The Official August 16, 2013 MCAT Thread

[gettheleadout]

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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[Platonic]

question about TBR physics example 5.1b.

For a spring undergoing damped motion, option b says

The potential energy at consecutive maxima decreases while the kinetic energy at consecutive maxima remains constant.

I think this is correct, although the solution says that it's wrong because the kinetic energy at consecutive maxima decreases.

However, the velocity and thus kinetic energy at the maxima should be zero. So the KE at consecutive maxima should be zero, making A correct, right??

 

[gettheleadout]

question about TBR physics example 5.1b.

For a spring undergoing damped motion, option b says

The potential energy at consecutive maxima decreases while the kinetic energy at consecutive maxima remains constant.

I think this is correct, although the solution says that it's wrong because the kinetic energy at consecutive maxima decreases.

However, the velocity and thus kinetic energy at the maxima should be zero. So the KE at consecutive maxima should be zero, making A correct, right??

Ah, interesting question! So, the issue here is that you're misreading the question by assuming that when it refers to "maxima" it's referring to the displacement from equilibrium, i.e. the amplitude. In fact, it's referring to the individual maxima of the plots of kinetic and potential energy (see the plot with the Energy axis in Figure 5-3.)

 

[Platonic]

Ah, interesting question! So, the issue here is that you're misreading the question by assuming that when it refers to "maxima" it's referring to the displacement from equilibrium, i.e. the amplitude. In fact, it's referring to the individual maxima of the plots of kinetic and potential energy (see the plot with the Energy axis in Figure 5-3.)

Oops. I guess that's what I get for skipping the content and going straight to the questions

 

[Roxas]

How's everyone doing? Anyone else hitting kind of a slump right now? This is about where I got in content review first time around when I got really burned out and never quite got back into it, and I'm feeling the same way again. I've been pushing pretty hard up until now but I just can't seem to get my focus back. It probably doesn't help that I'm sick, and leaving for a vacation in a few days.

 

[inycepoo]

How's everyone doing? Anyone else hitting kind of a slump right now? This is about where I got in content review first time around when I got really burned out and never quite got back into it, and I'm feeling the same way again. I've been pushing pretty hard up until now but I just can't seem to get my focus back. It probably doesn't help that I'm sick, and leaving for a vacation in a few days.

I've just been skipping all the reading and going directly to the problems. Even active reading puts me to sleep, let alone passive. Okay, I actually skim it, but spend no more than 10 min on a chapter. Then I eat something lol. But I guess this wouldn't work if you're iffy on the content and need to read everything, but I doubt that's more effective. Diving into passages keeps you focused and on your toes. Oh, VR practice wakes me up too, but it looks like I'm in he minority for that one...but I like reading random things ^___^

 

[Platonic]

How's everyone doing? Anyone else hitting kind of a slump right now? This is about where I got in content review first time around when I got really burned out and never quite got back into it, and I'm feeling the same way again. I've been pushing pretty hard up until now but I just can't seem to get my focus back. It probably doesn't help that I'm sick, and leaving for a vacation in a few days.

I'm feeling a little bit burned out myself TBH. I finished TBR gen chem a couple days ago, and it took like a week and a half of full devotion to the book. I'm doing content review in sections, and taking EXCELLENT (and let's just say a little unscrupulous) notes. I have finished the first 4.5 chapters of EK Bio, and will mostly skim the rest of the book, since I just took organismal Bio (bio 2) and we went over pretty much everything in the prep books. I will also be finishing up TBR physics within the next week, although I skip right to the questions.

After I'm done with physics, I'll skim OChem and the 2nd half of bio, then use self assessment package (and another round of phases in BR books) to gauge my preparedness in content. If I do well there, I'll be moving on to FL's within the next 3 weeks. Might take a 1 week break after content review to recharge.

