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What characterizes a pure organic compound?
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For what type of compounds is recrystallization the major mode of purification?
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CH 337: Unit 2
| Question | Answer |
|---|---|
| What characterizes a pure organic compound? | no detectable impurities |
| For what type of compounds is recrystallization the major mode of purification? | Solids |
| In its simplest form what does the process of recrystallization involved? | recrystallization involves the preparation of a hot saturated solution which is then allowed to cool |
| What occurs as a hot saturated solution gradually cools? | The solute precipitates |
| In order to obtain a purer sample what must be true about the impurities? | the impurities dissolve in the hot solvent and remain in solution even as the hot saturated solution is cooled. |
| How does the rate of cooling affect crystal growth? | Slow cooling promotes slow crystal growth which promotes the formation of larger, pure crystals. Rapid cooling promotes rapid crystal growth promotes the formation of smaller, less pure crystals. |
| What useful adage can guide one during the trial-and-error process of solvent selection? | The adage "like-dissolves-like" |
| As the number of functional groups present increases, solubility increases | statement |
| What two high-polarity solvents are commonly used for recrystallization? | Methanol and ethanol (water is too polar) |
| What two low-polarity solvents are commonly used for recrystallization? | diethyl ether and ethyl acetate |
| Why should one use a steam bath or hot-water bath, rather than a hot plate or open flame, to heat solvents such as ether, hexane and petroleum ether? | These solvents are very flammable |
| What is the most crucial aspect of recrystallization? | solvent selection |
| What characterizes an effective recrystallization solvent? | Has to be all 3: Be chemically inert, have a very high capacity at the boiling point of the solvent and have a low capacity at the solvent's freezing point and dissolve impurities |
| What rule of thumb should be followed in order to select an Erlenmeyer flask of appropriate size? | The flask should be 2 to 3 times larger than the volume of solvent one expects to use |
| In the absence of specific solubility data what rule of thumb should be followed in order to select an appropriate quantity of solvent? | Solvent volume should be 20 to 40 times the mass of the sample to be recrystallized |
| What characterizes the manner of solvent addition? | added incrementally |
| How are insoluble impurities removed from the hot, saturated solution? | hot gravity filtration |
| Under what circumstances are seed crystals used? | when the solute doesn't crystalize from the cooling saturated solution |
| Aside from mechanical losses during recrystallization what other factors might lead to low recovery of material? | a) too much solvent used b) crystallization during hot, gravity filtration c) vacuum filtration preformed prior to complete crystallization |
| For how long should one dry a produce that has been recrystallized from a high boiling-point solvent? | At least over night (a week in our class) |
| What is the 1st step to judge if a solvent will be suitable to use as a recrystallization solvent? | Combine a small quantity of the test solvent (5-10 drops) and a small quantity of sample (20-30 mg) in a test tube at room temp. Mix by shaking. If the sample dissolves then the solvent is not good. If the sample doesn't dissolve go to step 2. |
| What is the 2st step to judge if a solvent will be suitable to use as a recrystallization solvent? | Heat the contents of the test tube to the boiling point of the solvent. If the sample doesn't dissolve the solvent will not be a good solvent. If the sample dissolves go to step 3. |
| What is the 3st step to judge if a solvent will be suitable to use as a recrystallization solvent? | Allow the contents of the test tube to cool. If crystals don't form within 10-20 minutes the solvent isn't a good choice. If crystals do form it is likely to be an effective recrystallization solvent. |
| What are the 3 basic steps in recrystallization? | 1) preparation of a hot, saturated solution at the solvent's boiling point followed by removal of any insoluble impurities 2) gradual cooling of the hot, saturated solution 3) collection of crystalline material by vacuum filtration |
| Identify two methods for transferring a solid to the recrystallization flask? | a) the use of creased weighing paper b) the use of creased weighing paper and powder funnel |
| Why is it advisable to set aside a small amount of the crude material before preforming a recrystallization? | For use as seed crystals and to use to get melting point |
| After the flask has cooled to room temp is there any advantage to cooling it in an ice-water bath? | Yes. The advantage to cooling in an ice-water bath is that it facilities maximum crystallization |
| What actions could one take in the event that no material crystalizes after 15 minutes of cooling in an ice-water bath? | Add seed crystals |
| What is the standard system for collecting products purified by recrystallization? | Vacuum filtration |
| Why is it advisable to employ a trap flask during vacuum filtration? | To prevent the flow of water from the water aspirator to the filter flask |
| Why should one pull some of the recrystallizing solvent through the filter paper, by application of the vacuum source, prior to filtration? | To seat the filter paper |
| What quantity of water should be used to recrystallize a 1.2g quantity of salicylic acid (solubility of salicylic acid in water: 1g/460mL at 25C, 1g/15mL at 100C) | Volume of water at 100C required to dissolve. 1.2g x 15 mL/g = 18mL |
| How much salicylic acid will crystalize when the solution prepared above is allowed to cool to 25C? | nass remaining in solution after cooling to 25C = 18mL x 1g/460mL = .04g Mass of salicylic acid that crystalized after cooling = 1.2 - .04 = 1.16g |
| What is the maximum percent recovery of salicylic acid in this scenario? | 1.16/1.20 x 100% = 96.7% |
| How to increase the percent recovered? | ice-bath (lowers temp) |
| Assuming 38.2 mL of water was used in the preceding questions, what is the maximum percent recovery of salicylic acid that the solution was cooled to 25C? | 38.2 mL (1g/460mL) = .083g 1.20-.083 = 1.12 1.12/1.2 x 100% = 93.3% |
Created by:
leelow13