Chapter 8 - Lab Techniques

Question 1

What is the general idea behind separation techniques?

Answer 1

In order to separate components of a mixture, you must convert the system into a state where the components are in different phases.

Question 2

What is the general idea behind distillation?

Answer 2

Upon heating, the most volatile component converts to a gas more readily than the less volatile components.

Question 3

What is the difference between simple and fractional distillations?

Answer 3

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).

Question 4

What is vacuum distillation?

Answer 4

It lowers the atmospheric pressure in the distillation apparatus to lower the boiling point of compounds that typically have a very high boiling point.

Question 5

What is the general idea behind chromatography?

Answer 5

It separates components by exploiting differences in their:

• solubility in a migrating solvent
• affinity for a polymer

Question 6

In chromatography, what is the adsorbent?

Answer 6

the stationary phase that will attract a mobile phase (solvent)

Question 7

What materials are typically used for the stationary phase of chromatography?

Answer 7

Polar materials:

• alumina gel
• silica gel

Question 8

How can a compound be identified using thin layer chromatography?

Answer 8

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.

Question 9

In chromatography, do nonpolar species move slower or faster?

Answer 9

They move faster.

Question 10

What is the R_f value in thin layer chromatography?

Answer 10

It is a “ratio of the fronts,” where the distance a spot travels is compared to the distance the solvent travelled.

R_f = distance_spot/distance_solvent

Question 11

What can be concluded by a spot that has the greatest R_f value in TLC?

Answer 11

It is the component that has the least affinity for the stationary phase, and is most soluble in that solvent.

Question 12

What is the driving force that separates compounds in column chromatography?

Answer 12

gravity

Question 13

If a racemic mixture is eluted through a column filled with S-enantiomer, which will be present in the collected aliquot?

Answer 13

The S-enantiomer will be present because the R-enantiomer was attracted to the column.

Question 14

How are components of gas chromatography analyzed?

Answer 14

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.

Question 15

Do compounds with lower or higher boiling points elute through a gas column faster? Why?

Answer 15

Those with lower boiling points elute faster. This is because they normally have less intermolecular (polar) forces and are lighter in mass.

Question 16

What is the general idea behind extraction?

Answer 16

It takes advantage of drastic differences in the solubilites of copmonents in two difference immiscible solvents.

Question 17

In extraction, what is the partition coefficient?

Answer 17

the ratio of a compound’s maximum solubility in one solvent compared to its maximum solubility in another solvent

Question 18

Acid-base extraction cannot be used for what type of compounds?

Answer 18

water-sensitive compounds that may be hydrolyzed: acid anhydrides, acid halides, esters

Question 19

What is recrystallization?

Answer 19

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.

Question 20

Charcoal has a high affinity for what type of molecule?

Answer 20

organic molecules, specifically those that are colored (have conjugated π-networks)

Question 21

How does vacuum filtration speed up the filtering process compared to gravity filtration?

Answer 21

It lowers the pressure below the filter, allowing solvent to flow through the filter faster than it would with just gravity.

Question 22

Why can’t salt solutes be filtered?

Answer 22

They are in the aqueous phase. They must be a solid to be filtered.

Question 23

What is the best solvent wash to use on crystals in cold filtratrion?

Answer 23

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.

Question 24

What is the mixed melting point test?

Answer 24

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.

Question 25

What 3 things must be true in order for a chemical identity test to be effective?

Answer 25

• the chemical testing reagent must react with very few functional groups
• the chemical testing reagent must undergo some visible change (color, phase)
• the chemical testing reagent must be limiting

Question 26

What is a good test for methyl ketones?

Answer 26

iodoform test

Question 27

How can 2,4-dinitrophenylhydrazine derivatives be distinguished?

Answer 27

by melting points