Possible Quiz Questions Flashcards
(24 cards)
Most commercial samples are only ___ to ___% pure
95-96
Why is recrystallization an important method of purifying compounds?
It is rapid and convenient
What are the four forces that hold organic solids together?
Ionic attraction, hydrogen bonds, dipole-dipole interactions, and Van der Waals forces
List the four forces that hold organic solids together in order of decreasing strength.
- Ionic attraction (similar to covalent bonding)
- Hydrogen bonding
- Dipole-dipole forces
- Van der Waals forces
Where does hydrogen bonding exist?
Between H and N, O, or F
What is required for dipole-dipole interactions?
Polar molecules. Permanent dipoles are able to interact with one another, and exist between molecules that are polar
All molecules experience _______ forces.
Van der Waal
Van der Waals forces exist between (permanent, induced) dipoles
Induced
What is melting point?
The temperature at which the vapor pressures of the solid and liquid are equal
Melting points are _____ properties and can be used to ________.
Physical properties, to determine unknown solids.
Why are melting points useful?
To characterize and determine the purity of a compound (impure compounds tend to melt within a wide temperature range).
Recrystallization of the compound will purify the compound and the melting point range will (increase, decrease)
Decrease
Temperature and solubility of a given solvent are ________ proportional.
Directly
Explain how the relationship between temperature and solubility is related to the process of recrystallization
As temperature increases, the solubility of the solvent increases. The solute is dissolved in a minimal amount of hot solvent. When the solution is cooled, the solubility decreases, and the solute will begin to crystallize out of solution.
Hydrogen bonds (increase, decrease) boiling point
Increase
What are the 7 properties of an ideal solvent?
- Should not react with the solid
- Should be nonflammable
- Should be nontoxic
- Should be inexpensive
- Should be volatile at a lower boiling point so it can be removed from the crystals
- The solvent should dissolve the solid when the mixture is heated; no appreciable solubility should occur when the solvent is chilled
- The solid compound should crystallize when the hot solution is cooled to room temperature or when placed in an ice bath.
“Like dissolves _____”
Like
What are the types of impurities?
- Soluble: Impurities that are dissolved by the solvent, which are removed by recrystallization
- Insoluble: Impurities that are not dissolved by the solvent, which are removed by hot gravity filtration
How much solvent should be added for the recrystallization process?
Enough to cover the solid
What would happen if too much solvent is added to the solid?
The solute in solution would be diluted too much for recrystallization to occur
What technique will help induce recrystallization? Why?
Scratching with a glass rod. Recrystallization must start on some nucleation center
How can melting points be used to analyze purity?
1) Impurities usually cause the melting point of a compound to decrease
2) Reaction did not go to completion
What is the formula for % recovery?
% recovery=(amount of pure solid/amount of impure solid)x100
What is the formula for percent difference?
[|T(ref)-T(sample)|/T(ref)] x 100
