The Official August 16, 2013 MCAT Thread

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gettheleadout

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Welcome everyone! For those following the SN2ed or a similar 3-month study schedule, prep should begin in the next few weeks.

Hope to see some ambitious and optimistic people here with me, and shout out to the 3/23'ers from before I bailed on that test date.

Let's go!

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What does that have to do with your blanket statement that it's "energy releasing?"
The breakage of the bond and the formation of the new intermolecular interactions between ions and the solvent is not a two step process. The old interactions are broken simultaneously and smoothly as the new ones form. So it's really one process. And if it's spontaneous, then its exergonic and thus is energy releasing.
 
The breakage of the bond and the formation of the new intermolecular interactions between ions and the solvent is not a two step process. The old interactions are broken simultaneously and smoothly as the new ones form. So it's really one process. And if it's spontaneous, then its exergonic and thus is energy releasing.

It's actually modeled as a three-step process, and the relative values for each of the steps can be determined. You're trying to back up your statement that bond-breaking can be exergonic without any new bonds being formed. Unfortunately, in solvation the new intermolecular interactions that are formed constitute distinct exothermic processes that contribute to the overall enthalpy change for the reaction. There is no discrete bond-breaking reaction in the absence of any new interactions (the formation of which would be exothermic) that I am aware of. All you've concluded is that spontaneous solvation processes exist. Yeah, that's obvious. The fact remains that every time a bond (or intermolecular interaction) is disrupted, that discrete thermodynamic process is endothermic. To prove it may also be exergonic would require an instance that I don't believe exists and that has not been presented.

You can contort solvation however you want, but they way you're attempting to describe it is not in agreement with modern chemistry. This is all explained in depth in the chemguide link I posted.
 
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Can someone help me with this bloody problem! I think Kaplan made a mistake.

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The explanation claims that at:
Cathode 1: copper is reduced
Cathode 2: OH- is oxidized

But, the reduction potential of water is higher than copper and the oxidation potential of sulfate is higher than OH-. So, given their reduction potentials, water should be reduced at cathode 1 and sulfate should be oxidized at cathode 2 right? Is this an error on their part or am I not seeing something?
 
It's actually modeled as a three-step process, and the relative values for each of the steps can be determined. You're trying to back up your statement that bond-breaking can be exergonic without any new bonds being formed. Unfortunately, in solvation the new intermolecular interactions that are formed constitute distinct exothermic processes that contribute to the overall enthalpy change for the reaction. There is no discrete bond-breaking reaction in the absence of any new interactions (the formation of which would be exothermic) that I am aware of. All you've concluded is that spontaneous solvation processes exist. Yeah, that's obvious. The fact remains that every time a bond (or intermolecular interaction) is disrupted, that discrete thermodynamic process is endothermic. To prove it may also be exergonic would require an instance that I don't believe exists and that has not been presented.

You can contort solvation however you want, but they way you're attempting to describe it is not in agreement with modern chemistry. This is all explained in depth in the chemguide link I posted.
from the chem guide you posted,

You can think of an imaginary process where the crystal lattice is first broken up into its separate gaseous ions, and then those ions have water molecules wrapped around them. That is how they exist in the final solution.


This can be broken down mathematically into distinct processes, but physically, I believe it's occurring as one smooth, continuous process. It's not like step 1 endergonic, pause, step two exergonic. It's a smooth, continuous process. Now if you can provide evidence that the steps are physically distinct and consecutive rather than smooth, continuous and simultaneous, then I can relent.
 
from the chem guide you posted,

You can think of an imaginary process where the crystal lattice is first broken up into its separate gaseous ions, and then those ions have water molecules wrapped around them. That is how they exist in the final solution.


This can be broken down mathematically into distinct processes, but physically, I believe it's occurring as one smooth, continuous process. It's not like step 1 endergonic, pause, step two exergonic. It's a smooth, continuous process. Now if you can provide evidence that the steps are physically distinct and consecutive rather than smoothing, continuous and simultaneous, then I can relent.

It has nothing to do with them being temporally distinct, its the fact that we can demonstrate mathematically that the individual process of breaking any intermolecular interaction would be, by itself, endothermic.

Yes, obviously things happen smoothly, I haven't been contesting that at all, transition states exist, yada yada. But you cannot use the process of solvation or hydration to argue that the bond breaking is an exception where it releases energy net. You would have to prove that in the absence of formation of any sorts of new interactions, energy is still released. I don't believe 1) you can actually demonstrate that in any way or 2) that that's even true.
 
Can someone help me with this bloody problem! I think Kaplan made a mistake.

