General chem question!!

[allday0109]

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Is Electromotive force E0 different to E cell?
because I knew the Ecell = Ecatode- E anode, but in some questions they just add up each E value.
When do I use this equation?

 

[allday0109]

nobody knows? please help me!!!

 

[doc toothache]

Is Electromotive force E0 different to E cell?
because I knew the Ecell = Ecatode- E anode, but in some questions they just add up each E value.
When do I use this equation?

Emf (E cell) = E cathode+ E anode
Eo = half cell reaction (Eo have positive or negative values)

Tables of half cell reactions are usually listed as oxidation potentials (oxidation reactions). If the electrode is used as a left-hand half-cell, the Eo values are used just as they are found in a table. If the electrodes are used as right-hand half-cells, the sign of the value must be changed.

for example Zn;Zn++//Cu++; Cu vs
Cu++; Cu//Zn; Zn++

 

[rose786]

Maybe this link will be helpful. It's the 2nd question down.
http://forums.studentdoctor.net/showpost.php?p=2862982&postcount=9

 

[allday0109]

I understand how to get the Ecell but in some questions, they didn't follow this rule, but just add up both E values.
I am really confused when I should get Ecell as above or just add up two E values.
E cell= E cathod - E anode or

E*= sum of both E values.

 

[rose786]

Well, like it said in that link I posted, it's best not to use formulas. Use the problem and work backwards. Read up on the "rules" of electrochemistry. Like galvanic cells will always be positive for Ecell because they are spontaneous. Recognize when a reaction gives a reduction half potential vs an oxidation half potential. There are a couple rules like these to know, and then you can solve electrochemistry problems without those formulas.

 

[poc91nc]

I understand how to get the Ecell but in some questions, they didn't follow this rule, but just add up both E values.
I am really confused when I should get Ecell as above or just add up two E values.
E cell= E cathod - E anode or

E*= sum of both E values.

You need to understand the idea...and quit thinking in terms of plug and chug...

I don't understand why kaplan writes the equation as Eox - Ered...but unlearn that since it is confusing you.

Most of the problems you see will give you a full redox reaction composed of two half reactions..one oxidation and one reduction.

Then...you will usually be given two reduction potentials. Now, look at the full redox reaction...identify the half reaction that is a reduction. Ok...now you have one value; E red.

Then...you find the half reaction that is oxidation. If you were given reduction potentials....you have to find the OXIDATION POTENTIAL. The oxidation potential for a reaction that is a reduction is just the reverse. Hence, you just flip the sign.

Once you have that...just add the two value. As you can see...you think additive...and worry about the signs accordingly.

Ok, what if you were given oxidation potentials to start with, instead of reduction potentials...which half reaction is going to have its sign reversed?

 

[doc toothache]

I understand how to get the Ecell but in some questions, they didn't follow this rule, but just add up both E values.
I am really confused when I should get Ecell as above or just add up two E values.
E cell= E cathod - E anode or

E*= sum of both E values.

By definition the cell reaction is the sum of the two half-cell reactions.

Let's try with an examples.

1. Calculate the Emf of the following cell at 25 C.

Ag;Ag+//Cu++; Cu
Eo=Eo left + Eo right
E= (-0.789) + (+0.377)=-0.462 volt
Since the copper half-cell appears at the right, the sign of value from Table 1 must be changed.

Table 1

Electrode .....Eo
Ag; Ag+ ..... -0.789
Cu: Cu++..... -0.337

 

[allday0109]

Thank you very much you all!!!! I love you guys!!^^