Chapter 8 - Lab Techniques Flashcards
(27 cards)
What is the general idea behind separation techniques?
In order to separate components of a mixture, you must convert the system into a state where the components are in different phases.
What is the general idea behind distillation?
Upon heating, the most volatile component converts to a gas more readily than the less volatile components.
What is the difference between simple and fractional distillations?
simple distillation: for liquids that have large differences in boiling points (and separating solute from solvent). It is faster and has a higher yield.
fractional distillation: for liquids that have small differences in boiling points (< 30º difference). It is slower and more pure. It uses a distillation column with greater surface area (glass beads).
What is vacuum distillation?
It lowers the atmospheric pressure in the distillation apparatus to lower the boiling point of compounds that typically have a very high boiling point.
What is the general idea behind chromatography?
It separates components by exploiting differences in their:
- solubility in a migrating solvent
- affinity for a polymer
In chromatography, what is the adsorbent?
the stationary phase that will attract a mobile phase (solvent)
What materials are typically used for the stationary phase of chromatography?
Polar materials:
- alumina gel
- silica gel
How can a compound be identified using thin layer chromatography?
The migratory spot of the unknown compound can be compared to the spot of known compounds. It should migrate to the same distance as the known compound if it is identical.
In chromatography, do nonpolar species move slower or faster?
They move faster.
What is the Rf value in thin layer chromatography?
It is a “ratio of the fronts,” where the distance a spot travels is compared to the distance the solvent travelled.
Rf = distancespot/distancesolvent
What can be concluded by a spot that has the greatest Rf value in TLC?
It is the component that has the least affinity for the stationary phase, and is most soluble in that solvent.
What is the driving force that separates compounds in column chromatography?
gravity
If a racemic mixture is eluted through a column filled with S-enantiomer, which will be present in the collected aliquot?
The S-enantiomer will be present because the R-enantiomer was attracted to the column.
How are components of gas chromatography analyzed?
A graph is obtained that shows peaks of collision after a gas has been forced through a column. The height and area under the peak corresponds to the amount of the component.
Do compounds with lower or higher boiling points elute through a gas column faster? Why?
Those with lower boiling points elute faster. This is because they normally have less intermolecular (polar) forces and are lighter in mass.
What is the general idea behind extraction?
It takes advantage of drastic differences in the solubilites of copmonents in two difference immiscible solvents.
In extraction, what is the partition coefficient?
the ratio of a compound’s maximum solubility in one solvent compared to its maximum solubility in another solvent
Acid-base extraction cannot be used for what type of compounds?
water-sensitive compounds that may be hydrolyzed: acid anhydrides, acid halides, esters
What is recrystallization?
It involves dissolving a solid into hot solvent, filtering out the insoluble solid impurities, and then slowly cooling the solution to precipitate the purified crystals.
Charcoal has a high affinity for what type of molecule?
organic molecules, specifically those that are colored (have conjugated π-networks)
How does vacuum filtration speed up the filtering process compared to gravity filtration?
It lowers the pressure below the filter, allowing solvent to flow through the filter faster than it would with just gravity.
Why can’t salt solutes be filtered?
They are in the aqueous phase. They must be a solid to be filtered.
What is the best solvent wash to use on crystals in cold filtratrion?
To wash off any residual solvent on the crystals, you must use a solvent in which the residual solvent is soluble, but in which the crystals are insoluble. Also use a solvent with a low boiling point so that it readily evaporates away from the solid.
What is the mixed melting point test?
It is the mixing of an unknown compound with a known compound to form a homogenous mixture, and then testing the mixture’s melting point. If it is sharp (within about 2 degrees), it is pure. If it is broad, the unknown is a different compound.
