Random TPRH Questions

[dennis-brodmann]

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I've been working from TPRH and have a few random questions since the topic/explanations didn't make sense to me:

Biology Discretes

#28: Does a (+) RNA Virus lead to a (-) strand? and vice versa?

#89:
How does countercurrent exchange with blood control urine composition?

Organic:

Passage 4
Question 4: I understand that enantiomers have different absolute configurations, but I was taught in Organic Chemistry that one cannot say whether an enantiomer is (+) or (-) unless an optical rotation confirms it?

 

[Laureatebarrel]

#89: I believe countercurrent exchange sets up concentration gradients in the renal medulla, thus makes it possible for urine to be concentrated in the collecting ducts.
Organic: (+) or (-) are optical rotation and that is different from enationmer (R) and (S). R, S you can figure out with the formula but (+) or (-) you have to experiment. Enantiomers will have the same optical rotation but opposite sign so if you know the optical rotation of one you know the optical rotation of the other. Hope that makes sense.

 

[dennis-brodmann]

#89: I believe countercurrent exchange sets up concentration gradients in the renal medulla, thus makes it possible for urine to be concentrated in the collecting ducts.
Organic: (+) or (-) are optical rotation and that is different from enationmer (R) and (S). R, S you can figure out with the formula but (+) or (-) you have to experiment. Enantiomers will have the same optical rotation but opposite sign so if you know the optical rotation of one you know the optical rotation of the other. Hope that makes sense.

What do you mean by formula?
Do you mean using priority rules to figure out R and S configurations?
Okay, so if I have an R compound with a (+) optical rotation, its enantiomer
will be S with a (-)?

I guess that makes sense for one chiral center, but what about multiple chiral centers?

e.g.) I have a 3 chiral centered-compound which implies I have 8 different stereoisomers that can result (from # stereoisomers = 2^n, where n is # of stereocenters).

RRR; SRR; RSR; RRS
SSS; RSS; SRS; SSR

If an optical rotation experiment was done on the RRR compound that showed each chiral center was (+), does that imply SSS is (-)?

If that's true, does that mean we wouldn't have to go back and do tests on the other 6 molecules?

Would that mean we would automatically know that SRR is (+), (-), (-)?


Thanks truongta1990!

 

[dennis-brodmann]

I don't quite understand the logic behind the computation for this question:

The space shuttle's liquid rockets use a fuel of liquid oxygen and hydrogen which react to give water vapor. The most efficient burn occurs when the ratio of hydrogen to oxygen gas is about 2 to 1. If one-fourth of the gas in the rocket nozzle is always water vapor, and the pressure in the nozzle is 10 atm, what H2 partial pressure should be maintained for an efficient burn?

The way I thought you calculated was as follows:

There is a total pressure of 10 atm. A fourth of this is the pressure due to water vapor,
i.e. (1/4)*(10 atm) = 5/2 atm = 2.5 atm

Because the ratio of H2:O2 is 2:1, I thought the molar ratio for H2 is 2/3.
=> The partial pressure due to hydrogen gas is
(Molar Ratio H2)*(Pressure due to Water Vapor)=(2/3)*(2.5)=5/3=1.6667

Anyway, that's not the answer.
The correct answer is 5.0 atm.

The explanation in the back said that the total pressure of hydrogen and oxygen is 7.5 atm.
I feel like they're using the 2:1 ratio such that 2.5 atm H2 + 2.5 atm H2+ 2.5 atm O2 = 7.5 atm H2 and O2.
This is throwing me off though because 2.5 atm represents water vapor, i.e. H2O(g), not H2 or O2.

Help!!!!!

 

[sdm33]

I think Countercurrent exchange could mean anything if you not specific, I have never seen Renal Q that vague?

 

[dennis-brodmann]

Passage 11 Question #6:

To show that ethylene has a higher heat of hydrogenation than cis-2-butene, a scientist notes that the catalytic hydrogenation of ethylene:
A. is stereospecific.
B. has a positive heat of formation.
C. requires a much greater energy of activation.
D. releases more energy than does hydrogenation of cis-2-butene.

The answer is D, but I was really wondering why C isn't true. I thought that for exothermic reactions, the greater the energy of the reaction (deltaH), the higher the activation energy.

Is that not necessarily true? Can two compounds have the same activation energy but differ in energy released for the same reaction type?

Passage 12 Question #5:
1-Pentene + HI --> (-)-2-Iodopentane + (+)-2-Iodopentane (Given as a figure)
The mechanism of this reaction is such that equal amounts of each product are formed. The reaction mechanism proceeds via a planar intermediate which can be attacked by an iodide ion on either side of the plane to yield equal amounts of (+) and (-) products.

Which of the following statements is NOT correct concerning the reaction of 1-pentene with hydrogen iodide?
A. The slow step of the reaction is the formation of the intermediate carbocation.
B. The overall rate of the reaction depends on the concentrations of both 1-pentene and hydrogen iodide.
C. The products form a racemic mixture.
D. (+)- and (-)-2-Iodopentane have differing boiling points.

