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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
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