9 days till exam ahh- some chem questions

[mgdent]

---

Members do not see this ad.

42. If 1 mole of N2 and 1 mole of H2 are
mixed and allowed to react according to
the equation N2 + 3H2 --> 2NH3. What
is the maximum number of moles of NH3
that could be produced?
Answer: 2/3


62. Given the following half-cell reactions:
Cl2(g) + 2e- 2Cl-(aq) E°= +1.36v
Cu2+(aq) + 2e- Cu(s) E°= +0.34v
What is the value of E° for the following
reaction?
Cu2+(aq) + 2Cl-(aq) Cu(s) + Cl2(g)
A. -2.38v
B. -1.70v
C. -1.02v
D. +1.02v
E. +1.70v

It says the answer is C but I thought it was D. 0.34 is more negative and therefore would be oxidized so I changed the sign and did 1.36 + (-.34)= 1.02.

62. Just wanted to confirm that H2Se is not linear but rather bent?

67. The electronic configuration of a
particular neutral atom is
1s22s22p63s23p2. What is the number of
unpaired electrons in this atom?
A. 1
B. 2
C. 3
D. 4
E. 0

I have seen this discussed on other threads about a possible mistake in the answer key so I wanted to go over this here. The answer key says the correct answer is C. I thought the answer was D, 4. Anyone know what the correct answer is?

83. Which structure represents a trans (E)
isomer?

How is this possible. Trans and E are different.. Trans would have 2 H's in opposite directions and E would be opposite priority substituents. They say the answer is
F-C-H
H-C-Br (double bond between the C's)


Lastly, are enantiomers always racemic mixtures? I posted this before and someone answered:

Racemic:
CHO
H--OH --->> r configuration
OH--H --->> r configuration
CHO

and

CHO
OH-H --->> s configuration
H-OH --->> s configuration
CHO
--these are racemixtures because they are both exactly the opposite configurations (enantiomers)
-if they were, say RS and the other was SR, they would be a racemic once again as they are enantiomers.
--If one was, say RR and the other RS, they would diastereomers because they different at exactly one charilty center

Now what confuses me is the following are enantionmers.

H
HO--CH3 --->> s configuration
HO--CH2CH3 --->> R configuration
H

H
CH3-OH --->> R configuration
CH2CH3-OH --->> S configuration
H

So are a pair of enationmers also a racemic mixture? I know this post is long but test day is coming up fast and I need some help with this stuff. Thanks A LOT!

 

[UCB05]

42. If 1 mole of N2 and 1 mole of H2 are
mixed and allowed to react according to
the equation N2 + 3H2 --> 2NH3. What
is the maximum number of moles of NH3
that could be produced?
Answer: 2/3

Simple stoich. H2 is the limiting reagent and the conversion ratio is 2mols NH3 per 3 mols H2

62. Given the following half-cell reactions:
Cl2(g) + 2e- 2Cl-(aq) E°= +1.36v
Cu2+(aq) + 2e- Cu(s) E°= +0.34v
What is the value of E° for the following
reaction?
Cu2+(aq) + 2Cl-(aq) Cu(s) + Cl2(g)
A. -2.38v
B. -1.70v
C. -1.02v
D. +1.02v
E. +1.70v

It says the answer is C but I thought it was D. 0.34 is more negative and therefore would be oxidized so I changed the sign and did 1.36 + (-.34)= 1.02.

The question isn't concerned with spontaneity. It gives you the equation to calculate. Cu2+ is reduced and Cl- is oxidized. The sign must be flipped on the Cl reaction to get -1.36 + 0.34 = -1.02. +1.02v is the potential of the spontaneous reverse reaction, but that wasn't what was asked.

62. Just wanted to confirm that H2Se is not linear but rather bent?

Correct. Like water.

67. The electronic configuration of a
particular neutral atom is
1s22s22p63s23p2. What is the number of
unpaired electrons in this atom?
A. 1
B. 2
C. 3
D. 4
E. 0

I have seen this discussed on other threads about a possible mistake in the answer key so I wanted to go over this here. The answer key says the correct answer is C. I thought the answer was D, 4. Anyone know what the correct answer is?

The correct answer is 2. 4 is the number of electrons in the valence shell, of which two are paired in an s orbital and two are unpaired in their own p orbitals.

