Aug 28, 2015
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Hi guys,

Can anyone help me understand why choice C is wrong? I've been assuming that the release of hydrogen is the "products" or the right side of the equation, so wouldn't Keq increase at high temperatures?

Here is the passage


Hydrogen is seen by many as the fuel of the future since its electrochemical use, or combustion, yields useful energy yet produces only water. Unfortunately storage of gaseous and/or liquid hydrogen requires extremely high pressures and low temperatures. Therefore, efforts have been made to store hydrogen in solid form. One strategy involves complexation of molecular hydrogen within metal-organic frameworks, or MOFs. MOFs consist of an organic scaffold bearing transition metal atoms, or ions where absorption of H2 takes place in what’s known as a Kubas-complex. In such a complex the σ bond of H2 donates electron density to the metal, forming a bonding interaction, as shown in Equation 1.



A different strategy involves the addition and removal of H2 from a boron-nitrogen (known as amine-borane, or AB) material as shown in Equation 2 above. Release of hydrogen from ammonia-borane (NH3BH3) relies on the fact that the molecule juxtaposes hydrogen atoms with opposite charges. The hydrogen on N is partially-positively charged, or protic, due to the high electronegativity of nitrogen. The hydrogen on boron dominates the electrons of the bond leading to a partial-negative charge and classification as hydridic. When protic hydrogen on one molecule of ammonia-borane approaches hydridic hydrogen on a second molecule, H2 is released and the B—N backbone extended (Figure 1).



Figure 1
For any storage scheme to be viable, a facile, controllable equilibrium between bound and free hydrogen must exist. More specifically, the equilibrium constant, Keq, must be tunable by variables, such as temperature, such that hydrogen is only released when required.


This is the question:

The release of hydrogen, in both strategies presented in the passage, is favored at high temperatures while sequestration of hydrogen is favored at low temperatures. Which of the following best explains this observation?

A) ΔS for the release of H2 is positive, and favored at high temperatures.

B) ΔH° for the sequestration of H2 is more favorable at low temperatures.

C) Keq for Equations 1 and 2 becomes large at high temperatures.

D) ΔH° for the sequestration of H2 is more favorable at high temperatures.

The answer is A, and this is the explaination:
A. Reactions that lead to the release of gas from a solid have a positive ΔS. Since ΔG = ΔH – TΔS, positive values of ΔS lead to negative values of ΔG at high temperatures. Choices B and D can be eliminated since ΔH° is a value defined at standard state, and doesn’t change at high or low temperatures. Choice C can be eliminated because if Keq for the reactions were large at high temperatures we’d expect more hydrogen sequestration and less release.

What are the reactants and products in this equation? and why is answer C wrong? Thanks guys
 
Dec 10, 2013
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344
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Medical Student
Did you figure this one out? I have the exact same question. As far as I can tell both A. and C. are correct choices.... Keq = (Products/Reactants) therefore at a higher temperature you would also have a higher Keq because the reaction would be favoring the products... right?
 

QuentinT88

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Sep 12, 2013
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The fact that it states that the Keq is high at large temperatures tells you that K > 1. So it would favor the product, or H sequestration. But the passage states that an equilibrium should exist. In my mind, this means that the Keq would shift back towards the reactants until K=1. Thus C would be wrong because Keq would not be > 1 at high temperatures but in equilibrium.

I was between A and B but A made the most sense to me.
 
Dec 10, 2013
474
344
Status
Medical Student
So at high temperatures the Keq is close to 1? Would this be the same for lower temperatures then; at low temperatures the Keq would also be close to 1?

I see why A is correct, that is the answer I ended up choosing, I just can't seem to grasp why C is wrong....
 

QuentinT88

5+ Year Member
Sep 12, 2013
457
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Status
Medical Student (Accepted)
My way of viewing this is that the mechanism can be in equilibrium both ways, the sequestration ( solid H2) AND the free form of H2 (gas) could both be the product. I think the key concept to remember is that the only way for this mechanism to work is if equilibrium is maintained.