 

[Platonic]

I've just been skipping all the reading and going directly to the problems. Even active reading puts me to sleep, let alone passive. Okay, I actually skim it, but spend no more than 10 min on a chapter. Then I eat something lol. But I guess this wouldn't work if you're iffy on the content and need to read everything, but I doubt that's more effective. Diving into passages keeps you focused and on your toes. Oh, VR practice wakes me up too, but it looks like I'm in he minority for that one...but I like reading random things ^___^

glad to see I'm not alone. I was a clean slate in most of the gen chem, so i read that book carefully. There's no way in hell I'm going to devote a month worth of studying going through content review in orgo and the second half of Biology when I just took the classes. And there's no way in hell I'm doing content review in physics, since that is my major. I spent a couple days before my official studying began to go through EK verbal techniques book. Haven't studied verbal since, and likely won't until I see a disappointing score in the assessment package or FL's.

We'll see when I start doing FL's if my strategy is smart, or just lazy

 

[HotHamH2O]

How's everyone doing? Anyone else hitting kind of a slump right now? This is about where I got in content review first time around when I got really burned out and never quite got back into it, and I'm feeling the same way again. I've been pushing pretty hard up until now but I just can't seem to get my focus back. It probably doesn't help that I'm sick, and leaving for a vacation in a few days.

Not to be that guy, but I'm doing really well. This study method I'm using is really helping to keep things from getting dull. Chad's videos are so amazing. I should have used these the first time around.

 

[gettheleadout]

Hey Ham do we get an answer explanation for your last discrete?

 

[inycepoo]

Hey Ham do we get an answer explanation for your last discrete?

I believe everyone who answered got it right, no? *eyebrows* Plus I swear that came out of the Kaplan Qbank lol

 

[gettheleadout]

I believe everyone who answered got it right, no? *eyebrows* Plus I swear that came out of the Kaplan Qbank lol

But what's the proper reasoning for answering the question? Surely you're not supposed to know progesterone raises BBT.

inb4don'tcallmeshirley

 

[HotHamH2O]

Everyone dreads female reproduction system questions so here's one for you guys!


A patient with at risk for thromboembolic blood disorders is advised by her physician to refrain from taking her birth control pill. The patient is sexually active and not desiring pregnancy so she asks her physician if there are any alternative to oral contraceptives. Her physician tells her about the sympothermal method. She and her partner are instructed to refrain from sexual intercourse until an incrase in her basal body temperature.
This rise occurs 24 to 48 hours after ovulation and is most likely associated with an increase in the release of:

A. FSH
B. LH
C. estrogen
D. progesterone

Please respond in white! Answer will be provided tomorrow!

Sorry about that guys. I've been on nights for 3 in a row so I've been exhausted.

Answer is D. The point that is being driven home here is not that you need to know that progesterone raises BBT but that it raises AFTER ovulation.

 

[Roxas]

Not to be that guy, but I'm doing really well. This study method I'm using is really helping to keep things from getting dull. Chad's videos are so amazing. I should have used these the first time around.

Nah man, I'm happy for you. After your frustration last time around you deserve a killer score this time. So what are you doing differently besides chad's?

 

[gettheleadout]

Sorry about that guys. I've been on nights for 3 in a row so I've been exhausted.

Answer is D. The point that is being driven home here is not that you need to know that progesterone raises BBT but that it raises AFTER ovulation.

But estrogen and progesterone rise in tandem after ovulation.

Edit: Ah the point is if it were estrogen the BBT increase would occur before ovulation as well.

 

[HotHamH2O]

Nah man, I'm happy for you. After your frustration last time around you deserve a killer score this time. So what are you doing differently besides chad's?

I'll break down my days for you.

Preface: Everyday I wake up at 8am (unless I was on night shift since I get OFF at 7am), have breakfast, spend time with my kids, walk to library (right across the street) and am studying by 10am. Done at 3 or 4pm, hit the gym. Spend time with family, eat dinner. Review note cards for 2 hours from 9-11pm.