83. Which structure represents a trans (E)
isomer?

How is this possible. Trans and E are different.. Trans would have 2 H's in opposite directions and E would be opposite priority substituents. They say the answer is
F-C-H
H-C-Br (double bond between the C's)

Perhaps you're looking at the wrong substituents. Cis/trans, E/Z are determined by the high-priority substituents. You should be looking at the positions of F and Br, not the hydrogens. This molecule is trans because F and Br on on opposite sides, and it is E for the same reason. E/Z matters when there are more than two substituents. If the molecule were
F-C-H
F-C-Br
Then cis/trans doesn't really make sense. F has higher priority than H on top, Br has higher priority than F on the bottom. This molecule would be E.

Lastly, are enantiomers always racemic mixtures? I posted this before and someone answered:

Racemic:
CHO
H--OH --->> r configuration
OH--H --->> r configuration
CHO

and

CHO
OH-H --->> s configuration
H-OH --->> s configuration
CHO
--these are racemixtures because they are both exactly the opposite configurations (enantiomers)
-if they were, say RS and the other was SR, they would be a racemic once again as they are enantiomers.
--If one was, say RR and the other RS, they would diastereomers because they different at exactly one charilty center

"Racemic" is an adjective used to describe the optical property of a solution, not one or both entantiomers. You can have a non-racemic solution of one or both enantiomers. A mixture is only racemic when both enantiomers are present in exact quantities. This allows the two enantiomers to rotate light in opposite directions to the same degree, canceling out each other's optical activity. Diastereomers have different physical and chemical properties, including optical rotation, which will make it highly unlikely that equal concentrations of two will result in an optically inactive mixture.


Now what confuses me is the following are enantionmers.

H
HO--CH3 --->> s configuration
HO--CH2CH3 --->> R configuration
H

H
CH3-OH --->> R configuration
CH2CH3-OH --->> S configuration
H

So are a pair of enationmers also a racemic mixture? I know this post is long but test day is coming up fast and I need some help with this stuff. Thanks A LOT!

As I said above, two enantiomers aren't "racemic." The solution they exist in is racemic if they are present in equal quantities.

 

[mgdent]

Thanks for those answers, that helped. One question though. I am confused about number 62. I see that Cu2+ is reduced and Cl is oxidized so just flip the sign for oxidation? Maybe I am beginning to mix things up lol. I have only seen problems given a - and + and then I flip the more negative to oxidation and get a + (spon). But this makes sense too- so basically, oxidation FLIP sign?

So if it were both - values, flip the ox to +? Also, what would happen in an electrolytic cell. Since the anode is + what changes would occur in the answers. Thanks a lot again!

 

[mgdent]

any thoughts? thanks guys

 

[cm8a12321]

Thanks for those answers, that helped. One question though. I am confused about number 62. I see that Cu2+ is reduced and Cl is oxidized so just flip the sign for oxidation? Maybe I am beginning to mix things up lol. I have only seen problems given a - and + and then I flip the more negative to oxidation and get a + (spon). But this makes sense too- so basically, oxidation FLIP sign?

So if it were both - values, flip the ox to +? Also, what would happen in an electrolytic cell. Since the anode is + what changes would occur in the answers. Thanks a lot again!

This is a trick to approach this type of question: Think of this like a Hess' Law type question, but note that coefficient of the half reaction DO NOT apply to electrochemistry (so, no multiplying or dividing to match up the total equation) if you want to use this trick.

Eg.

"Cl2(g) + 2e- 2Cl-(aq) E°= +1.36v
Cu2+(aq) + 2e- Cu(s) E°= +0.34v

What is the value of E° for the following
reaction?

Cu2+(aq) + 2Cl-(aq) Cu(s) + Cl2(g)" : GIVEN

What I do first is look at the given and play w/ my half rxn to get to this equation.

For the first half rxn, You see that you want the 2Cl- to go to the left to match up the GIVEN; Thus, you switch this half rxn to the left. And by doing this, you must also switch the sign of E° to -1.36V.

For the second half rxn, you see that Cu2+(aq) is on the left and matches with the GIVEN, so you do not need to change E°.

To find E°, all you have to do is add the new -1.36+0.34, which gives you -1.02 V.

This trick always work. Just remember that coefficient of the half reaction DO NOT apply to electrochemistry (so, no multiplying or dividing to match up the total equation) if you want to use this trick.

 

[cm8a12321]

42. If 1 mole of N2 and 1 mole of H2 are
mixed and allowed to react according to
the equation N2 + 3H2 --> 2NH3. What
is the maximum number of moles of NH3
that could be produced?
Answer: 2/3

N2 + 3H2 --> 2NH3 says that 1 mol N2 and 3 mol H2 make 2 mol NH3.