Capture.png


The explanation claims that at:
Cathode 1: copper is reduced
Cathode 2: OH- is oxidized

But, the reduction potential of water is higher than copper and the oxidation potential of sulfate is higher than OH-. So, given their reduction potentials, water should be reduced at cathode 1 and sulfate should be oxidized at cathode 2 right? Is this an error on their part or am I not seeing something?

I concur.
 
It has nothing to do with them being temporally distinct, its the fact that we can demonstrate mathematically that the individual process of breaking any intermolecular interaction would be, by itself, endothermic.

Yes, obviously things happen smoothly, I haven't been contesting that at all, transition states exist, yada yada. But you cannot use the process of solvation or hydration to argue that the bond breaking is an exception where it releases energy net. You would have to prove that in the absence of formation of any sorts of new interactions, energy is still released. I don't believe 1) you can actually demonstrate that in any way or 2) that that's even true.
I disagree. Separation seems to be a mathematical tool in this case, not a true representation of the process. That's why the word hypothetical is used in the quote above. The breakage and new formation are not independent processes, they're theoretical, mathematical parts of one physical process. And I think this process does fit under a general umbrella of "bond breaking," and it is exergonic.
 
I disagree. Separation seems to be a mathematical tool in this case, not a true representation of the process. That's why the word hypothetical is used in the quote above. The breakage and new formation are not independent processes, they're theoretical, mathematical parts of one physical process. And I think this process does fit under a general umbrella of "bond breaking," and it is exergonic.

Just go ask a chemistry professor if you really want to push this so hard. You're trying to argue an exception to a universal rule in chemistry by including under the "general umbrella of 'bond breaking'" a process that necessarily includes the breakage and formation of several interactions. If you think that has enough merit to be worth your time, when it's never going to play in role in answering an MCAT question, go right ahead. This conversation is no longer worth mine.
 

I shall restrain myself in saying something that I should not about this seemingly abrupt uproar in response to a very logical attempt to discuss a matter. Mr. Patrick mod knows what I'm talking about.

The word hypocrite comes to mind. (Damn it, so much for resisting...)
 
I shall restrain myself in saying something that I should not about this seemingly abrupt uproar in response to a very logical attempt to discuss a matter. Mr. Patrick mod knows what I'm talking about.

The word hypocrite comes to mind. (Damn it, so much for resisting...)
Uproar? I exited the conversation, stating that it wasn't worth my time anymore. Feel free to elaborate on what's wrong with that.
 
Uproar? I exited the conversation, stating that it wasn't worth my time anymore. Feel free to elaborate on what's wrong with that.

Yes, uproar. HotHam felt it too. I shall now adopt the above strategy. (Firemen learn quickly, don't they?)
 
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Yes, uproar. HotHam felt it too. I shall now adopt the above strategy. (Firemen learn quickly, don't they?)

To put it plainly, I tried for like 7 posts to logically walk through the issue with Platonic, but if you read through our entire conversation, it becomes circular. Yes, of course I was frustrated and I wanted to convey that. That's entirely different from insulting someone, using unnecessary, inappropriate language, etc...
 
Yes, uproar. HotHam felt it too. I shall now adopt the above strategy. (Firemen learn quickly, don't they?)

whoa whoa whoa, don't drag me into this. I merely posted an attempt at humor to lighten the mood. As far as who is to blame - that's none of my business; I wasn't involved in the debate.
 
I'm still waiting for someone to weigh in on my electrolytic cell problem.
 
Just go ask a chemistry professor if you really want to push this so hard. You're trying to argue an exception to a universal rule in chemistry by including under the "general umbrella of 'bond breaking'" a process that necessarily includes the breakage and formation of several interactions. If you think that has enough merit to be worth your time, when it's never going to play in role in answering an MCAT question, go right ahead. This conversation is no longer worth mine.
This actually is a unique situation as compared to a typical bond breaking process, since the breakage of a bond and the formation of a more stable ionic configuration seems to occur in the same process. So I think it can be argued that this is one process (that can be theoretically broken into distinct processes for mathematical analysis) where a bond is not simply broken, but transferred, with energy release being a result.

Anyway, we have clearly reached an impasse on this issue, and this discussion is cluttering the thread.
 
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Can someone help me with this bloody problem! I think Kaplan made a mistake.

