1) Heat your solvent close to the boiling point. 2) Add 1.00 gram of the impure unknown to a 250 ml Erlenmeyer flask and heat this mixture with your solvent on the hot plate. Add just enough hot water until all your unknown has dissolved in hot water. 3) Continue heating the water/unknown to continue heating for 3 to 5 minutes. Try not to add more than 20 mL of solvent. Remove the mixture from the hot plate and carefully add a spatula tip full of decolorizing carbon (charcoal). 4) Perform a “hot filtration” over a steamed fluted piece of filter paper. 5) If you need to, this is the point where you will boil off excess solvent. 6) Allow your solution to cool slowly. Once this flask has cooled off (cool to your touch!), you should see crystals form. This material will then get chilled in an ice bath for ~ 10 minutes. 7) The recrystallized unknown solution is filtered with a Buchner funnel. 2) What are some of the characteristics of a good solvent for recrystallization? 1. The substance to be purified must have a high solubility in the solvent at its boiling point, and a significantly diminished solubility at lower temperatures (limited only by the freezing temperature of the solvent). Obviously, the first part is necessary so that you can get the material into solution, and the second part is required so that the purified compound will come back out of solution. 2. The solvent should dissolve the impurities at all temperatures. Actually, its OK if the impurity dissolves readily or doesn't dissolve at all, as we'll see. The important point here is that purification will be accomplished only if the impurities remain in solution as the mixture is cooled. This isn't too hard to accomplish if the amount of impurity is small. Obviously, it's a bit more difficult if a large amount of an impurity is present, but if the solubility characteristics of the
Sample Questions Final Exam - Chem 2130 - Ochem Lab
Nov 03, 2014
Sample Questions Final Exam - Chem 2130 - Ochem Lab
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SAMPLE QUESTIONS FOR THE FINAL EXAM FOR CHEM 2050
1) List the steps that need to be taken to properly perform a recrystallization.
1) Heat your solvent close to the boiling point. 2) Add 1.00 gram of the impure unknown to a 250 ml Erlenmeyer flask and heat this mixture with your solvent on the hot plate. Add just enough hot water until all your unknown has dissolved in hot water. 3) Continue heating the water/unknown to continue heating for 3 to 5 minutes. Try not to add more than 20 mL of solvent. Remove the mixture from the hot plate and carefully add a spatula tip full of decolorizing carbon (charcoal).4) Perform a “hot filtration” over a steamed fluted piece of filter paper. 5) If you need to, this is the point where you will boil off excess solvent. 6) Allow your solution to cool slowly. Once this flask has cooled off (cool to your touch!), you should see crystals form. This material will then get chilled in an ice bath for ~ 10 minutes. 7) The recrystallized unknown solution is filtered with a Buchner funnel.
2) What are some of the characteristics of a good solvent for recrystallization?
1. The substance to be purified must have a high solubility in the solvent at its boiling point, and a significantly diminished solubility at lower temperatures (limited only by the freezing temperature of the solvent). Obviously, the first part is necessary so that you can get the material into solution, and the second part is required so that the purified compound will come back out of solution.
2. The solvent should dissolve the impurities at all temperatures. Actually, its OK if the impurity dissolves readily or doesn't dissolve at all, as we'll see. The important point here is that purification will be accomplished only if the impurities remain in solution as the mixture is cooled. This isn't too hard to accomplish if the amount of impurity is small. Obviously, it's a bit more difficult if a large amount of an impurity is present, but if the solubility characteristics of the desired substance are sufficiently different from the impurity, as is often the case, a suitable solvent can usually be found.
3. The solvent must not react with the substance to be purified. It's hard to get your compound to come back out of solution when it is consumed by a reaction.
4. A reasonably volatile solvent is preferred. This consideration is dictated by the need to ultimately remove traces of solvent from the crystals, and this is most easily accomplished by evaporation, either at room temperature or in an oven. Even with an oven to aid in the drying, only moderate temperatures can be used, since you should stay below the melting temperature of the solid, and definitely below temperatures where the material will decompose. Generally speaking, solvents with boiling temperatures below ~150°C are OK.
5. Ideally, the solvent should be inexpensive, non-toxic, and non-flammable. These are not absolutely required, but desirable characteristics, which can help to determine the choice of a solvent. Obviously, water is an ideal choice based on these criteria, and is the preferred solvent if it satisfies conditions 1-4.
3) Draw pictures of a reflux, simple distillation, and fractional distillation apparatus. Label all of the parts and describe the use for the technique.
Reflux apparatus Fractional Distillation apparatus
Simple Distillation apparatus
4) Describe the formation of an electrophile and show the mechanism for the substitution of an electrophile on an aromatic system.
5) Discuss and describe chromatography and explain the difference b/t column chromatography and thin layer chromatography.
Chromatography is an effective and very useful method for separation and purification of organic compounds that can be used even for complex mixtures. Chromatography separates components of a mixture based upon how well they are adsorbed on an inert phase (the stationary phase) versus how well they dissolve in a liquid phase (the mobile phase).
Thin layer chromatography: the stationary phase is coated on a plate of glass or plastic.Column chromatography: the stationary phase is packed into a glass column.
6) Show the mechanisms and products formed in various Diels Alder adducts. An example would be to have 1,3 butadiene react with maleic anhydride.
The Diels-Alder reaction is a cycloaddition reaction between a conjugated diene and an alkene. This reaction produces a 1,4-addition product.
Experiment: Reaction of Anthracene with Maleic Anhydride
7) Show the mechanisms and products formed in various Aldol reactions. An example would be to have acetone react with another molecule of acetone.
8) Discuss oxidation and reduction reactions. Show products and identify reagents that could be used to carry out the specified reaction.
9) Understand the basic information associated with the Fisher esterification and the mechanism associated with making an ester.
Fischer esterification is the esterification of a carboxylic acid by heating it with an alcohol in the presence of a strong acid as the catalyst.
Example:
The overall reaction is reversible; to drive the reaction to completion, it is necessary to exploit Le Châteliers principle, which can be done either by continuously removing the water formed from the system or by using a large excess of the alcohol.
Mechanism:
10) Show the mechanisms for SN1, SN2, E1, and E2 reactions and show different side reactions given a specific substrate. Identify conditions and reactant/solvents for each type of reaction.
11) Look at reagents and post-lab questions.
12) Be able to calculate both percent yield and identify limiting reagents.
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