Question from BS

[w1ll]

This question was kind of weird and appeared in a BS passage. I don't think you really need the passage to answer the question. I'm using EK books and I can't really think of where this is explained besides possibly... (i'll say later as it might be a hint). Here goes:

Which of the following rationales explains which compound is the product of kinetic control?

A) Its semicarbazone has the lower melting point.
B) It forms faster at all temperatures because its rate of formation is independent of its path.
C) Its reaction profile has the lower energy of activation.
D) Its reaction profile has the higher energy of activation

It was then followed by a question about what the Thermodynamically controlled product was, but you needed passage info for that.

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[RPedigo]

The kinetic product is the one that forms fastest, which would be the one with the lowest activation energy. The thermodynamic product is the one, regardless of activation energy, makes the most stable endproduct. So I'd guess (c) on this one.

 

[Sam212]

This question was kind of weird and appeared in a BS passage. I don't think you really need the passage to answer the question. I'm using EK books and I can't really think of where this is explained besides possibly... (i'll say later as it might be a hint). Here goes:

Which of the following rationales explains which compound is the product of kinetic control?

A) Its semicarbazone has the lower melting point.
B) It forms faster at all temperatures because its rate of formation is independent of its path.
C) Its reaction profile has the lower energy of activation.
D) Its reaction profile has the higher energy of activation

It was then followed by a question about what the Thermodynamically controlled product was, but you needed passage info for that.

Kinetic product forms fast, so it must have lower energy of actiivation, thus the answer is C.

 

[BloodySurgeon]

ditto answer choice c... lower Ea means that it is the easiest/quickest path to take. The problem with most thermodynamic products is that it requires more energy and if enough energy is not supplied then it cant pass that large Ea bump and the major product will be the kinetic product.

 

[w1ll]

Cool thanks guys, that was easy enough. For some reason I can't find this info in the EK books. Would this be something discussed in Orgo, or Bio Enzymes or something?

 

[w1ll]

Also I didn't want to open another thread but I had a question from the PS section I don't understand.

A sparingly soluble metal hydroxide, M(OH)2 has a molar solubility of S mol/L at 25°C. Its Ksp value is:

A) S2.
B) 2S2.
C) 2S3.
D) 4S3.

 

[BloodySurgeon]

Also I didn't want to open another thread but I had a question from the PS section I don't understand.

A sparingly soluble metal hydroxide, M(OH)2 has a molar solubility of S mol/L at 25°C. Its Ksp value is:

A) S2.
B) 2S2.
C) 2S3.
D) 4S3.

the answer is D--> 4S^3

molar solubility = S

M(OH)2 <---> M2+ + 2OH-

Ksp = [M2+][OH-]
Ksp = (S) (2S)^2
Ksp= (S) (4S^2)
Ksp= 4S^3


Hope that helps.. best of luck!

 

[w1ll]

the answer is D--> 4S^3

molar solubility = S

M(OH)2 <---> M2+ + 2OH-

Ksp = [M2+][OH-]
Ksp = (S) (2S)^2
Ksp= (S) (4S^2)
Ksp= 4S^3


Hope that helps.. best of luck!

Thanks that makes sense. This may be a stupid question but, why did you set M2+ = S when all they tell you is that the whole compound = S?

 

[BloodySurgeon]

Thanks that makes sense. This may be a stupid question but, why did you set M2+ = S when all they tell you is that the whole compound = S?

A sparingly soluble metal hydroxide, M(OH)2 has a molar solubility of S mol/L at 25°C. Its Ksp value is:

I think you answered your own question. S is only a letter that represents the molar solubility. It could be any letter and by convention its actually usually X.

 

[w1ll]

I think you answered your own question. S is only a letter that represents the molar solubility. It could be any letter and by convention its actually usually X.

Hmm I think the wording threw me off, I was thinking that S = whole compound, so M2+ is only a fraction of S.... which it is in a way since its only a fraction of the Ksp. Maybe I'm over thinking this? Its late here and I've been in the library for what seems like an eternity the last few months lol.

 

[BloodySurgeon]

Np.. we've all been there. I can't tell you how many times i've came out of a library with no brain cells to spare

 

[SketchLazy]

Hmm I think the wording threw me off, I was thinking that S = whole compound, so M2+ is only a fraction of S.... which it is in a way since its only a fraction of the Ksp. Maybe I'm over thinking this? Its late here and I've been in the library for what seems like an eternity the last few months lol.

Just to be clear, S does equal the amount of moles for the whole compound. But when the compound dissociates into ions in solution, the total number of moles of an ion in solution is S times the number of ions in the chemical formula. That's why you use S/L for the concentration of Mg and 2S/L for the concentration of OH to calculate the solubility product for Mg(OH)2.