Capture.png


The explanation claims that at:
Cathode 1: copper is reduced
Cathode 2: OH- is oxidized

But, the reduction potential of water is higher than copper and the oxidation potential of sulfate is higher than OH-. So, given their reduction potentials, water should be reduced at cathode 1 and sulfate should be oxidized at cathode 2 right? Is this an error on their part or am I not seeing something?
I've only read the electrochemistry chapter without yet attempting the end of chapter problems, but I'll give it a shot. For cathode 1, they might be saying that there's no O2 for the reduction of water. For "cathode 2," I am uncertain about which electrode you are referring to. I believe the electrode labeled "2" is an anode, which would make the second cathode the electrode labeled "3." In that case, it makes sense that OH- would get oxidized there. There's a negative potential for its oxidation, but I guess that's what the external voltage is there for, to drive unfavorable reactions?
 
I've only read the electrochemistry chapter without yet attempting the end of chapter problems, but I'll give it a shot. For cathode 1, they might be saying that there's no O2 for the reduction of water. For "cathode 2," I am uncertain about which electrode you are referring to. I believe the electrode labeled "2" is an anode, which would make the second cathode the electrode labeled "3." In that case, it makes sense that OH- would get oxidized there. There's a negative potential for its oxidation, but I guess that's what the external voltage is there for, to drive unfavorable reactions?

Despite it being an electrolytic cell driving unfavorable, non-spontaneous reactions, does't the reactions still have to compete with those most "favorable" non-spontaneous reactions. In this case, the oxidation of sulfate is the more favorable non-spontaneous reaction. Right? I can't grasp this for some reason.
 
I just confirmed this. Despite electrolysis driving non-spontaneous redox reactions, it is still the most easily oxidized species that is oxidized and the most easily reduced species that is reduced. So this question is **** and sulfate would be oxidized at the anode.
 
I just confirmed this. Despite electrolysis driving non-spontaneous redox reactions, it is still the most easily oxidized species that is oxidized and the most easily reduced species that is reduced. So this question is **** and sulfate would be oxidized at the anode.

I concur.
 
I just confirmed this. Despite electrolysis driving non-spontaneous redox reactions, it is still the most easily oxidized species that is oxidized and the most easily reduced species that is reduced. So this question is **** and sulfate would be oxidized at the anode.
Isn't sulfate fully oxidized already? Table 1 shows a reaction where it is reduced to sulfurous acid, but I don't think it can be oxidized. I thought the cell does something like this:

1.)electrons go to electrode 1 , where they are conducted to reduce copper cation.

2.)since the region near electrode 1 gets more negative, the sulfate floats over to electrode 2.

3.)In response to the negative charge of sulfate building up, Pt gives up its electrons to electrode 3 and falls into solution.

4.)The electrons at electrode 3 reduce OH-, doing the reverse of the 4th reaction presented in Table 1.

This might be way off though. I plan on working the electrochemistry problems after I do the circuits section of the physics book.
 
Isn't sulfate fully oxidized already? Table 1 shows a reaction where it is reduced to sulfurous acid, but I don't think it can be oxidized. I thought the cell does something like this:

1.)electrons go to electrode 1 , where they are conducted to reduce copper cation.

2.)since the region near electrode 1 gets more negative, the sulfate floats over to electrode 2.

3.)In response to the negative charge of sulfate building up, Pt gives up its electrons to electrode 3 and falls into solution.

4.)The electrons at electrode 3 reduce OH-, doing the reverse of the 4th reaction presented in Table 1.

This might be way off though. I plan on working the electrochemistry problems after I do the circuits section of the physics book.

The explanation says that OH- is oxidized at the electrode labeled 2 though.

edit: I do see what you are saying about sulfate already being in an oxidated state due to it's solvation. I still hate this problem.
 
Man! I just came across this thread and read through every single post.. I feel like I've been with you guys since the beginning now. (Actually, to be totally honest, I totally skipped over that long bond-breaking debate.)

I rescheduled my MCAT from 3/23 to 6/20 to 8/16 (I know...) and I feel like if I can't get my $**# together this time around I don't deserve to be a doctor. So I've been really trying to invest in intensive studying (HotHam, you inspire me!) but am a bit iffy on my approach.. I have all the Kaplan books and online resources and have been doing content review since the very end of May, when I graduated. My plan is to be done with it by this coming Wednesday, spend 1.5-2 weeks on loose practice passages, and then spend all of July doing practice tests (I think I have around 16 of them) and focus on the post-game analysis.

SN2ed looked like a solid schedule but it meant buying a lot of books, and because I got Kaplan for free I'm relying on that. Is that a bad decision, though? From what it seems, TBR and TPR seem to be the most widely-used resources.. it may be a bit late to get TBR in time since don't they make you order by mail? but should I go for TPR?

