AAMC Phys/Chem/BC Section pack q#8

[piscean92]

Hello guys,

I am having a hard time understanding the reasoning behind this question. I got it right somehow. What I was thinking during this question was the question asks about "energy meter" which is mentioned in the last line of the 2nd para, and that same line talks about "work function" and a number is given. So, I picked a number lol. Also, I thought of things I know about work like it is a process function (path-dependent) and the work function equation, but I am not sure if those two apply here (or how). I shall greatly appreciate it if someone can help me with this question. Thanks in advance.
(Please ignore all the highlighting, it just works best for me.)

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[aldol16]

I'm not personally familiar with this technique, but based on the information in the passage, the work function is irrelevant here. The work function is basically like a physical "activation energy" for the energy meter before it can begin detecting. Anyway, the key to this question is to recognize the function of the energy meter. Ideally, you would shine a light on the sample and all the photons would either cause the bonds to break or heat up the sample. In this ideal case, the difference between the laser pulse energy and the energy detected by your oscilloscope will be the bond enthalpy. The problem is, this doesn't happen because photochemical processes aren't 100% efficient. That is, not all the photons will be doing one of those two things. Some photons get transmitted through and don't "react" in the way you want. That's what the energy meter is designed to detect. And so at the end, you have to correct for the photochemical efficiency, which is what it says in the next paragraph.

Now, if an appropriate laser is used such that photochemical efficiency is 100%, then you won't get transmittance and all the pulse energy will be used to either break bonds or heat up the solution.

 

[piscean92]

I'm not personally familiar with this technique, but based on the information in the passage, the work function is irrelevant here. The work function is basically like a physical "activation energy" for the energy meter before it can begin detecting. Anyway, the key to this question is to recognize the function of the energy meter. Ideally, you would shine a light on the sample and all the photons would either cause the bonds to break or heat up the sample. In this ideal case, the difference between the laser pulse energy and the energy detected by your oscilloscope will be the bond enthalpy. The problem is, this doesn't happen because photochemical processes aren't 100% efficient. That is, not all the photons will be doing one of those two things. Some photons get transmitted through and don't "react" in the way you want. That's what the energy meter is designed to detect. And so at the end, you have to correct for the photochemical efficiency, which is what it says in the next paragraph.

Now, if an appropriate laser is used such that photochemical efficiency is 100%, then you won't get transmittance and all the pulse energy will be used to either break bonds or heat up the solution.

Oh, so "appropriate laser" was supposed to mean 100% efficiency? I wouldn't have been able to guess that. Thank you very much! Great explanation, the whole passage makes sense now.

 

[aldol16]

Oh, so "appropriate laser" was supposed to mean 100% efficiency? I wouldn't have been able to guess that. Thank you very much! Great explanation, the whole passage makes sense now.

I think so. That's what I make of it, anyway. The idea is that you want 100% efficiency because you want all the incident energy to be distributed among only the bond breaking processes and the heating - and not transmitted.

 

[piscean92]

I think so. That's what I make of it, anyway. The idea is that you want 100% efficiency because you want all the incident energy to be distributed among only the bond breaking processes and the heating - and not transmitted.

yea makes sense