B/c you have 1 mole N2 and 1 mole H2, you know that your 1 mole of H2 limits you from making more NH3 (1 mole known H2<1 mole NH3); thus, you can use this 1 mole H2 to find max amount of NH3 by applying dimensional analysis: 1 mole H2 (2 mol NH3/ 3 mole H2)= 2/3 mol NH3

62. Given the following half-cell reactions:
Cl2(g) + 2e- 2Cl-(aq) E°= +1.36v
Cu2+(aq) + 2e- Cu(s) E°= +0.34v
What is the value of E° for the following
reaction?
Cu2+(aq) + 2Cl-(aq) Cu(s) + Cl2(g)
A. -2.38v
B. -1.70v
C. -1.02v
D. +1.02v
E. +1.70v

It says the answer is C but I thought it was D. 0.34 is more negative and therefore would be oxidized so I changed the sign and did 1.36 + (-.34)= 1.02.

CHECK MY POST #4 ABOVE

62. Just wanted to confirm that H2Se is not linear but rather bent?

Yes. H2Se contains a maximum of 8 valence electrons (6 from Se and 2 from the 2 H's). So, if you draw the figure one, Se is connected to 2 H, where the former is on the left and the latter is on the right. However, you have two lone pairs on the Se atom, which helps to make this overall structure a bent shape.

67. The electronic configuration of a
particular neutral atom is
1s22s22p63s23p2. What is the number of
unpaired electrons in this atom?
A. 1
B. 2
C. 3
D. 4
E. 0

I have seen this discussed on other threads about a possible mistake in the answer key so I wanted to go over this here. The answer key says the correct answer is C. I thought the answer was D, 4. Anyone know what the correct answer is?

This configuration actually represents Silicon (Si). Si is a metalloid that does not have an electron jump like the three infamous transition metals: Cu, Cr, and Mo. Thus, at ground state, their configuration remains the same.

So, when finding how many unpaired electrons, you pick the valence electron of Si, which is 4 (3s^2 3p^2); The s shell (which contains a max of one orbital) is filled completely according to Hund's rule and the p shell (which have a max of 3 orbitals) is filled halves for two orbitals (Note: Each orbital contains a maximum of 2 electrons). Therefore, the answer to this question is 2 unpaired electrons, not 4 or 3.

83. Which structure represents a trans (E)
isomer?

How is this possible. Trans and E are different.. Trans would have 2 H's in opposite directions and E would be opposite priority substituents. They say the answer is
F-C-H
H-C-Br (double bond between the C's)

Z=means Cis conformation
E=means trans conformation

Priority for E/Z substitutient are based on molecular weight.

Eg.

Me-C-isopropyl
Me-C-Br (double bond between the C's; Me=methyl)

The answer for this is Z (MW of Br>MW of Me)



Lastly, are enantiomers always racemic mixtures? I posted this before and someone answered:

Racemic:
CHO
H--OH --->> r configuration
OH--H --->> r configuration
CHO

and

CHO
OH-H --->> s configuration
H-OH --->> s configuration
CHO
--these are racemixtures because they are both exactly the opposite configurations (enantiomers)
-if they were, say RS and the other was SR, they would be a racemic once again as they are enantiomers.
--If one was, say RR and the other RS, they would diastereomers because they different at exactly one charilty center

Now what confuses me is the following are enantionmers.

H
HO--CH3 --->> s configuration
HO--CH2CH3 --->> R configuration
H

H
CH3-OH --->> R configuration
CH2CH3-OH --->> S configuration
H

So are a pair of enationmers also a racemic mixture? I know this post is long but test day is coming up fast and I need some help with this stuff. Thanks A LOT!

Racemic mixture, in plain English, is the tendency to switch from R to S or vice versa. Eg. if you do a SN1 rxn, you would not only have a back attack but a front attack as well. This tendency to attack on either side will switch your R/S configuration.

So if you have a container that is R and S in a fischer projection, the enantionmer for this fischer projection is S and R.

In order to be a racemate, I would have to equal amounts of it (2 mole of R and S and 2 mole of S and R). Just flat out fisher projection of the enationmers is hard to tell.

Enantiomers are mirror images: like one is RS--->SR, but diastereomers are images that are chiral and retain one config but the others does not, such as RS--->RR or RS--->SS

I hope this help clarify your problems.

 

[mgdent]

thanks so much, that was great. one thing though- if given an electrolytic cell instead of galvanic, what changes would I have to do. I know that anode and cathode signs change but I havent seen any problems yet asking for the emf or Eo for electrolytic.

 

[cm8a12321]

The DAT would not give something like electrolytic calculation. But, if they would, all you need to know is that an electrolytic cell is non-spontaneous and does not follow the norm of a galvanic (or voltaic) cell. Thus, you would need to apply the Nernst equation: E=E°+(RT/nF) ln(Keq).