Solution to AAMC 9 #29

[Sol Rosenberg]

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Can anyone explain the solution to AAMC 9 #29 in a less confusing way than the solutions? Why would plotting the concentration of M on a nonlinear scale change whether or not there was an excess of HC?

I still just don't get this one....

Thanks a lot,

Jota

 

[Mr. Tee]

Woops I missed this question too.

jota_jota are you an engineer by any chance?

(sorry I didn't have anything constructive to say 😳 )

 

[Sol Rosenberg]

Woops I missed this question too.

jota_jota are you an engineer by any chance?

(sorry I didn't have anything constructive to say 😳 )

Yes

 

[oxeye]

This was one I missed - and one that I still don't understand! I hope someone here does. 😉

 

[Lests55]

Sorry guys, I missed that one too. The only thing I can think of is back to gen chem rate laws where we graphed ln A vs. t and got slope of -k (or something like that)....If I remember correctly in such circumstances you have a first order reaction... I don't know, just reaching on that one.

There seemed to be two bs answers on that section of 9. That one and the one that we were supposed to figure out that it was an adiabatic process. I was reading the solutions and just like "how was I supposed to know you were looking for that?".

 

[Mr. Tee]

There seemed to be two bs answers on that section of 9. That one and the one that we were supposed to figure out that it was an adiabatic process. I was reading the solutions and just like "how was I supposed to know you were looking for that?".

Are you talking about the one where the compression happens too quickly for heat to escape? 😛 (the other answer choices didn't make much sense)

 

[EMH]

When working back through a practice test, most of the ones I miss I say darn I should have got that. Then there are problems like this one where I don't feel bad if I miss it.

I got this right but I sort of guessed, couldn't give a decent explanation. Except that in that equasion in the passage the rate appeared to be determined by K1. So rate 1 = slow step.

 

[EMH]

Are you talking about the one where the compression happens too quickly for heat to escape? (the other answer choices didn't make much sense)

Nope this one is is on passage V, I'd state it but I beleive that's against the rules.

 

[Lests55]

Are you talking about the one where the compression happens too quickly for heat to escape? 😛 (the other answer choices didn't make much sense)

Yeah, that's the one. Never made the connection.

 

[Sol Rosenberg]

Sorry guys, I missed that one too. The only thing I can think of is back to gen chem rate laws where we graphed ln A vs. t and got slope of -k (or something like that)....If I remember correctly in such circumstances you have a first order reaction... I don't know, just reaching on that one.

There seemed to be two bs answers on that section of 9. That one and the one that we were supposed to figure out that it was an adiabatic process. I was reading the solutions and just like "how was I supposed to know you were looking for that?".

Are you talking about #58? I think that one was just a bad answer explanation. The answer was just the reverse of an expanding gas doing PV work (and losing heat as PV work is being done.) The question was just illustrating the opposite -- when the volume was decreased, work was being done on the gas, and its temperature would increase.

 

[Mr. Tee]

I was talking about #57...

 

[Lests55]

I was talking about #57...

I was talking about 57

 

[Sol Rosenberg]

I was talking about #57...

Yeah, I had to answer that one by eliminating the other choices, because the correct answer wasn't obvious to me, but the other answer choices just didn't make sense....

 

[Lests55]

I was talking about 57

How about the one on the bio section where the answer was something like give a fetus a virus carrying a certain gene. Got that one right, but I don't think I'd have the cajones to put "give a fetus a virus" on Saturday.

 

[nekrogg]

Can anyone explain the solution to AAMC 9 #29 in a less confusing way than the solutions? Why would plotting the concentration of M on a nonlinear scale change whether or not there was an excess of HC?

I still just don't get this one....

Thanks a lot,

Jota

[disregard my explanation!]



i made 4 foolish mistakes on this section (including the one in the back with the Aluminum in the P group.. HOW CAN I MISS THINGS LIKE THAT?!) dropped me from a 12 to a 10 😡 i blame the loud people in the library!!

 

[xanthomondo]

Can anyone explain the solution to AAMC 9 #29 in a less confusing way than the solutions? Why would plotting the concentration of M on a nonlinear scale change whether or not there was an excess of HC?

I still just don't get this one....