The correct answer is D, which I chose. I'm kind of confused as to why B is true.
I originally thought this was an SN1 reaction in which the rate of the reaction would only depend on the formation of the planar intermediate from the substrate and not the nucleophile. Now I realize that one of the reactants is an alkene and NOT an alkyl halide. Is THIS why the rate is dependent on both reactants?

For an SN1 reaction, only an alkyl halide influences the rate (i.e. rate=k[alkyl halide]); this describes the slow step. The slow step determines the rate. The answer choice does say overall rate - doesn't this statement imply the slow step as well?

 

[dennis-brodmann]

bump

 

[circulus vitios]

The answer is D, but I was really wondering why C isn't true. I thought that for exothermic reactions, the greater the energy of the reaction (deltaH), the higher the activation energy.

Is that not necessarily true? Can two compounds have the same activation energy but differ in energy released for the same reaction type?

Activation energy is kinetics. They're asking about thermodynamics. Look at these reactions where the thermodynamics (delta H) are the same but the kinetics (activation energy) are vastly different.

 

[circulus vitios]

I don't quite understand the logic behind the computation for this question:

The space shuttle's liquid rockets use a fuel of liquid oxygen and hydrogen which react to give water vapor. The most efficient burn occurs when the ratio of hydrogen to oxygen gas is about 2 to 1. If one-fourth of the gas in the rocket nozzle is always water vapor, and the pressure in the nozzle is 10 atm, what H2 partial pressure should be maintained for an efficient burn?

The way I thought you calculated was as follows:

There is a total pressure of 10 atm. A fourth of this is the pressure due to water vapor,
i.e. (1/4)*(10 atm) = 5/2 atm = 2.5 atm

Because the ratio of H2:O2 is 2:1, I thought the molar ratio for H2 is 2/3.
=> The partial pressure due to hydrogen gas is
(Molar Ratio H2)*(Pressure due to Water Vapor)=(2/3)*(2.5)=5/3=1.6667

Anyway, that's not the answer.
The correct answer is 5.0 atm.

The explanation in the back said that the total pressure of hydrogen and oxygen is 7.5 atm.
I feel like they're using the 2:1 ratio such that 2.5 atm H2 + 2.5 atm H2+ 2.5 atm O2 = 7.5 atm H2 and O2.
This is throwing me off though because 2.5 atm represents water vapor, i.e. H2O(g), not H2 or O2.

Help!!!!!

You're given the mole ratio of hydrogen with respect to oxygen, or vice versa. You don't know anything about the mole ratio of hydrogen and oxygen with respect to water vapor, so you cannot multiply (2/3) by the pressure of water vapor.

10 atm * 25% water vapor = 2.5 atm water vapor
10 atm gas - 2.5 atm water vapor = 7.5 atm hydrogen and oxygen gases.
Hydrogen😳xygen ratio is 2:1 or 2/3.
7.5 atm * (2/3) = 5.0 atm hydrogen gas
7.5 atm * (1/3) = 2.5 atm oxygen gas

 

[dennis-brodmann]

Activation energy is kinetics. They're asking about thermodynamics. Look at these reactions where the thermodynamics (delta H) are the same but the kinetics (activation energy) are vastly different.

When I took Biochemistry I, the activation energy - the energy required to reach the transition state - was always expressed as

, so I thought it was as a thermodynamic variable. Anyway, your explanation and figure make more sense to me, so I'll just go with that lol

You're given the mole ratio of hydrogen with respect to oxygen, or vice versa. You don't know anything about the mole ratio of hydrogen and oxygen with respect to water vapor, so you cannot multiply (2/3) by the pressure of water vapor.

10 atm * 25% water vapor = 2.5 atm water vapor
10 atm gas - 2.5 atm water vapor = 7.5 atm hydrogen and oxygen gases.
Hydrogen😳xygen ratio is 2:1 or 2/3.
7.5 atm * (2/3) = 5.0 atm hydrogen gas
7.5 atm * (1/3) = 2.5 atm oxygen gas

I guess what threw me off was that they said The space shuttle's liquid rockets use a fuel of liquid oxygen and hydrogen which react to give water vapor.
I didn't think that the H2 and O2 populated the rest of the shuttle, but I guess you can't assume that there are other gases in the shuttle unless the question mentions it.

Thanks for your help circulus vitios!

 

[image187]

countercurrent exchange which takes place in the medullary part of the loop of henle, basically sets up a medullary osmolarity which becomes the basis for the kidneys ability to concentrate the urine. At the end of the collecting duct system when water is drawn out of the filtrate, this medullary osmolarity allows the kidney to concentrate the urine to about 1200 mosm, which is pretty amazing considering this basically exceeds the concentration of any other substance in the body by a lot.