br gchem 2, thermodynamics passage 3 #20

[Rosalindbungs]

Which change results in LESS energy being released in the overall process?
a. using a metal that is easier to sublime than lithium
b. using a hlogen that forms a stronger bond as a diatomic molecule than fluorine.
c. using a halogen atom with a greater electron affinity than fluorine
d. using a cation that can easily lose a 2nd electron

answer: B

I don't understand why the answer key says that when a process releases more energy when you input less energy...

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[Pamplemousse123]

Which change results in LESS energy being released in the overall process?
a. using a metal that is easier to sublime than lithium
b. using a hlogen that forms a stronger bond as a diatomic molecule than fluorine.
c. using a halogen atom with a greater electron affinity than fluorine
d. using a cation that can easily lose a 2nd electron

answer: B

I don't understand why the answer key says that when a process releases more energy when you input less energy...

Hi there, so this question is essentially asking for the enthalpy of the reaction (heat released or added in a reaction). The answer should be B in this case because if you form a stronger bond, you need more energy to break the bond as compared to a molecule that doesn't require as much energy to break. This of it in this way: if you are a strong muscular man, it will take more energy to know that guy down relative to a weak man (i could've added humor to the analogy, but i think this is best for understanding purposes). I have a afeeling that you are not correlating strong bond with more energy required to break the bond. THis answer can also be represented with the equation Hrxn = hbonds broken - hbonds formed. if Hrxn is positive, energy needs to be added to the system in order for it to proceed. If you have a bigger hbonds broken (by increasing the strength of the bonds of the reactants), you will need more energy added to the system. i hope i am being clear!

 

[BerkReviewTeach]

Which change results in LESS energy being released in the overall process?
a. using a metal that is easier to sublime than lithium
b. using a hlogen that forms a stronger bond as a diatomic molecule than fluorine.
c. using a halogen atom with a greater electron affinity than fluorine
d. using a cation that can easily lose a 2nd electron

answer: B

I don't understand why the answer key says that when a process releases more energy when you input less energy...

For this question, you should note that they are asking about the overall energy of the five steps (of the Born-Haber cycle) combined. Less energy released overall, would mean that deltaH for the overall process was a less negative number. The change that will cause the overall deltaH to increase (become less negative) is the one that makes the deltaH for the corresponding step either more positive or less negative.

Choice A is input energy (converting the reactant solid into a reactant gas), so reducing the energy of that step means we invest less energy, so we should get more out overall. That is not our answer.

Choice B is also input energy (breaking the bond within a reactant molecule), so increasing the strength of that bond means we invest more energy, so we should get less out overall. That must be our answer.

Choice C is output energy (the neutral halogen atom gaining an electron releases energy--thus the negative deltaH value), so using a halogen that wants to gain an electron even more in that step means we would get more energy released, so we should get more out overall. That is not our answer.

Choice D is input energy (removing an electron from the gaseous metal), so reducing the energy of that step means we invest less energy, so we should get more out overall. That is not our answer.

• Going through the changes one-by-one shows:
  Choice A: less positive deltaH
  Choice B: more positive deltaH
  Choice C: more negative deltaH
  Choice D: less positive deltaH

Choice B stands out as different.