So since both can be the product, and if K > 1 ( favoring products of release/ gaseous H2) then this would not be consistent with high temperature. Instead, it would shift back towards K=1 towards equilibrium. Or since there were already a lot of lot of gas released, it would shift back to the reactants, towards more solid form of H2 to form at high temperatures.

Overall I think what the question is trying to get out is the main driving force behind the mechanism is thermodynamics so A is the best answer.
 

ThePrincetonReview

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Hi guys,

Can anyone help me understand why choice C is wrong? I've been assuming that the release of hydrogen is the "products" or the right side of the equation, so wouldn't Keq increase at high temperatures?

Here is the passage


Hydrogen is seen by many as the fuel of the future since its electrochemical use, or combustion, yields useful energy yet produces only water. Unfortunately storage of gaseous and/or liquid hydrogen requires extremely high pressures and low temperatures. Therefore, efforts have been made to store hydrogen in solid form. One strategy involves complexation of molecular hydrogen within metal-organic frameworks, or MOFs. MOFs consist of an organic scaffold bearing transition metal atoms, or ions where absorption of H2 takes place in what’s known as a Kubas-complex. In such a complex the σ bond of H2 donates electron density to the metal, forming a bonding interaction, as shown in Equation 1.



A different strategy involves the addition and removal of H2 from a boron-nitrogen (known as amine-borane, or AB) material as shown in Equation 2 above. Release of hydrogen from ammonia-borane (NH3BH3) relies on the fact that the molecule juxtaposes hydrogen atoms with opposite charges. The hydrogen on N is partially-positively charged, or protic, due to the high electronegativity of nitrogen. The hydrogen on boron dominates the electrons of the bond leading to a partial-negative charge and classification as hydridic. When protic hydrogen on one molecule of ammonia-borane approaches hydridic hydrogen on a second molecule, H2 is released and the B—N backbone extended (Figure 1).



Figure 1
For any storage scheme to be viable, a facile, controllable equilibrium between bound and free hydrogen must exist. More specifically, the equilibrium constant, Keq, must be tunable by variables, such as temperature, such that hydrogen is only released when required.


This is the question:

The release of hydrogen, in both strategies presented in the passage, is favored at high temperatures while sequestration of hydrogen is favored at low temperatures. Which of the following best explains this observation?

A) ΔS for the release of H2 is positive, and favored at high temperatures.

B) ΔH° for the sequestration of H2 is more favorable at low temperatures.

C) Keq for Equations 1 and 2 becomes large at high temperatures.

D) ΔH° for the sequestration of H2 is more favorable at high temperatures.

The answer is A, and this is the explaination:
A. Reactions that lead to the release of gas from a solid have a positive ΔS. Since ΔG = ΔH – TΔS, positive values of ΔS lead to negative values of ΔG at high temperatures. Choices B and D can be eliminated since ΔH° is a value defined at standard state, and doesn’t change at high or low temperatures. Choice C can be eliminated because if Keq for the reactions were large at high temperatures we’d expect more hydrogen sequestration and less release.

What are the reactants and products in this equation? and why is answer C wrong? Thanks guys

The key concept to understand here is the difference between ΔG and ΔG° (you may need to review this concept again for comprehension). The question asks why the release of hydrogen is favored at high temperatures. This is equivalent to asking why is the release of hydrogen is spontaneous at high temperatures. Spontaneity is determined by ΔG, not ΔG°. As the equation for ΔG° is ΔG°=-RTln(Keq), you can see that Keq is related with ΔG°, which only determines whether there is more reactant or product at equilibrium but does not explain whether the reaction is spontaneous or not. This is why choice C is incorrect. The correct answer of A uses the equation for ΔG, which does explain why the reaction is spontaneous at high temperatures.


If you would like further assistance with understanding thermodynamics more thoroughly for the MCAT exam, feel free to PM me!