half reactions and potential

[frochocinco]

i'm really struggling with this conceptually. i understand what happens in a galvanic cell pretty much just fine, however, when it comes to understanding these half reactions, im pretty lost.

1) first off, the one thing on galvanic cells im a little iffy about is understanding the spontaneity of the reactions...if the half reactions add up to a positive number, the reaction will occur? is that correct, or should they add to a negative potential

2)when they give you a table with potential values of half reactions, how do you know whether they are reduction potentials or oxidation potentials? some are losing electrons and some are gaining electrons
--> i thought that an overall positive potential allows for a spontaneous reaction, but it seems from the tables that half reactions prefer negative potentials

3) If it asks for example, which is the strongest reducing agent, how do you know whether it is the half reaction with the most positive or most negative potential?


sorry if this is convoluted, im just pretty confused and would TRULY appreciate some help and clarification!

thanks

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[The Kraken]

i'm really struggling with this conceptually. i understand what happens in a galvanic cell pretty much just fine, however, when it comes to understanding these half reactions, im pretty lost.

1) first off, the one thing on galvanic cells im a little iffy about is understanding the spontaneity of the reactions...if the half reactions add up to a positive number, the reaction will occur? is that correct, or should they add to a negative potential

2)when they give you a table with potential values of half reactions, how do you know whether they are reduction potentials or oxidation potentials? some are losing electrons and some are gaining electrons
--> i thought that an overall positive potential allows for a spontaneous reaction, but it seems from the tables that half reactions prefer negative potentials

3) If it asks for example, which is the strongest reducing agent, how do you know whether it is the half reaction with the most positive or most negative potential?


sorry if this is convoluted, im just pretty confused and would TRULY appreciate some help and clarification!

thanks

1) Positive E = spontaneous. Erxn = Ereduction-Eoxidation. The "-Eoxidation" part is for an oxidation written as a reduction, aka electrons on the left side of the arrow.

EXAMPLE: Li+ + e- -> Li, E= -3.05.
-3.05 = NOT spontaneous, in fact, it really really wants to go the other way.

2) On any physical chemistry course/textbook table, all of the half reactions will be written as reductions, aka electrons on the left side of the reaction. On the MCAT, you have to see what side the electrons are on. Most of the time it seems that they are written as reduction potentials, aka electrons on the left side. Just take the time and look. Electrons on the right = oxidation potential, and you dont have to flip signs.

FOR EXAMPLE IF YOUR TABLE LOOKS LIKE THIS:
H2O2 + 2H+ + 2e- -> 2H2O(l), E = +1.77
Li -> Li+ + e- E = +3.05
Erxn = 1.77 + 3.05, since the oxidation half reaction is already written as an oxidation.

Yes it is confusing at first, just take your time and master this and its really simple.



3) Strongest reducing agent = gives electrons up easy, not taking them in. Back to the lithium vs hydrogen peroxide, which one wants to give up electrons? Look at Li -> Li+ + e-, E=+3.05. In the reaction, it gives it electrons away, and is very VERY favorable. Hence, Li metal or whatever = best reducing agent.
TL;DR for #3 - highest E for electrons on right side of reaction = strongest reducing agent.

 

[frochocinco]

great. thanks a lot. appreciate the reply