Question 15

[sharra.26428]

I thought that the Rate law is dependent on the concentration of reactants only, so could you please explain this question involving the products?

[srraghav1542]

Yeah, is there any reply to this. I got the answer right, but I also don't understand why the rate is 2nd order with respect to the product?

[admin]

It is true that the routine that you should do for the MCAT is to express the rate law in terms of the reactants. However, as you correctly worked out, if you are given information otherwise (a common MCAT trick is to take something you know well and just alter it slightly to see if you can go with the change), then you adapt.

"The rate of a chemical reaction can be measured in terms of the rate of disappearance of one of the reactants or in terms of the rate of the appearance of one of the products. For example in the hypothetical reaction, A + 2B ----> AB2 the rate could be measured in terms of the disappearance of either reactant A or B, or in terms of the appearance of the product AB2."

http://chem.lapeer.org/Chem2Docs/Rate.Eq.html

[jeronash5705]

It is one thing to take something we know and alter it slightly. It is an entirely different thing to present us with a physical impossibility and expect us to go with it. Those of us who actually understand kinetics will automatically discount the answer you claim is right. If your problem is to be believed, the reaction would not ever occur in the first place, because there is no X to begin with, and the rate law is entirely dependent on X. On the other hand, the rate law would be increasing (to the power of two) even when there are no reactants left. How can a reaction occur when there are no reactants? It can't, but your problem asks us to suspend reality and believe that it can.

You are right that a rate of reaction can be measured by observing the rate of appearance of product, but that has nothing to do with the rate law in this problem. Whoever wrote this problem has obviously never done kinetics experiments in real life.

[nedaa.asba6809]

Given that the reaction is a second order, then acording to (CHM 9.2) if the rate of the reaction is given by : rate= k[A]^m[B]^n then the overall reaction order is (m+n).
So back to the question if it is of a second order then we can express the the rate as
rate= k[x]^2 then from the figures it's clear that both answeres B, D are impossible. Then by graphing the rate versus the concentration it's a graph of a parabola and the only possible answer is C.

[quadalpha]

Aside from jeronash's objection, would it be possible to make the labels on the x-axes rather larger? They look like little infinity symbols.

[klr861276]

Any way you want to look at this poorly worded problem, the supposedly correct answer is a graph indicating that as the concentration of X (the triatomic product) increases, the rate of the reaction increases. This could be true if we're talking about the reverse reaction, but not the forward reaction. This is one of those GS problems that I just throw out in my mind because it's flat wrong.

[mcat_premed3832]

This question was . . . unpretty.

The diagram is being made nice (legible on the x axis) and the question has been adjusted. To maintain context, the question will be moved down to question 18 later this week.