AAMC Official Guide Chem/Phys Question 9

Lionroar
Posts: 21
Joined: Thu Sep 28, 2017 3:02 pm

AAMC Official Guide Chem/Phys Question 9

Postby Lionroar » Sun Dec 24, 2017 7:17 pm

I am not sure where to begin with this one. The AAMC explanation does not address how the passage indicated through which mechanism the unfolding proceeded. And should we just memorize that the units for the unimolecular rate constant are s^-1? Thank you very much.
Lionroar
Posts: 21
Joined: Thu Sep 28, 2017 3:02 pm

Re: AAMC Official Guide Chem/Phys Question 9

Postby Lionroar » Mon Dec 25, 2017 6:47 pm

I found the Next Step review video of the OG questions, so thank you for addressing my second point. However, what still remains unclear is how the passage told us that the mechanism was unimolecular.

Also, later in that section in question 24 - the AAMC explanation says that the M(C17H32CO2)2 precipitate was the salt of a weak base. How do we know that? Thank you very much.
NS_Tutor_Andrew
Site Admin
Posts: 521
Joined: Mon May 23, 2016 1:47 pm

Re: AAMC Official Guide Chem/Phys Question 9

Postby NS_Tutor_Andrew » Tue Dec 26, 2017 1:27 pm

Hi lionroar,

Thanks for the question! Let me go one by one here.

For Q9, it is a little tricky to figure out that it's a first-order reaction mechanism, because the passage doesn't give you that information very directly. I think the best way to approach this is through process of elimination, and figuring out which order makes the most sense given the information that we're given. We have to choose between zeroth-order (choice C), first-order (choice D), and second-order (choice A).

For the kinetics to be second-order, the rate would have to depend on the concentration of two reactants, and there's no indication in the passage that there even is a second reactant in addition to the folded protein. So we can eliminate that.

A zeroth-order mechanism is a little tricker to assess -- in such a mechanism, the rate doesn't depend on the concentration of any reactant. This may sound bizarre, but it can happen in cases where the reaction is being catalyzed by an enzyme present in low concentrations such that only a tiny proportion of the reactant is in contact with the enzyme at any given time -- therefore, variations in the overall concentration of the reactant will not meaningfully affect the amount of reactant that's actually able to undergo the reaction. This can also be the case if the reactants have to be adsorbed onto a catalyst with limited surface area. Similarly, there's nothing in the passage suggesting that this would be the case.

In a first-order mechanism, the rate is dependent on the concentration of a single reactant. These mechanisms often take place when something just "happens" to a reactant -- a classic example is spontaneous radioactive decay. This is the best fit for protein unfolding conceptually, and even though the passage doesn't really confirm it directly, it's certainly consistent w/ passage info, so it's the best choice.

Turning to Q24, recognizing that M(C17H32CO2)2 is the salt of a weak base is a piece of background knowledge about acid-base chemistry that the MCAT is just expecting you to know. Carboxylic acids (-COOH) are very common organic acids that are technically weak acids, but are relatively strong for organic compounds, and their conjugate bases have the functional group -COO- (also written as -CO2-). These are known as carboxylate anions, and whenever you see them, you should think "weak base".

Hope this helps!!
Andrew D.
Content Manager, Next Step Test Prep.

Return to “AAMC Resources”