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### Topic: Lab: Galvanic Cells - How to calculate standard cell potential  (Read 23016 times)

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#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #15 on: May 07, 2013, 08:03:45 PM »
okay I figured out the cell potential but how would I get the percentage difference. And when it says potential from the book what potential would I use?
Calculated cell potential

Anode: Zn(s) Zn2+(aq) + 2e-
Eo = -0.76v
Cathode: Cu2+(aq) + 2e-  Cu(s)
Eo = 0.34v
Ecell = Ecath - Eanode
= 0.34 – (-0.76)
= 1.1v

Anode: Zn Zn2+ + 2e-
Eo = -0.76v
Cathode: Ni2+(aq) + 2e-  Ni(s)
Eo = -0.23v
Ecell = Ecath - Eanode
= -0.23 – (-0.76)
= 0.53v

Anode: Zn Zn2+ + 2e-
Eo = -0.76v
Cathode: Fe2+(aq) + 2e-  Fe(s)
Eo = -0.41v
Ecell = Ecath - Eanode
= -0.41 – (-0.76)
= 0.35v

Anode: Zn Zn2+ + 2e-
Eo = -0.76v
Cathode: Ag+(aq) + e-  Ag(s)
Eo = 0.80v
Ecell = Ecath - Eanode
= 0.80 – (-0.76)
= 1.56v

#### Borek

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #16 on: May 08, 2013, 02:58:33 AM »
I believe by "potential in the book" kriggy meant theoretical potential, calculated using table data.

Note that the problem asks for "% difference". That means if the ratio of measured and calculated potential is 105%, potential difference is 5%.
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#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #17 on: May 08, 2013, 09:33:22 AM »
I believe by "potential in the book" kriggy meant theoretical potential, calculated using table data.

Note that the problem asks for "% difference". That means if the ratio of measured and calculated potential is 105%, potential difference is 5%.
So would the theoretical potential in this case be my voltmeter reading? I am just not understanding where the theoretical value can be calculated or found somewhere.

#### Borek

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #18 on: May 08, 2013, 09:49:31 AM »
Theoretical value is the one calculated using standard potentials, as you did two posts ago:

Calculated cell potential

Anode: Zn(s) Zn2+(aq) + 2e-
Eo = -0.76v
Cathode: Cu2+(aq) + 2e-  Cu(s)
Eo = 0.34v
Ecell = Ecath - Eanode
= 0.34 – (-0.76)
= 1.1v
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#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #19 on: May 08, 2013, 10:04:31 AM »
Theoretical value is the one calculated using standard potentials, as you did two posts ago:

Calculated cell potential

Anode: Zn(s) Zn2+(aq) + 2e-
Eo = -0.76v
Cathode: Cu2+(aq) + 2e-  Cu(s)
Eo = 0.34v
Ecell = Ecath - Eanode
= 0.34 – (-0.76)
= 1.1v
So the equation would look something like this

[(potential calculated (I am not sure if this is supposed to be the voltmeter readings) - theoretical potential)/theoretical potential]*100

#### Borek

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #20 on: May 08, 2013, 11:48:16 AM »
[(potential calculated (I am not sure if this is supposed to be the voltmeter readings) - theoretical potential)/theoretical potential]*100

No.
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#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #21 on: May 08, 2013, 04:45:53 PM »
[(potential calculated (I am not sure if this is supposed to be the voltmeter readings) - theoretical potential)/theoretical potential]*100

No.

well thanks for that really ...

#### Borek

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #22 on: May 08, 2013, 04:55:03 PM »
Sorry, but each time I try to explain anything in the next post you mix everything as if you were not paying attention and I have a feeling I am wasting my time. I told you that the theoretical voltage is the one you have calculated, then you propose that the error is between theoretical voltage and the calculated one - in other words, error is between the theoretical and the theoretical. Does it make any sense?

