Question 41

[behii208456]

the second reaction is not reversed but the sign has?
please explain the reasoning behind this since as written the half reactions will give the final reaction desired!
thanks

[mcat_premed3832]

I guess the short answer is that weird expression "there are many ways to skin a cat"! If you look carefully, the second reaction was indeed reversed. However, it was not done the way you are used to (in fact, you should do it on paper and you will see that you need to reverse the equation).

The way we did it in the explanation is to put a negative sign throughout the bottom reaction. Thus we subtracted one reaction from the other which is the same as reversing the second reaction then adding the two reactions.

[sashavazir7501]

I don't understand your backslash convention on these practice tests-

Its confusing for the sake of being confusing.


use standard iupac convention.

[mcat_premed3832]

The objective of our exams is not simply to teach; rather, it's to prepare you for the real MCAT. Our team has experience with over 100 real past MCAT tests. Our objective is to follow the newest trends on real tests and to be sure that you are aware of the different presentations of problems that they may have. Of all the questions we prepared, most follow standard convention but some follow other conventions that we have witnessed the AAMC using as well.

[larubio25277]

I am still having difficulty understanding this question. I do not understand why the second equation's E0 is negative? I noticed that in the explanation that VO2+ was negative which would then make the E0 negative, but why?

[jellywing_2058]

To clarify, the reason the reaction, -VO^2+ + 2H^+ + e- ↔ H2O + V^3+ -(Eo = 0.34 V) is negative is that in a cell, one electrode will have a reduction occurring while the other electrode of a cell will have an oxidation occurring.

Moreover, for a cell to be spontaneous it does require a positive overall potential but the cell also requires both a reduction and an oxidation side to be fully functional. So, if one is to calculate the overall cell potential, by using the table E half cell potential values, one is to note that all half cells are in their reduction forms (electrons are all in the reactant side of the equations).

So, to calculate an overall cell potential, you need to keep the more positive half cell as the reduction half reaction and the more negative (or least positive) of the two half cells as the oxidation half cell. The equation is reversed to which the electrons are on the product side and as such, the potential sign is then inverted as is the case here.

[larubio25277]

Oh ok, that makes alot more sense than the explanation given in the test. Thank you.