Thanks a lot,

Jota

When you have a rate reaction...rate = k[x][y]
If you have y in sooo much excess (say 1000000 molar, and your reaction only proceeds to .0001 molar), y pretty much doesn't change

Since y doesn't change it is pretty much a constant, so you can factor it into k and now you have a first order rate equation, where rate = k'[x]

this is a very popular procedure for determining the rate equation...its called "psuedo-first order"

so the ln[x] vs time would be linear since its first order in x.....

But, if the plot isnt linear, that means something went wrong....but its first order in x!....that means that [y] didn't hold as truly constant as we originally thought....therefore y is not in excess and we can't factor it into the constant

hope that helps

 

[Sol Rosenberg]

When you have a rate reaction...rate = k[x][y]
If you have y in sooo much excess (say 1000000 molar, and your reaction only proceeds to .0001 molar), y pretty much doesn't change

Since y doesn't change it is pretty much a constant, so you can factor it into k and now you have a first order rate equation, where rate = k'[x]

this is a very popular procedure for determining the rate equation...its called "psuedo-first order"

so the ln[x] vs time would be linear since its first order in x.....

But, if the plot isnt linear, that means something went wrong....but its first order in x!....that means that [y] didn't hold as truly constant as we originally thought....therefore y is not in excess and we can't factor it into the constant

hope that helps

Ahhhhh -- The way you explain it, it makes perfect sense. You should write solutions for AAMC!

Thanks a lot!

Jota

 

[daydrmgirl]

When you have a rate reaction...rate = k[x][y]
If you have y in sooo much excess (say 1000000 molar, and your reaction only proceeds to .0001 molar), y pretty much doesn't change

Since y doesn't change it is pretty much a constant, so you can factor it into k and now you have a first order rate equation, where rate = k'[x]

this is a very popular procedure for determining the rate equation...its called "psuedo-first order"

so the ln[x] vs time would be linear since its first order in x.....

But, if the plot isnt linear, that means something went wrong....but its first order in x!....that means that [y] didn't hold as truly constant as we originally thought....therefore y is not in excess and we can't factor it into the constant

hope that helps

Thanks, that is very helpful.

I eliminated the answer down to A and C (b/c the question says that it's nonlinear, and in the text, it says that "If the HC(g) is in a large excess... [then] Equation 2 is obtained") but still don't know why HC has to be in the rate-determining slow step... can someone guide me through the logic? Thanks!

 

[albaniandoc]

I did 9R yesterday and got a 30. PS 11, VC 9 and BS 10. I looked it over a little bit and found really careless mistakes and some mistakes that were due to unclear answer choices, kinda like this one. It seems that other people have made the same mistakes as well.

 

[xanthomondo]

Thanks, that is very helpful.

I eliminated the answer down to A and C (b/c the question says that it's nonlinear, and in the text, it says that "If the HC(g) is in a large excess... [then] Equation 2 is obtained") but still don't know why HC has to be in the rate-determining slow step... can someone guide me through the logic? Thanks!

If HC wasn't involved in the rate determining step, then its concentration would be irrelevant to the entire kinetics study....

if you had A+B+C -> D, where your rate only depends upon A and B, you can think of a mechanism such as

A+B-->AB
AB + C --> D

the rate determining step is what determines the rate equation...

if the first reaction took days to react, but the second reaction occured in nanoseconds, then what does your overall reaction depend on, the formation of AB or reaction of AB with C?
In this case as soon as get some AB, you instantly have D, therefore C wont factor into the kinetics of the overall reaction

Since we've determined that HC is part of the reaction rate because it wasn't constant (from before), it can be inferred that being part of the reaction rate involves being part of the slowest step (the rate determining step)

only 48 hours! 🙂 you should be relaxing!

 

[Whatsamatta U]

I did 9R yesterday and got a 30. PS 11, VC 9 and BS 10. I looked it over a little bit and found really careless mistakes and some mistakes that were due to unclear answer choices, kinda like this one. It seems that other people have made the same mistakes as well.

Yeah, I made a lot of stupid mistakes in the bio section on 9. Maybe this is because we are all getting so sick of this and are acting carelessly...something we hopefully won't do on the real thing!

By the way, OnmywayRN, best...avatar...ever.