Day 1: Take a full length (been doing the R versions of kaplan and TPR); if any energy left, go ahead and begin to review concepts I knew I was weak on during the test.
Day 2: Spend entire day doing a post-game analysis of each question I got wrong and each question I got right that I wasn't so sure on. I think I linked you to my strategy for post-game before. Check up in the thread. Basically, I just rewite the question and answer in my own words and keep them in a composition book. I also have an excel spreadsheet that I use to log the question #, subject, and reason for each wrong question.
Day 3: Watch chad and do topical passages and/or discretes for subjects that I did poorly on from the FL. This is where the spreadsheet comes in handy because all my subjects are right there in a list with a reason of why I failed to answer the question correctly. I just go down the list and beat the living hell out of each of my weaknesses until I have enough knowledge to teach it to someone else (my wife is ready to kill me) and write passages on it.
Day 4. Repeat

Sometimes I can combine days 2 and 3 depending on how poorly I did on the FL. But when I say I spend all day doing post-game, I really do. Every question I get wrong, I read everything I can find on it. I rewrite the question in great detail. Maybe even make a few more questions on that same subject. When I'm done, I make damn sure I will never get a question on that subject wrong again. It's also important to go back through the test and look at those questions you were not so sure about but you still got right. I have another excel sheet that keeps track of what I did on those "guess" questions that helped me to get them right. I make sure I understand my logic here and make sure to repeat it. So far so good. This week was much slower going because I'm still just restarting my studies.

I think this is how someone can become a better MCAT taker. I played a lot of sports and now I do a lot of endurance athletics (I'm an avid cyclist and tri-athlete). This kind of training is the recipe of champions. I'm not saying I am one, just trying to do what they do. Gotta fake it 'till you make it!

 

[krogers21]

I'll break down my days for you.

Preface: Everyday I wake up at 8am (unless I was on night shift since I get OFF at 7am), have breakfast, spend time with my kids, walk to library (right across the street) and am studying by 10am. Done at 3 or 4pm, hit the gym. Spend time with family, eat dinner. Review note cards for 2 hours from 9-11pm.

Day 1: Take a full length (been doing the R versions of kaplan and TPR); if any energy left, go ahead and begin to review concepts I knew I was weak on during the test.
Day 2: Spend entire day doing a post-game analysis of each question I got wrong and each question I got right that I wasn't so sure on. I think I linked you to my strategy for post-game before. Check up in the thread. Basically, I just rewite the question and answer in my own words and keep them in a composition book. I also have an excel spreadsheet that I use to log the question #, subject, and reason for each wrong question.
Day 3: Watch chad and do topical passages and/or discretes for subjects that I did poorly on from the FL. This is where the spreadsheet comes in handy because all my subjects are right there in a list with a reason of why I failed to answer the question correctly. I just go down the list and beat the living hell out of each of my weaknesses until I have enough knowledge to teach it to someone else (my wife is ready to kill me) and write passages on it.
Day 4. Repeat

Sometimes I can combine days 2 and 3 depending on how poorly I did on the FL. But when I say I spend all day doing post-game, I really do. Every question I get wrong, I read everything I can find on it. I rewrite the question in great detail. Maybe even make a few more questions on that same subject. When I'm done, I make damn sure I will never get a question on that subject wrong again. It's also important to go back through the test and look at those questions you were not so sure about but you still got right. I have another excel sheet that keeps track of what I did on those "guess" questions that helped me to get them right. I make sure I understand my logic here and make sure to repeat it. So far so good. This week was much slower going because I'm still just restarting my studies.

I think this is how someone can become a better MCAT taker. I played a lot of sports and now I do a lot of endurance athletics (I'm an avid cyclist and tri-athlete). This kind of training is the recipe of champions. I'm not saying I am one, just trying to do what they do. Gotta fake it 'till you make it!

Using this method, which FLs are you planning to do (and in what order if it isn't too much trouble). Sounds like a great idea; however, I may be a tad late to the game. Also, what are you planning to do about Bio with Chad's videos lacking this portion of the MCAT?

