AAMC Chemistry Section Bank #8, #12, #20

Ray
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Joined: Fri Mar 02, 2018 11:51 pm

AAMC Chemistry Section Bank #8, #12, #20

Postby Ray » Tue Apr 24, 2018 12:27 am

Hi,

For question #8, I feel like it should be zero, but I don't quite understand the reason behind it.

For question #12, why does it subtract the work function from hf?

For question #20, I actually chose C which is opposite to the right answer. Could you explain this one more clearly?

Thank you very much.
NS_Tutor_Andrew
Site Admin
Posts: 520
Joined: Mon May 23, 2016 1:47 pm

Re: AAMC Chemistry Section Bank #8, #12, #20

Postby NS_Tutor_Andrew » Tue Apr 24, 2018 12:05 pm

Hi Ray,

For questions 8 & 12, the common thread is the photoelectric effect and the work function. (The photoelectric effect applies directly to Q12, and indirectly to Q8, because breaking a bond means ejecting electrons out of it by definition, so the underlying concept is the same). If you're a course student, I'd suggest reviewing pg. 231-234 of your physics textbook for an overview of this concept. If not, the Khan Academy video on the topic can do the job. I'd suggest reviewing this material carefully b/c both of these questions deal w/ these concepts, so missing both may suggest that you could benefit from a thorough review of the material.

With that in mind, in a nutshell, the reason why the answer to 8 is zero is because if only just enough energy is used to break the bond, no energy is "left over", so to speak. For Q12, the Q refers to a photoelectron (i.e. an electron that has been liberated from an atom by EM radiation), and the work function can be thought of as the "start-up cost" in terms of energy of getting an electron out and moving.

Re: Q20, this question is notoriously puzzling and comes up a lot. If you Google it, you can find several discussions on Reddit/SDN, but my approach to solving this question is a little bit different from those explanations, which tend to get caught up in intricate reasoning about reversibility, which I think makes this more complicated than it needs to be.

The first step in answering this question is to figure out what exactly they're asking you to explain: in this case, the key observation is that the liposomes from Compound 1 were stable to mixing, while those from Compound 2 formed new liposomes after mixing with "an average size expected for the effective final lipid concentration." The next step is to figure out, based on that, whether you can eliminate any answer choices. A and D can both be eliminated without even understanding the substance of the question, because the question is asking you to explain a difference between the Compound 1 liposomes and the Compound 2 liposomes, and A and D state that they both are under the same kind of control, which wouldn't explain the presence of different outcomes. Now you're at the point of a 50:50 guess, and you can choose to either make that guess or to dig into the question to tease these two options apart. Depending on how much time you have and how you've been doing on the C/P section, the best strategic move may actually be to just guess between B and C and move on, as it took very little time/energy/insight to eliminate A and D.

The key idea with thermodynamic control is that the reaction gets to its final, most energetically preferable state, whereas the basic idea of kinetic control is that "stuff just happens"—things move quick, and whichever reaction pathway happens most easily just happens. The fact that all of the liposomes made with Compound 2 have the same average size suggests that they have all reached a common stable endpoint (which is what we'd expect for thermodynamic control), whereas the fact that the liposomes from Compound 1 have different sizes suggest that more or less "whatever happened, happened", which is what you'd associate with kinetic control.

Hope this helps, & best of luck as you study!!
Andrew D.
Content Manager, Next Step Test Prep.

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