# Please: Calculating the emf of a cell, how do I know if a cell is non standard?

Discussion in 'Chemistry' started by Cat_with_no_eyes, Oct 30, 2010.

1. ### Cat_with_no_eyesRegistered Senior Member

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The question says:

Calculate the standard e.m.f. of the cell at 298 K?

1) Ni (s) | Ni 2+^ (aq)|| Sn 2+^ (aq), Sn 4+^ (aq) |pt

.....

Will I have to use the Nernst equation? The questi does not even state if its standard or non standard? do i ignore the 298 K??

Last edited: Oct 30, 2010

3. ### Green DestinyBannedBanned

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Are you aware you have to calculate the reduction and oxidation? For instance, you would have $E_0=E_{red}+E_{oxi}$ and will measure the voltage of the cell.

5. ### Cat_with_no_eyesRegistered Senior Member

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yes I know I need to calculate E cell which is E cathode - E anode

I'm not entirely sure of the next steps I must take alltogether and how? Please could you guide me on this? thanks

7. ### Green DestinyBannedBanned

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Well, this is never easy stuff - atleast I never found it very easy. I even had to look it up again: here is an example:

$Zn(s) + Fe^2+ -----> Zn^2+ + Fe(s)$

To calculate the cell voltage, you need to split the equation (hopefully your are aware of how to do this, which gives us:

$Zn(s) -----> Zn2+ + 2e$
$Fe^2+ +2e -----> Fe(s)$

Which are the two half-reactions as we have gathered you know about. You need to know the potentials required, so

$Zn^2+ + 2e -----> Z^n$ where
$E_0 = -0.76V$

This equation needs to be reversed, so $E_0$ will be reversed also.

$Zn(s) -----> Zn^2+ + 2e$
$E_0 = +0.76V$
$Fe^2+ +2e -----> Fe(s)$
$E_0 = -0.41V$

You then add the two equations together.

ps. this is just an example. You will need to work this out yourself.

8. ### arfa branecall me arfValued Senior Member

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298K is a standard temperature so the question is about standard potentials. Pt is a standard "inert" electrode too--it doesn't get reduced or oxidized, but acts as a source or sink of electrons.

9. ### Green DestinyBannedBanned

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Yes, that's right. I've just explained that.

10. ### arfa branecall me arfValued Senior Member

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The Wiki page I referenced has a link to a list of standard electrode potentials, which will probably also be useful. These are often located in Chemistry textbooks, in the chapters that cover electrochemistry.

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