Anyway-- if you have feedback on my strategy I'd love to hear it! You guys seem like you know what you're talking about, so I'm all ears. Can't wait to join in on all the fun of this thread :)
 
The explanation says that OH- is oxidized at the electrode labeled 2 though.

edit: I do see what you are saying about sulfate already being in an oxidated state due to it's solvation. I still hate this problem.

I take back what I said in my previous post. I assumed Pt would get oxidized and fall back into solution, like Zn in the galvin zycle. But Pt stands for platinum, an expensive metal that probably is very difficult to oxidize. So SOMETHING in the solution has to get oxidized at electrode 2 for the circuit to be complete, and OH- seems to be the only candidate. So it has to get oxizided in the reverse of the 4th reaction in Table 1.
 
Man! I just came across this thread and read through every single post.. I feel like I've been with you guys since the beginning now. (Actually, to be totally honest, I totally skipped over that long bond-breaking debate.)

I rescheduled my MCAT from 3/23 to 6/20 to 8/16 (I know...) and I feel like if I can't get my $**# together this time around I don't deserve to be a doctor. So I've been really trying to invest in intensive studying (HotHam, you inspire me!) but am a bit iffy on my approach.. I have all the Kaplan books and online resources and have been doing content review since the very end of May, when I graduated. My plan is to be done with it by this coming Wednesday, spend 1.5-2 weeks on loose practice passages, and then spend all of July doing practice tests (I think I have around 16 of them) and focus on the post-game analysis.

SN2ed looked like a solid schedule but it meant buying a lot of books, and because I got Kaplan for free I'm relying on that. Is that a bad decision, though? From what it seems, TBR and TPR seem to be the most widely-used resources.. it may be a bit late to get TBR in time since don't they make you order by mail? but should I go for TPR?

Anyway-- if you have feedback on my strategy I'd love to hear it! You guys seem like you know what you're talking about, so I'm all ears. Can't wait to join in on all the fun of this thread :)

Welcome. Glad my experiences could help.

As far as Kaplan materials, I'm not too sure of their effectiveness. I'm using their practice FL's and they seem to be pretty good. As far as content review, I've found Chads Videos, TPR and EK to be the most effective. TBR was too esoteric for me and I would get overloaded with information and lose interest. I need straight to the point explanation and then go work problems. I'm not sure what your study style is so I can't really tell you what will work best for you. When it comes to science I have to practice and make tons of mistakes to learn anything. I can't learn from reading a book.
 
Awesome, thanks. Just to clarify, are Chad's videos the ones from ***********.com, the ones you have to buy?
 
Welcome. Glad my experiences could help.

As far as Kaplan materials, I'm not too sure of their effectiveness. I'm using their practice FL's and they seem to be pretty good. As far as content review, I've found Chads Videos, TPR and EK to be the most effective. TBR was too esoteric for me and I would get overloaded with information and lose interest. I need straight to the point explanation and then go work problems. I'm not sure what your study style is so I can't really tell you what will work best for you. When it comes to science I have to practice and make tons of mistakes to learn anything. I can't learn from reading a book.

See, I'm a case in point as the exact opposite. I've always found videos and A/V-heavy study materials as gimmicky. Instead, I prefer reading a book. Since you don't have TBR, you don't know what it's like or if it's your style, so maybe buy the gen chem or physics (the 2 best ones) set and see if it caters to your learning? Then decide if you want to buy the rest.
 
Awesome, thanks. Just to clarify, are Chad's videos the ones from ***********.com, the ones you have to buy?

Correct. But $100 for 90 days compared to $2k from TPR or Kaplan...it's a no brainier. If you do we'll with audio video, his are the best.
 
Hey Ham,

If you still have TBR Gen Chem, in Ex. 6.18 they say that different gases (of very different molecular size) all have the same mean free path when released at the same temperature.

But the gases are in an open environment of which the volume is undefined so the mean free path (or distance between particles) is also undefined because nothing is constraining their movement.

Size of a molecule is absolutely what determines mean free path whether TBR highlights it or not. There probably a good amount of info on it in your undergrad chem book. Mine basically explains it the same as TBR saying its the average distance between two molecules. If you increase the size of those molecules you decrease the distance between them.
 
But the gases are in an open environment of which the volume is undefined so the mean free path (or distance between particles) is also undefined because nothing is constraining their movement.

Size of a molecule is absolutely what determines mean free path whether TBR highlights it or not. There probably a good amount of info on it in your undergrad chem book. Mine basically explains it the same as TBR saying its the average distance between two molecules. If you increase the size of those molecules you decrease the distance between them.