 

[HotHamH2O]

Using this method, which FLs are you planning to do (and in what order if it isn't too much trouble). Sounds like a great idea; however, I may be a tad late to the game. Also, what are you planning to do about Bio with Chad's videos lacking this portion of the MCAT?

I'm a retaker so I don't suggest this schedule for a first timer. I just finished SN2ed 4 month in April so I don't really need a lot of content review. My issue on test day was not lack of knowledge, it was lack of test taking ability. I know the material, I just have a hard time decoding what I am being asked in the question in a reasonable amount of time.

As far as materials - for review of weaknesses I have TPRH, TBR, and EK. Between those materials and google, I always get plenty of information. Chads vids just better suit me because I am 100% an audio/visual learner. My memory on things I read has a half life of about 10 minutes.

I exhausted my AAMC FL in the SN2ed schedule so I'm using Kaplan, TBR CBTs, and TPR CBTs.

If this is your first time I would just suggest you take more time practicing actual FL than content review. First time through I spent 3 months reading content and then a month taking actual tests. This didn't work for me. It gave me a false sense of confidence. Answering passages regarding the topics you just read an entire chapter on is not indicative of test day. My greatest weakness on test day was figuring out what subject the question was pertaining too. So, do lots of FLs in a simulated testing environment and take your post-game analysis seriously. Don't ever tell yourself, "Oh I know that, that was just a silly mistake I won't make again." Trust me, you'll make it again unless you beat it out of yourself with serious post-game work.

 

[inycepoo]

Officially registered for the 0800 time on 8/9! There was a location close to my home so I guess I'll wake up early then haha.

P.S. The 8/9 thread is being boring so I will just hang out here with you guys

 

[HotHamH2O]

Krogers,

I should mention also that I am no expert at this stuff. When I scored substantially lower than my AAMC average I knew I had to get back to the drawing board. I found a study plan here on SDN that matched my study habits during undergraduate best, which is what I should have done in the first place. It was naive of me to think that reading all my content review and not doing a ton of practice would be sufficient.

Best advice I can give is find out what works best for you and matches how you've succeeded this far.

I haven't retaken yet obviously, but I can say that in two weeks I'm already tackling FL's with much improvement. My biggest improvement is coming to logical conclusions on those really esoteric questions. Good luck finding what works for you bud!

 

[IcemanMD]

Anyone know if the oval window is considered part of the inner ear or the middle ear or a bridge between the two?

 

[gettheleadout]

Anyone know if the oval window is considered part of the inner ear or the middle ear or a bridge between the two?

It's an orifice of the cochlea, so part of the inner ear.

Edit: okay, apparently this isn't actually very clear-cut, so I'm just going to wager that it won't be asked. Just know what it does.

That said, "The footplate [of the stapes], shaped like the sole of a steam iron, is held by a ringlike ligament in an opening called the oval window, where the inner ear begins. (Saladin, 2004)

Edit 2: this is a funny question when you think about it, because it's a bit like asking if the ear drum is part of the outer or middle ear. Well, it can be viewed from either side and divides the two, which is it? See? Not a worthy point of contention for the MCAT.

 

[Ansar]

67. Which of the following is a violation of the law of conservation
of energy?
A. Heat can be changed completely to work in cyclical
process.
B. A system undergoing a reaction with constant enthalpy
experiences a temperature change.
C. After sliding to a stop, a box with initial kinetic energy
K has only thermal energy in an amount less
than K.
D. A bond is broken and energy is released.

Y'all know why the answer is not A?? Is it because A is due to 2nd law of Thermodynamics, not conservation of energy principles??

 

[gettheleadout]

The answer is B, right? And yeah I would say that since total energy is constant in A it's not a conservation violation, it's just impossible based on the 2nd Law.

 

[Ansar]

The answer is B, right? And yeah I would say that since total energy is constant in A it's not a conservation violation, it's just impossible based on the 2nd Law.

