CHEM 220L LAB FINAL UPDATED ACTUAL QUESTIONS
AND CORRECT ANSWERS
Question:
1. What is meant by "green chemistry"?
Answer:
Green chemistry are chemical processes that are designed to lower or eliminate making or using substances
that are hazardous to the environment
Question:
2. What is one way to make a chemical experiment more "green"?
Answer:
Using catalysts in your reactions to reduce waste. This reduces waste because catalysts can carry out single
reactions multiple times by using very small amounts. Stoichiometric reagents are used in larger amounts,
and carried out only once. This reduces waste
Question:
3. If you don't know the density of your solvent, how might you figure out which layer is organic?
Answer:
By observing which layer is on top. Organic layers are less dense than water, and will be the layer sitting
on top of water
Question:
4. Why might bromide be a better leaving group than chloride? What makes for a better leaving group?
Answer:
A good leaving group would be largest in size, a weak base, and more electronegative
Question:
5. Why is the melting point for vanillyl alcohol so much higher than that of vanillin?
Answer:
Vanillyl alcohol has a higher melting point than vanillin because of the intermolecular hydrogen bonds that
lead to strong crystal formation.
Question:
6. In an SN2 reaction, what do you expect will happen to the rate of the reaction as the amount of steric
crowding increases
Answer:
As steric crowding increases in an SN2 reaction, the reaction rate becomes slower
Question:
7. In an SN1 reaction, what do you expect will happen to the reaction rate as the stability of the carbocation
formed (recall: 3' > 2' > 1') increases?
Answer:
As the carbocation stability increases, the reaction rate increases. SN1 reactions are fastest when the
carbocation is the most stable
, Question:
8. Ideal bond angle for sp3?
Answer:
109.5'
Question:
9. Ideal bond angle for sp2?
Answer:
120'
Question:
10. The melting point range of an impure compound will be _____ and _____ than the melting point range
for the pure compound
Answer:
melting point of an impure compound will be lower, and have a broader range
Question:
11. What general chemistry principle is responsible for the difference in melting points of pure and impure
compounds?
Answer:
Melting point depression. Impurities in a solvent will lower the melting point.
Question:
12. Why is it important to control the rate of heating close to the melting point?
Answer:
We must control the rate so that we can have a more precise range calculated for the substance
Question:
13. Johnny is very careless in the lab. He purified two white solids (A and B) by recrystallization, but
forgot to label the vials. Both A and B have a melting point range of 102-104°C, so he is not sure which
solid is which. He has a small amount of an authentic sample of pure A available in the lab. What should
he do?
Answer:
He should determine the melting point of just pure A and then mix some of pure A with solids A & B and
then see which one has the lower and broader range.
Question:
14. What is the stationary phase of TLC?
Answer:
Stationary phase is the silica or alumina gel layer on an inert backing. It is called stationary because it is
not moving in this phase
Question:
15. What is the mobile phase of TLC?
Answer:
The mobile ("moving") phase is a suitable liquid solvent, and draws up the plate through capillary action
AND CORRECT ANSWERS
Question:
1. What is meant by "green chemistry"?
Answer:
Green chemistry are chemical processes that are designed to lower or eliminate making or using substances
that are hazardous to the environment
Question:
2. What is one way to make a chemical experiment more "green"?
Answer:
Using catalysts in your reactions to reduce waste. This reduces waste because catalysts can carry out single
reactions multiple times by using very small amounts. Stoichiometric reagents are used in larger amounts,
and carried out only once. This reduces waste
Question:
3. If you don't know the density of your solvent, how might you figure out which layer is organic?
Answer:
By observing which layer is on top. Organic layers are less dense than water, and will be the layer sitting
on top of water
Question:
4. Why might bromide be a better leaving group than chloride? What makes for a better leaving group?
Answer:
A good leaving group would be largest in size, a weak base, and more electronegative
Question:
5. Why is the melting point for vanillyl alcohol so much higher than that of vanillin?
Answer:
Vanillyl alcohol has a higher melting point than vanillin because of the intermolecular hydrogen bonds that
lead to strong crystal formation.
Question:
6. In an SN2 reaction, what do you expect will happen to the rate of the reaction as the amount of steric
crowding increases
Answer:
As steric crowding increases in an SN2 reaction, the reaction rate becomes slower
Question:
7. In an SN1 reaction, what do you expect will happen to the reaction rate as the stability of the carbocation
formed (recall: 3' > 2' > 1') increases?
Answer:
As the carbocation stability increases, the reaction rate increases. SN1 reactions are fastest when the
carbocation is the most stable
, Question:
8. Ideal bond angle for sp3?
Answer:
109.5'
Question:
9. Ideal bond angle for sp2?
Answer:
120'
Question:
10. The melting point range of an impure compound will be _____ and _____ than the melting point range
for the pure compound
Answer:
melting point of an impure compound will be lower, and have a broader range
Question:
11. What general chemistry principle is responsible for the difference in melting points of pure and impure
compounds?
Answer:
Melting point depression. Impurities in a solvent will lower the melting point.
Question:
12. Why is it important to control the rate of heating close to the melting point?
Answer:
We must control the rate so that we can have a more precise range calculated for the substance
Question:
13. Johnny is very careless in the lab. He purified two white solids (A and B) by recrystallization, but
forgot to label the vials. Both A and B have a melting point range of 102-104°C, so he is not sure which
solid is which. He has a small amount of an authentic sample of pure A available in the lab. What should
he do?
Answer:
He should determine the melting point of just pure A and then mix some of pure A with solids A & B and
then see which one has the lower and broader range.
Question:
14. What is the stationary phase of TLC?
Answer:
Stationary phase is the silica or alumina gel layer on an inert backing. It is called stationary because it is
not moving in this phase
Question:
15. What is the mobile phase of TLC?
Answer:
The mobile ("moving") phase is a suitable liquid solvent, and draws up the plate through capillary action