Well the question gives the volume as the same between the gases since they're all contained equally in the same room, so I wouldn't call that an "open environment."

Also, I just checked my chem book and it's literally only mentioned in one paragraph on one page. :laugh: (Zumdahl & Zumdahl, 2010)
 
Well the question gives the volume as the same between the gases since they're all contained equally in the same room, so I wouldn't call that an "open environment."

Also, I just checked my chem book and it's literally only mentioned in one paragraph on one page. :laugh: (Zumdahl & Zumdahl, 2010)

It does make logical sense to you though right? If I have two balloons both blown up to a radius of 6 inches and spaced 2 ft apart. If I blow them up so they are now 1 ft radius the distance between them has decreased.

edit: PS that whole open environment thing I got from the first 3 pages from TBR on the chapter you referenced too.
 
It does make logical sense to you though right? If I have two balloons both blown up to a radius of 6 inches and spaced 2 ft apart. If I blow them up so they are now 1 ft radius the distance between them has decreased.

edit: PS that whole open environment thing I got from the first 3 pages from TBR on the chapter you referenced too.

Yeah, it makes perfect sense, I'm just very wary of trusting intuition on this because I can't find a single thing anywhere saying you can use the calculation of mean free path to compare separate components of a mixture of gases. I'm worried that the terms only apply to the average molecular size of the particles in a mixture, similar to how you can calculate the mean free path of air, or something like that.

And apparently, TPR didn't intend for them to be mixed together either! :p
 
Yeah, it makes perfect sense, I'm just very wary of trusting intuition on this because I can't find a single thing anywhere saying you can use the calculation of mean free path to compare separate components of a mixture of gases. I'm worried that the terms only apply to the average molecular size of the particles in a mixture, similar to how you can calculate the mean free path of air, or something like that.

And apparently, TPR didn't intend for them to be mixed together either! :p

ohhhh....I see what you are saying. You are referring to a pure sample. Well, perhaps my question should have been more clear about being a pure sample. ;)
 
Still no resolution on mixed gases. I'm going to laugh my ass off if a mixed gases question like this pops up on the MCAT.:laugh:
 
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But really though, imagine a gas like this, with several small fast particles and one huge slow one. The large particle is so large that if it moves a particular distance, it is much more likely to encounter a collision than if a smaller particle had traveled the same distance. Not to mention the fact that it's so slow that it has surely been rammed many times by the small particles by the time it has gone very far.

2j2n.jpg
 
But really though, imagine a gas like this, with several small fast particles and one huge slow one. The large particle is so large that if it moves a particular distance, it is much more likely to encounter a collision than if a smaller particle had traveled the same distance. Not to mention the fact that it's so slow that it has surely been rammed many times by the small particles by the time it has gone very far.

2j2n.jpg

I concur.
 
My guess is A, because at higher pressures fewer moles of gas are favored (and you have 1 mol of CO2 vs. 2 moles of CO). But Gen Chem is also my worst area, so this reasoning could be totally off.
 
Can anyone fill me in on this question? Don't get the BR response for it...

6VQp7fc


I know fight or flight applies but the answer was...

D. Blushing and erections seem like PNS responses to me because of the blood rushing to cheeks/junk.
 
Can anyone fill me in on this question? Don't get the BR response for it...

6VQp7fc


I know fight or flight applies but the answer was...

D. Blushing and erections seem like PNS responses to me because of the blood rushing to cheeks/junk.

I just ignored that question. Apparently blushing is usually sympathetic but there's no way you're expected to know that.

The erection thing you just have to know as well. This question is one of TBR's stupid, unrealistic discretes. The smooth muscle in the erectile tissue of the penis is actually contracted while the penis is flaccid, and relaxed while the penis is erect, which seems odd at first.

Just remember PeNiS = PNS.
 
Can anyone fill me in on this question? Don't get the BR response for it...

6VQp7fc


I know fight or flight applies but the answer was...

D. Blushing and erections seem like PNS responses to me because of the blood rushing to cheeks/junk.

You're getting sympathetic and parasympathetic mixed up. The answer is D, penile erection. Sympathetic is fight or flight while parasympathetic is rest or digest. Erection is parasympathetic, while ejaculation and orgasm is sympathetic.
 
You're getting sympathetic and parasympathetic mixed up. The answer is D, penile erection. Sympathetic is fight or flight while parasympathetic is rest or digest. Erection is parasympathetic, while ejaculation and orgasm is sympathetic.

Point and Shoot. :naughty:
 
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