The posted answer is D, because "Energy is always required to break a bond." (I know it does, but I guess I didn't see how answer D implied it didn't)

 

[gettheleadout]

The posted answer is D, because "Energy is always required to break a bond." (I know it does, but I guess I didn't see how answer D implied it didn't)

Oh lol I didn't even read D correctly. Whoops. So how does B work? (Haven't reviewed thermo yet)

 

[IcemanMD]

The posted answer is D, because "Energy is always required to break a bond." (I know it does, but I guess I didn't see how answer D implied it didn't)

Better wording would have been ->Energy is released as a result of a bond being broken.

Just trying to trick you I guess.

Also thanks for the ear clarification gtlo

 

[Platonic]

67. Which of the following is a violation of the law of conservation
of energy?
A. Heat can be changed completely to work in cyclical
process.
B. A system undergoing a reaction with constant enthalpy
experiences a temperature change.
C. After sliding to a stop, a box with initial kinetic energy
K has only thermal energy in an amount less
than K.
D. A bond is broken and energy is released.

Y'all know why the answer is not A?? Is it because A is due to 2nd law of Thermodynamics, not conservation of energy principles??

A and D are wrong, C is wrong because the ground will take some energy. So B wins by default.

But I don't see why a reaction couldn't have constant enthalpy and a changing temperature. Enthalpy is U+PV, and internal energy U is a function of temperature. If Internal energy decreases and PV increases, you can have a net enthalpy change of zero, but a decrease in internal energy and thus temperature.

 

[Platonic]

The posted answer is D, because "Energy is always required to break a bond." (I know it does, but I guess I didn't see how answer D implied it didn't)

ATP --> ADP + P releases energy though.

EDIT: I believe all exergonic reactions (reactions in which gibbs free energy decreases) release energy. And many exergonic reactions have bonds being broken.

 

[gettheleadout]

ATP --> ADP + P releases energy though.

EDIT: I believe all exergonic reactions (reactions in which gibbs free energy decreases) release energy. And many exergonic reactions have bonds being broken.

http://forums.studentdoctor.net/showthread.php?t=837342

 

[krogers21]

67. Which of the following is a violation of the law of conservation
of energy?
A. Heat can be changed completely to work in cyclical
process.
B. A system undergoing a reaction with constant enthalpy
experiences a temperature change.
C. After sliding to a stop, a box with initial kinetic energy
K has only thermal energy in an amount less
than K.
D. A bond is broken and energy is released.

Y'all know why the answer is not A?? Is it because A is due to 2nd law of Thermodynamics, not conservation of energy principles??

I guess I'm not understanding the "cyclical process" it is referring to. I'd like to know! Also, how is a bond breaking resulting in a violation of the law of conservation of energy? This makes no sense to me

 

[gettheleadout]

I guess I'm not understanding the "cyclical process" it is referring to. I'd like to know! Also, how is a bond breaking resulting in a violation of the law of conservation of energy? This makes no sense to me

I assume they're referring to something like an ideal Carnot heat engine.

For a single bond breaking, there cannot be a net release of energy, that's why it violates conservation.

 

[Platonic]

http://forums.studentdoctor.net/showthread.php?t=837342

"Hydrolysis of the phosphate groups in ATP is especially exergonic, because the resulting orthophosphate group is greatly stabilized by multiple resonance structures, making the products (ADP and Pi) much lower in energy than the reactant (ATP). The negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy. Hydrolysis relieves some of these electrostatic repulsions as well, liberating useful energy in the process."

http://en.wikipedia.org/wiki/ATP_hydrolysis

 

[balla121]

"Hydrolysis of the phosphate groups in ATP is especially exergonic, because the resulting orthophosphate group is greatly stabilized by multiple resonance structures, making the products (ADP and Pi) much lower in energy than the reactant (ATP). The negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy. Hydrolysis relieves some of these electrostatic repulsions as well, liberating useful energy in the process."

http://en.wikipedia.org/wiki/ATP_hydrolysis

check this out,

note* bonds formed or broken is marked within Parenthesis ()

