Melting Point Determination and Purification by Recrystallization Flashcards
What is recrystallization?
A way to purify solids
What are the four important concepts in recrystallization?
- Solubility
- Saturation Level
- Exclusion
- Nucleation
What is the most important thing when choosing a solvent?
Dissolves the solute well at high temps, but not at low temps
What is the solvent mixture used for the recrystallization?
Water and ethanol
What is second most important when choosing a solvent?
Will not react with the solute
What is nice, but not necessary, when choosing a solvent?
Nonflammable, nontoxic, inexpensive, volatile
What is important about saturation level?
We want to use a minimal amount of solvent to dissolve at boiling (saturated at boiling)
As we cool our saturated solution…
it becomes supersaturated.
Which compound crystallizes first?
The more pure, more present compound. The impurities are excluded.
Can you crystallize a 50/50 mixture?
No, you must have more of one compound than the other
Should we cool quickly or slowly?
Slowly, if we cool too fast we just get a precipitate
What is nucleation?
Something for the crystals to grow on, scratches, dust, seed crystal
Why is it not a melting point?
The is never an exact temperature, but a melting range
Benzoic acids melting range
118-120
What is the effect of impurities on a melting range?
Impurities lower and broaden the melting range
What is the melting range when a compound is pure?
1-2 degrees
What do we use to see the samples melt?
Capillary tubes
Do we need to heat the melting point apparatus slowly or fast? Why?
We need to heat it slowly. It takes time to transfer the heat to the thermometer. If it’s too fast, the melting range will be much lower.
What is the effect of ionic attractive forces on melting point?
Very high melting point
What kinds of compounds have the highest melting point?
Ionic bonded salts
What has the second highest melting points?
Hydrogen bonded compounds
What is the effect of size on the melting points of compounds?
The largest compound has the highest melting point
What is the effect of being cyclic on a compounds melting point?
Have a higher melting point
What is the only difference between melting points and boiling points?
Symmetry matters with melting points
What is the effect of symmetry on melting point?
Symmetrical molecules generally have higher melting points
