CHEM 281 Study Guide - Midterm Guide: Risk Assessment, Long Term Ecological Research Network, Steambath

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Recrystallization is the method of obtaining pure crystals by dissolving impure compound in a hot (boliling) solvent. In an ideal recrystallization, as the solution cools, compound will crystallize and impurities will stay in the solution. The pure crystals could be collected by ltration. Little hot solvent as possible is recommended because if there is too much, the compound will remain dissolved as the solvent cools. (it must be supersaturated) no crystals will form in this case. Add the minimum amount of hot solvent needed to dissolve the sample in to a ask. Note: add solvent before heating or your crystals will be cooked. In experiment: added around 12cm^3: heat to dissolve solute. Place the ask on a hot plate and turn it on to allow gentle heating of the solvent. Steam bath (putting the ask in boiling water) could also be used. Some solvent vapours are ammable, so avoid naked ames.

Related Questions

In a recrystallization, the crystals do not always form spontaneously after cooling, even though the solution is supersaturated. Which of the following will help crystals form? More than one answer may be correct.

1. Scrape the inside of the Erlenmeyer flask with a glass rod below the surface of the solvent.

2. Add a scrap of paper to nucleate the crystals.

3. Add some crystals of the compound you are trying to crystallize.

4. Add some crystals of any kind.

5. Scrape the outside of the Erlenmeyer flask with a glass rod.

A process of recrystallization involves dissolving the solid inan appropriate solvent at an elevated temperature and allowing thecrystals to re-form on cooling. Residual amount of solvent isremoved by drying the crystals and therefore usually does notconsider to be an impurity, although if the crystals are not driedproperly the residual solvent will affect the melting pointmeasurements.
The choice of solvent is the most critical step in the process ofrecrystallization. Which of the following criteria should besatisfied while choosing a solvent for recrystallization?

Select one:

a. A compound to be purified should be completely soluble in areasonable amount of hot solvent.

b. A compound to be purified should be insoluble, nearlyinsoluble, or slightly soluble in cold solvent.

c. Impurities should be either insoluble in the solvent at alltemperatures so they can be filtered out or soluble in a coldsolvent staying in it while the desired compound willprecipitate.

d. The boiling point of the solvent should be low enough so theresidual amount of it will be readily removable from the crystalsof pure compound.

e. The boinling point of the solvent should generally be lowerthan the melting point of the desired compound so the compound willnot melt while heating the solvent giving the impression that ithas been dissolved.

f. The solvent should not react chemically with the compoundbeing purified.

g. All of these considerations are correct.

CHEM 281 Study Guide - Midterm Guide: Risk Assessment, Long Term Ecological Research Network, Steambath

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