(ATP) Adenosine - p-p-p + H-O-H ---> Adenosine-p-p + pi + energy

bonds broken in reaction: Adenosine-p-p (-) p bond, and a H (-)O-H <--- these two bonds were broken and they needed energy input to make it happen

bonds formed in reaction: a phosphorus (-) O-H bond is formed on ADP, and a P (-) H bond is formed on the Pi <----- these bonds released energy upon formation

The fact that the energy input from breaking bonds is LESS THAN energy released upon bond formation, in our balanced equation, we have energy listed as a product

 

[gettheleadout]

"Hydrolysis of the phosphate groups in ATP is especially exergonic, because the resulting orthophosphate group is greatly stabilized by multiple resonance structures, making the products (ADP and Pi) much lower in energy than the reactant (ATP). The negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy. Hydrolysis relieves some of these electrostatic repulsions as well, liberating useful energy in the process."

http://en.wikipedia.org/wiki/ATP_hydrolysis

http://www.blobs.org/mobile/article.php?article=30

 

[Platonic]

check this out,

note* bonds formed or broken is marked within Parenthesis ()

(ATP) Adenosine - p-p-p + H-O-H ---> Adenosine-p-p + pi + energy

bonds broken in reaction: Adenosine-p-p (-) p bond, and a H (-)O-H <--- these two bonds were broken and they needed energy input to make it happen

bonds formed in reaction: a phosphorus (-) O-H bond is formed on ADP, and a P (-) H bond is formed on the Pi <----- these bonds released energy upon formation

The fact that the energy input from breaking bonds is LESS THAN energy released upon bond formation, in our balanced equation, we have energy listed as a product

I understand with the whole "high energy bond breaks, low energy bond forms, so net energy released" idea with ATP. But I was under the impression that the ions themselves, without covalently bonding to H+ and OH- , are more stable in solution than ATP. The quote in my post above references resonance stabilization of the charge. I believe the ions in solution would be more stable then ATP. Then again, I guess it's written ATP--->ADP + P rather than ATP-->ADP+ + P- for a reason.

Either way, I still think choice D in the original problem is a valid statement, since it doesn't preclude the formation of more stable bonds.

 

[gettheleadout]

I understand with the whole "high energy bond breaks, low energy bond forms, so net energy released" idea with ATP. But I was under the impression that the ions themselves, without covalently bonding to H+ and OH- , are more stable in solution than ATP. The quote in my post above references resonance stabilization of the charge. I believe the ions in solution would be more stable then ATP. Then again, I guess it's written ATP--->ADP + P rather than ATP-->ADP+ + P- for a reason.

Regardless of what makes you believe this, it's a non-issue because the actual mechanism of hydrolysis doesn't entail standalone homolytic cleavage of the phosphodiester bond. Such a thought experiment is artificial and inapplicable to considering the thermodynamics of ATP hydrolysis. See here: http://biowiki.ucdavis.edu/Biochemi...phorylation/ATP_and_Oxidative_Phosphorylation

Either way, I still think choice D in the original problem is a valid statement, since it doesn't preclude the formation of more stable bonds.

This type of thinking will get you the question wrong on the MCAT. The answer choice says "a bond is broken and energy is released." Anything beyond the two components of that statement is additional information you are inserting into the answer's meaning by speculation. For the answer to be correct, it must be sufficient that only "a bond is broken" and nothing else occurs and as a result "energy is released." If the statement as is does not fit the requirements of the question, it's not correct. As is, what that statement suggests is not possible, so D is the answer.

 

[localmotion2189]

The reaction is:

PCl3(g) + Cl2(g) <---> PCl5(g)

17.) "If a flask were filled with PCI3 and CI2 to a total pressure of 1.50 atm. such that the mole fraction of PCI3 is twice that of CI2, then what is the total pressure of the system once at equilibrium?"

In the back, the answer says:

"If everything reacted to the limiting reagent capacity, then x is 0.5. But the reaction does not go to completion, so the value of x falls between 0 and 0.5. This is because some, but not all, of the reactant is lost."

