Burning Boards question

[Miles Standisch]

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goes something like this.... Go is gibbs free energy:

a --> b Go = -3

b --> c Go = +1

c --> d Go = -5

d --> e Go = +6

At equilibrium, for the reaction a <--> b <--> c <--> d <--> e, which species will predominate?

A) a
B) b
C) c
D) d
E) e

And why?

Believe it or not, this is NOT an MCAT question, but one similar to a Step I question I had.

 

[Smurfette]

I believe the answer is D. Here's my unscientific explanation. It's been a long time since I had thermodynamics, so excuse me for my (lack of) proper terminology.

if Go is negative, a reaction will proceed.

C goes to D by Go=-5
E goes to D by Go=-6 (reverse D-->E and take the opposite of Go to find the "new" Go)

Thus, the product D is formed by Go=-11 (-6+-5=-11)

By contrast, Product B is formed from A (Go=-3) and from C (Go=-1) for a total of Go=-4.

Since a reaction with Go=-11 is more "spontaneous" than a reaction of Go=-4, product D will accumulate the most.

I hope I'm right on this...otherwise four years of chem classes down the drain!

 

[forbin]

I agree that the answer is D. I'm not so sure you can add the Go values from 2 different sides of D (i.e. C --> D and E --> D).

Think about it this way...

If you start with any of the other compounds, what is Go for the conversion to D?

A --> D Go = -3 + 1 - 5 = -7
B --> D Go = 1 - 5 = -4
C --> D Go = -5
E --> D Go = -6

So, since Go is negative between any other reactant and D, it must be the predominant species at equilibrium.

Hope this helps, and I'm not sure why this question is in any board review book... 😕

 

[babinski bob]

smurfette and forbin are both correct.. the intuitive way to approach the problem is to arbitrarily graph the points and connect them as a series of peaks and valleys. D is the lowest of the values/valleys and hence the lowest in energy. The species that would predominate at equilibrium ALWAYS one that is lowest in energy. Remember, thermodynamics deals with ENERGY and is irrespective of rate.

 

[Miles Standisch]

A young woman has had bone pain for a decade, with frequent bulky stools, as well as muscle weakness and distended abdomen. Imaging reveals generalized demineralization of bones, absence of lamina dura, and fracture lines in bones of the feet. Which of the below serum values is most likely?

A) Ca-low, Phosphorus-low, Alkaline phosphatase-hi, PTH-hi

B) C-low, P-hi, AP-hi, PTH-lo

C) C-low, P-hi, AP-hi, PTH-hi

D) C-hi, P-lo, AP-hi, PTH-lo

E) C-hi, P-lo, AP-lo, PTH-hi

 

[docuw]

Ok, this one I have to go with A.

Sounds like she has some GI disturbance - a malabsorption, so I am going to go with a Vitamin D defic (one of ADEK, fat soluble vitamins). She would then have Low Ca++ and Low Phos (not absorbing from GI). As a response, PTH will be high. This will activate the remodeling system, including, paradoxically, Osteoblasts which will increase Alk Phos activity. (granted, the osteoblasts won?t be happy b/c they don?t have enuf Ca++ available to them, but PTH still would have activated them).

Thus, low Ca++, low Phos, Hi Alk Phos, hi PTH

 

[WakeMedHeel]

I'm just glad I didn't have that as a question....

 

[lady bug]

Believe it or not, this is NOT an MCAT question, but one similar to a Step I question I had. [/B]

That looks fearfully similar to an MCAT question! 😱 Ahhhhh! That crap is gonna haunt me for the rest of my life. 😡