calculating EMF's (4 Viewers)

[wahashtini]

i seem to have a problem with these type of questions (i.e. calculating emf's for redox reaction and cells)
I dont understand the part where u make a negative potential on the chart to a positive one in order to work our the total EMF. (which ones do u change and which ones do u leave)

thanxxx

 

[Constip8edSkunk]

if the reaction is in opposite direction, you reverse the energy associated in the sense of changing the sign

 

[wahashtini]

i still don't get it, more info plz (how do u know if its in the opposite direction)?

 

[danno]

i dont know very much, im gonna have to study VERY hard after english tomorrow, but basically you go to the table at the back, and get the reduction potentials or whatever its called, for the specific ions your dealing with, and then you reverse the sign on one of them.

unfortunately i have no idea which one to reverse the sign on, so unless someone can help it will be a 50/50 chance for me

 

[ontherun]

yeh abood i think it sumthn like redox minus oxidation

when u look up wat ever it is for oxidation in the table and has a neg sign infront of it u take it off!

i think its right but not 100%

3AJEBA

 

[d_elmo]

that table if u look at the heading is standard REDUCTION potentials, so if you want to find the emf for oxidation then u have to reverse the sign and make it the opposite way

 

[beta-omega]

Ok, basically, figure out what which one is undergoing reduction, and which is undergoing oxidation. I suspect you know how to do that. Now with the EMF, the data sheet gives you a list of the standard potentials of reduction for that substance. The oxidation potentials is the negative of that number, basically, if its positive its now negative, and if its negative, its now positive. Change for the oxidation only! Then add your potentials together, and you should get your EMF.

 

[Brad]

It always has to be positive remeber that, so you reverse the one that will make the overall EMF >0 You can work out which one is reduced and oxidised that way too.

 

[L45er]

Here's a simple way. It's called the table of standard reduction potentials... hence they are the reduction reactions. Whatever is undergoing reduction, it is left the way it is in the table. The other half of the cell, the oxidation half equation, it obtained by reversing the reduction half equation, and swapping the symbol of the potential. You then put the two half equations together...
ie for Zn|Zn2+||Cu2+|Cu cell, the
Zn -> Zn2+ + 2e- (think its like -0.76V, but its undergoing oxidation so it becomes 0.76V)
Cu2+ 2e- -> Cu (is undergoing reduction, so voltage stays same, like 0.30 or something i think)
then
Zn + Cu2+ -> Cu + Zn2+ (0.76 + 0.30 = 1.06V total potential)


Easy!!!

 

[im_THEShazz]

i know this is 100% right!

1. inspect the half cell equations and find there E value in the standard potentials table.

2.write the half equation which is more positive E as the reduction reaction (so leave it how it is on the table)

3. reverse the direction of the other equation and change the sign of E so it becomes the oxidation reaction. so if its negetive in the table make it positive and vise versa.

4. E oof the Cell can now be calculated by adding the E of the oxidation and the E of the Reduction.

5. if you are writing it up as one equation balence the half-cell equations for electrons.

if you want an example just ask.

 

[Paroissien]

An idiot's guide is to take the negative of the half-reaction HIGHER on the table and add add that to other reaction.

 

[kheir]

how could such a simpl quest get so complex???

only wiv so much freaks ansering

 

[Paroissien]

It's not complex. It's basically the same answer over and over again cause everyone is bored I guess. I know I am

 

[tina_goes_doo]

It's so simple and i'm starting to get sick of the people thoughout this whole chem forum typing "wat u stoopid u no iz poztive so it go udder way round deer!"

 

[Paroissien]

That in reference to anyone in particular?

 

[helper]

Draw a S on your data sheet. Reverse the top half equation and keep the original half equation for the bottom.

 

[tina_goes_doo]

That in reference to anyone in particular?

No, just a general statement on how i hate netspeak. Just because the english exams are over doesn't mean you can kill the english language. It's both hard to read and most likely harder to type. Why bother?

 

[VVoody]

because ure stoopid tina

 

[charbar]

Tina_goes_doo if i were u i would consider chopping off my fingers since all u type is arrogant bullshit that i predict no one cares about. the reason im typing this msg to such an ignorant hoochy is because the concepts u may not understand in chem someone else may and cosider it to be a stupid question.. so next time u want to superior urself think twice about how much of a loser u are! if ure so bothered about the EMF question why post a thread, go find attenbtion elsewhere u conceited and narrowminded F*%$

 

[wind]

Easy charbar! lol

Anyways, back to the question!

Find your individual reactions on the table of standard potentials, which ever reaction is higher up, you reverse - consequently making it a positive.

To determine the EMF, you just add the two Eo values together