I understand how to set up the ICE table and that the total pressure of the system at equilibrium is (1.5-x), correct? However, my questions are:

A.) How do you know that some, but not all, of the reactant is lost?
B.) How do you determine when a reaction goes to completion (including all other reactions besides this one)?

I have been stuck on this problem for awhile so any help is appreciated. Thanks.

 

[gettheleadout]

The reaction is:

PCl3(g) + Cl2(g) <---> PCl5(g)

17.) "If a flask were filled with PCI3 and CI2 to a total pressure of 1.50 atm. such that the mole fraction of PCI3 is twice that of CI2, then what is the total pressure of the system once at equilibrium?"

In the back, the answer says:

"If everything reacted to the limiting reagent capacity, then x is 0.5. But the reaction does not go to completion, so the value of x falls between 0 and 0.5. This is because some, but not all, of the reactant is lost."

I understand how to set up the ICE table and that the total pressure of the system at equilibrium is (1.5-x), correct? However, my questions are:

A.) How do you know that some, but not all, of the reactant is lost?
B.) How do you determine when a reaction goes to completion (including all other reactions besides this one)?

I have been stuck on this problem for awhile so any help is appreciated. Thanks.

A) They're referring to some but not all of the limiting reactant being lost. If all was lost, the reaction would have run to completion. Which leads us to your second question...
B) Any reaction that represents a chemical equilibrium will not go all the way to completion. Running to completion means the maximal amount of product possible is formed, and this is only the case in essentially one-way reactions, such as strong acid dissociation.

 

[localmotion2189]

A) They're referring to some but not all of the limiting reactant being lost. If all was lost, the reaction would have run to completion. Which leads us to your second question...
B) Any reaction that represents a chemical equilibrium will not go all the way to completion. Running to completion means the maximal amount of product possible is formed, and this is only the case in essentially one-way reactions, such as strong acid dissociation.

Thanks! That cleared up a lot

 

[Platonic]

Regardless of what makes you believe this, it's a non-issue because the actual mechanism of hydrolysis doesn't entail standalone homolytic cleavage of the phosphodiester bond. Such a thought experiment is artificial and inapplicable to considering the thermodynamics of ATP hydrolysis. See here: http://biowiki.ucdavis.edu/Biochemi...phorylation/ATP_and_Oxidative_Phosphorylation

I will concede this point as it pertains to ATP. But I'm still not certain that lysis of some molecules into ions in a solvent can't be exergonic. Like when HCl is dissolved in water, but in a case where both ions behave as Cl- ions rather than bonding with the solving as with hydronium.

 

[gettheleadout]

I will concede this point as it pertains to ATP. But I'm still not certain that lysis of some molecules into ions in a solvent can't be exergonic. Like when HCl is dissolved in water, but in a case where both ions behave as Cl- ions rather than bonding with the solving as with hydronium.

You can still divide the steps of the dissociation into bond breaking and forming steps, and the bond breaking steps never release energy. Look up enthalpy of solution and enthalpy of hydration.

 

[Platonic]

You can still divide the steps of the dissociation into bond breaking and forming steps, and the bond breaking steps never release energy. Look up enthalpy of solution and enthalpy of hydration.

The exothermic step isn't bond forming though. They are intermolecular attractions, not bonds. My whole point here was that you can break a bond exergonically without forming another bond. I tried to illustrate this with ATP, but that involves bond formation in the hydrolysis. I would still say that D is the best MCAT answer though, for the reason in your previous post. The solvent condition can't be assumed if its not in the statement, even if there are conditions that make it valid.

 

[gettheleadout]

The exothermic step isn't bond forming though. They are intermolecular attractions, not bonds. My whole point here was that you can break a bond exergonically without forming another bond. I tried to illustrate this with ATP, but that involves bond formation in the hydrolysis. I would still say that D is the best MCAT answer though, for the reason in your previous post. The solvent condition can't be assumed if its not in the statement, even if there are conditions that make it valid.

