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sdsbama
Joined: 30 Jan 2008
Posts: 9
Location: birmingham, al
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 Posted: Thu Apr 03, 2008 2:53 pm Post subject: GS-2 Physical Sciences Question 22 |
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| why is it in the equation that the anode is turned into a negative when it shows in equation II as +.440? |
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asitmc2118
Joined: 06 Feb 2008
Posts: 2
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| Posted: Sat Apr 05, 2008 4:49 pm Post subject: |
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1st half reaction is oxidation reaction, and the number E=+0.440 represents an oxidation potential, not reduction potential.
We flip or reverse the whole reaction so we get the reduction potential of the reaction.(Why do we need this? read on)
If you reverse the reaction, then the sign of the E changes as well. so it goes from +0.440 to -0.440.
Then use the equation
[b]E(rxn) = E(cat) - E(anode)[/b]
and these E(cat) and E(anode) numbers have to be reduction potentials and this is why we had to switch everything(of the 1st half rxn) in the first place.
Now if you just plug it in,
E(rxn) =0.401 - (-0.440) = 0.401 + 0.440 = 0.841
I hope I am going in the right direction. |
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agent4474112
Joined: 22 Feb 2010
Posts: 2
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| Posted: Mon May 03, 2010 6:58 pm Post subject: |
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| It is already given in the passage...I don't know why they reversed the sign of the anode in the explanation? Doesn't make sense...Since it's given in the passage as it is. Just add them together!!! |
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geoffn045751
Joined: 06 Mar 2010
Posts: 5
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| Posted: Sat Jun 12, 2010 5:35 pm Post subject: |
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| Doesn't equation 1 show oxidation as written = +0.440 since the oxidation number of Fe increases to 2+ and equation 2 show reduction as written = +0.401? So according to Eoreaction = Eoreduction - Eooxidation shouldn't the answer be -0.039? |
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jellywing_2058
Joined: 04 May 2009
Posts: 179
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| Posted: Fri Jun 18, 2010 2:09 pm Post subject: |
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| agent4474112 wrote: | | It is already given in the passage...I don't know why they reversed the sign of the anode in the explanation? Doesn't make sense...Since it's given in the passage as it is. Just add them together!!! |
The overall reaction potential of the cell is essentially equal to the reduction potential of the substance reduced minus the reduction potential of the substance oxidized (E0 = E reduction - E oxidation). Hence, you do not simply add them up as it is a difference or subtraction of the two half cells. |
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jellywing_2058
Joined: 04 May 2009
Posts: 179
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| Posted: Fri Jun 18, 2010 2:14 pm Post subject: |
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| geoffn045751 wrote: | | Doesn't equation 1 show oxidation as written = +0.440 since the oxidation number of Fe increases to 2+ and equation 2 show reduction as written = +0.401? So according to Eoreaction = Eoreduction - Eooxidation shouldn't the answer be -0.039? |
The overall reaction potential does equal to E° (red) – E°(oxid) as stated above so essentially, the overall should be equal to the following: E° (cell) = 0.401 – (-0.440) = +0.841.
Moreover, as the oxidized half cell is given as +0.440, you need to invert the sign back to its original reduction half cell as the equation uses the standard reduction potentials for both the reduced and oxidized substances. So the answer is correct regardless of the fact that the oxidized potential had been given. |
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