You are expected to compare MEASURED with THEORETICAL, where THEORETICAL means CALCULATED from the table data.
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#### kriggy

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #23 on: May 08, 2013, 05:26:59 PM »
I believe by "potential in the book" kriggy meant theoretical potential, calculated using table data.

Yeah, I didnt know how to formulate it better.

[(potential calculated (I am not sure if this is supposed to be the voltmeter readings) - theoretical potential)/theoretical potential]*100
No.
well thanks for that really ...

Ok
The readings you get from your voltmeter is difference in potentials between your electrodes. You changed always 1 electrode and used Zn electrode as standart. If you look into table you can search for standart potential of Zn electrode. From the diference between Zn potential and measured voltage you get potential of the other electrode. By using known variables (potential, concentrations, temperature..) and Nernst equation you can calculate standart potential of measured electrode and then compare it with theoretical number and calculate error.
You can calculate absolute or relative error, it depends on what are you expected to do.

#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #24 on: May 08, 2013, 06:30:02 PM »
I believe by "potential in the book" kriggy meant theoretical potential, calculated using table data.

Yeah, I didnt know how to formulate it better.

[(potential calculated (I am not sure if this is supposed to be the voltmeter readings) - theoretical potential)/theoretical potential]*100
No.
well thanks for that really ...

Ok
The readings you get from your voltmeter is difference in potentials between your electrodes. You changed always 1 electrode and used Zn electrode as standart. If you look into table you can search for standart potential of Zn electrode. From the diference between Zn potential and measured voltage you get potential of the other electrode. By using known variables (potential, concentrations, temperature..) and Nernst equation you can calculate standart potential of measured electrode and then compare it with theoretical number and calculate error.
You can calculate absolute or relative error, it depends on what are you expected to do.

I dont get how the Nernst equation has anything to do with the percent difference

#### Borek

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #25 on: May 09, 2013, 03:04:31 AM »
By using known variables (potential, concentrations, temperature..) and Nernst equation you can calculate standart potential of measured electrode and then compare it with theoretical number and calculate error.

Now you are just adding to the confusion. Potential calculated with the Nernst equation would be the theoretical one, which can be compared with the real, measured one. Plus, you don't use Nernst equation to calculate standard potential, rather teh formal potential, using standard potentials from tables.
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#### kriggy

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #26 on: May 09, 2013, 04:37:50 AM »
Im sorry, I guess I understood the task wrong

#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #27 on: May 09, 2013, 09:38:48 AM »
Im sorry, I guess I understood the task wrong
No problem. I answered the question like this
% diff. = value 1-value 2(*100)
value 1+value 2
= 1.1 – 1.05    (*100)
1.1+1.05
= 0.05
2.15
=2.33%

#### Borek

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #28 on: May 09, 2013, 09:49:23 AM »
I answered the question like this
% diff. = value 1-value 2(*100)
value 1+value 2
= 1.1 – 1.05    (*100)
1.1+1.05
= 0.05
2.15
=2.33%

No idea what you are doing. Looks  to me like juggling numbers without any particular reason.

Let's start with the zinc/copper cell. State what is the theoretical potential and what is the measured potential.

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#### sallyhansen

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##### Re: Lab: Galvanic Cells - How to calculate standard cell potential
« Reply #29 on: May 09, 2013, 04:16:58 PM »
I answered the question like this
% diff. = value 1-value 2(*100)
value 1+value 2
= 1.1 – 1.05    (*100)
1.1+1.05
= 0.05
2.15
=2.33%

No idea what you are doing. Looks  to me like juggling numbers without any particular reason.

Let's start with the zinc/copper cell. State what is the theoretical potential and what is the measured potential.

For zinc/copper cell I did the steps like so
Anode: Zn(s) Zn2+(aq) + 2e-
Eo = -0.76v
Cathode: Cu2+(aq) + 2e-  Cu(s)
Eo = 0.34v
Ecell = Ecath - Eanode
= 0.34 – (-0.76)
= 1.1v