It's a semantic difference. The formation of the intermolecular attractions that are formed during solvation is exothermic, just as formation of covalent bonds is. Bond breaking is always exothermic. See: http://www.chemguide.co.uk/physical/energetics/solution.html

The only case where bond breaking alone would be exergonic would be where the increase in entropy is so great that it outweighs the always positive change in enthalpy. Can you provide any example of this sort of case? This would constitute a spontaneous decomposition of some molecule where there is no change in the intermolecular interactions with the surroundings or solvent between the reactant and products.

Edit: Actually I don't even think such a case is possible, because breaking any bond frees that electronic region of each of the atoms involved to engage in an additional intermolecular interaction. Basically I just don't see any evidence to back up what you're saying. Every spontaneous reaction I've ever seen involves an enthalpy change that is the sum of both bond breakage and formation, be it from hydrolysis, solvation, or whatever.

 

[BurntFlower]

I'm also taking the MCAT in August, but I wonder how badly it will hurt my chances to apply this cycle with a late MCAT. Anyone else in this position?

 

[Platonic]

It's a semantic difference. The formation of the intermolecular attractions that are formed during solvation is exothermic, just as formation of covalent bonds is. Bond breaking is always exothermic. See: http://www.chemguide.co.uk/physical/energetics/solution.html

The only case where bond breaking alone would be exergonic would be where the increase in entropy is so great that it outweighs the always positive change in enthalpy. Can you provide any example of this sort of case? This would constitute a spontaneous decomposition of some molecule where there is no change in the intermolecular interactions with the surroundings or solvent between the reactant and products.

Edit: Actually I don't even think such a case is possible, because breaking any bond frees that electronic region of each of the atoms involved to engage in an additional intermolecular interaction. Basically I just don't see any evidence to back up what you're saying. Every spontaneous reaction I've ever seen involves an enthalpy change that is the sum of both bond breakage and formation, be it from hydrolysis, solvation, or whatever.

Potential energy in a bond is negative, so removing the bond clearly requires energy input. My argument was not that the break itself could release energy, but that solvolysis of a bond can. Solvolysis of a molecule into separate ions can be exergonic and thus release energy. This is of course dependent on the presence of a solvent that can make the solvated state more stable than the molecular state.

 

[Platonic]

I'm also taking the MCAT in August, but I wonder how badly it will hurt my chances to apply this cycle with a late MCAT. Anyone else in this position?

Me. I initially planned on a gap year, but I'm thinking about throwing my name in the hat this year and hoping for the best. I haven't even started the application process though, no AMCAS, no nothing. Will worry about all of that after the MCAT.

 

[gettheleadout]

Potential energy in a bond is negative, so removing the bond clearly requires energy input. My argument was not that the break itself could release energy, but that solvolysis of a bond can. Solvolysis of a molecule into separate ions can be exergonic and thus release energy. This is of course dependent on the presence of a solvent that can make the solvated state more stable than the molecular state.

Solvation involves the exothermic formation of solvent-solute intermolecular interactions.

 

[Platonic]

Solvation involves the exothermic formation of solvent-solute intermolecular interactions.

the breakage and the formation of the new intermolecular interactions are simultaneous. Which, physically, means it's really one process, not two distinct processes. So the breakage and formation are part of the same (energy releasing) process.

 

[gettheleadout]

the breakage and the formation of the new intermolecular interactions are simultaneous. Which, physically, means it's really one process, not two distinct processes. So the breakage and formation are part of the same (energy releasing) process.

Incorrect. Enthalpy of hydration can be positive or negative, and the sign depends on the relative magnitude of the two steps, one of which is exothermic, the other endothermic.

 

[Platonic]

Incorrect. Enthalpy of hydration can be positive or negative, and the sign depends on the relative magnitude of the two steps, one of which is exothermic, the other endothermic.

It's a smooth process, not consecutive.