chromatography

Question 1

what is the purpose of chromtography

Answer 1

to separate compounds based on their partitioning between a mobile phase and a stationary phase

Question 2

in a TLC, what is the stationary phase?

Answer 2

it is a SOLID POLAR phase made of silica gel or Alumina gel, molecules with POLARITY will FAVOR the stationary phase and have lower Rf values and stay further down

Question 3

in TLC what is the mobile phase

Answer 3

it is the LIQUID, it is NONPOLAR so nonpolar molecules will travel UP the plate

Question 4

what travels faster up a plate? nonpolar solids or polar solids

Answer 4

nonpolar solvents have an affinity for the nonpolar mobile phase and will travel up faster by capillary action

Question 5

how does chromatography separate

Answer 5

by polarity

Question 6

how does gas chromatography separate

Answer 6

by boiling point

Question 7

why do we add blotting paper to the developing jar

Answer 7

to ensure that the atmosphere is saturated with the vapors of the developing solvent

if it is not saturated, it will evaporate off and the TLC will stop

Question 8

why do the spots on the starting line need to be above the liquid

Answer 8

they will otherwise dissolve off

Question 9

why can you not touch the TLC plate when it is traveling?

Answer 9

it will stop the TLC

Question 10

how can you visualize a TLC

Answer 10

either with your eye, I2 stain, or if its conjugated you can visualize it under UV

Question 11

if you have a polar compound, will it have a large or small Rf value

Answer 11

you will have a small Rf value, it will stick to the stationary phase and not move very far

Question 12

if you have a nonpolar compound, will you have a large or small Rf value

Answer 12

large, it will prefer the mobile phase and travel further up

Question 13

if a solvent is made less polar, what happens to the Rf

Answer 13

the Rf will decrease

Question 14

if the solvent is increased in polarity, what happens to the Rf

Answer 14

the Rf will increase

Question 15

what is the absorbant

Answer 15

it is a polar stationary phase

Question 16

what is the eluant

Answer 16

it is the nonpolar mobile phase

Question 17

functional groups that are more polar tend to elute

Answer 17

slower and have a smaller Rf value

Question 18

if the mobile phase is too polar

Answer 18

everything moves to the top because the mobile and stationary are too similar

Question 19

if mobile phase is too nonpolar

Answer 19

nothing moves, they are too dissimilar

Question 20

to determine reaction progress, what are the four lanes in TLC

Answer 20

the first lane is the starting material, with the polar molecules near the bottom

the second lane is a ixed co-spot

the third and fourth are product fractions that show the various polarities of products

Question 21

what is the two main differences with column chromatography and tlc

Answer 21

column allows larger scale separation and the mobile phase moves down with gravity rather than down

Question 22

in column chromatography, what elutes first

Answer 22

compounds with higher Rf, nonpolar, will elute first as they favor mobile phase

Question 23

in a column chromatography, what elutes later

Answer 23

the compounds with lower Rf, are polar, will elute later as they favor stationary