1. List three criteria that is important to consider when choosing a solvent for recrystallization
1) it should be unreactive to the substrate being purified; 2) it should dissolve a relatively large amount of the desired substance at a high temperature and a relatively small amount of it at a low temperature; 3) it should dissolve any impurities present well at all temperatures; 4) it should be reasonably volatile, so that excess solvent can be removed from the crystals after filtration. 5) The boiling point of the solvents needs to be less than the melting point of the solute
2. What basic concept allows us to use recrystallization as a purification technique?
The principle behind recrystallization is that the amount of solute that can be dissolved by a solvent increases with temperature (solubility). At this high temperature, the solute has a greatly increased solubility in the solvent, so a much smaller quantity of hot solvent is needed than when the solvent is at room temperature. At the cooler temperature, the solution is saturated at a much lower concentration of solute. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected.
3. Name two reasons why it is bad to rapidly cool your hot filtered solution in the recystillazation experiment
Cooling the solution too rapidly will result in formation of small crystals, which tend to be less pure compared to crystals formed from slow crystallization. The reason for this is that small crystals have more surface area and can pick up impurities
Cooling the hot filtrate quickly causes the crystal matrix to form rapidly trapping impurities rather than squeezing them out in slow cooling and have better over all packing of the crystal matrix,
4. If you are given 5.00 grams of unknown compound and 0.5 g of sand is added to it. You recrystallize the sample and obtain 3.85 g of compound. What is the percent yield?
(3.85 g of compound/5.00 grams of unknown) x 100 = 77.0% yield
5. What is the difference between crystallization and precipitation?
Crystallization is a slow, selective process in which a pure compound comes out of solution. Precipitation is a fast, non-selective process in which a mixture of compounds falls out of solution. In addition, crystallization results in the formation of well-ordered structures (crystals) while precipitation results in the formation of a disordered mass of molecules (a precipitate).
6. When collecting your pure crystals in the Büchner funnel, it is suggested that you rinse the crystals with solvent. Should the solvent be hot, cold, or room temperature? Why?
The solvent should be cold. Your crystals are highly soluble in hot solvent, but poorly soluble in cold solvent. By washing with cold solvent you will not lose an appreciable amount of your product during the washing process. Please note that the hot solvent will not melt your solute.
7. When determining the amount of solvent needed for your crystallization experiment, should you use the compound’s solubility at room temperature or at the boiling point of the liquid? Why?
You want to use the compound’s solubility at the boiling point of the solvent since you will be heating the solvent in order to dissolve all of your solid then cooling it down to start the crystallization process. If you use the room temperature solubility it will be very easy to get your solid to go into solution, but vevery difficult to get it to crystallize back out.
Melting Point
8. Define eutectic point?
Eutectic point is the solubility limit of B in A; thus, it is the lowest melting point of an A/B mixture (Note: sharp melting point – no range at eutectic point)
9. Name two observations you will observe if you take a melting point of a compound
