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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: sallyhansen on May 03, 2013, 08:43:39 PM

Title: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 03, 2013, 08:43:39 PM
Calculate the standard cell potentials listed below
1. Zn│Zn2+(1 M)║ Cu2+(1 M)│Cu
2. Zn│Zn2+(1 M)║ Ni2+(1 M)│Ni
3. Zn│Zn2+(1 M)║ Fe2+(1 M)│Fe
4. Zn│Zn2+(1 M)║ Ag+(1 M)│Ag

So the ones above are the ones I found for the cell notation
Voltmeter readings
1.05 V
0.42 V
0.31 V
0.42 V

Also how does the accuracy of each result (in terms of a percentage difference) need to be calculated for each one?

Would the equation just be this
% difference = Ka Experimental/ Ka Given * 100
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 04, 2013, 02:31:58 PM
Anyone please  ???
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek on May 04, 2013, 03:54:15 PM
I guess you won't get any help as your original post is confusing to the point where it is impossible to give any meaningful answer. You failed to explain what is given, what is calculated, what is measured and what you are doing.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 04, 2013, 06:24:09 PM
I guess you won't get any help as your original post is confusing to the point where it is impossible to give any meaningful answer. You failed to explain what is given, what is calculated, what is measured and what you are doing.
Purpose:  The purpose of this activity is to demonstrate the design and operation of a galvanic cell used in scientific research and to compare the measured potential difference of that cell to the calculated potential difference using a standard table.

Materials:
waste beaker      zinc electrode      zinc nitrate solution (1.00 M)
beaker         copper electrode   copper (II) nitrate solution (1.00 M)
Voltmeter      nickel electrode      nickel (II) nitrate solution (1.00 M)
wire leads (x2)          iron electrode      iron (II) nitrate solution (1.00 M)
porous cup      silver electrode      silver nitrate solution (1.00 M)

Procedure:
1.   Place the zinc nitrate solution into a beaker and connect the zinc electrode so that it is partially submerged in the solution.
2.   Place copper (II) nitrate solution into the porous cup and connect a copper electrode so that it is partially submerged in the solution.
3.   Connect each electrode up to the voltmeter.  If the needle moves left, change the connection so that the needle moves right.  Record your observations.
4.   Remove the copper electrode and porous cup.  Pour out the solution from the porous cup and thoroughly rinse and dry the porous cup.
5.   Repeat steps 2 to 4 using nickel/nickel (II) nitrate and zinc/zinc nitrate.
6.   Repeat steps 2 to 4 using iron/iron (II) nitrate and zinc/zinc nitrate.
7.   Repeat steps 2 to 4 using silver/silver nitrate and zinc/zinc nitrate.
8.   Remove the porous cup and pour out the contents of the beaker.
9.   Place the porous cup back into the beaker and use iron/iron (II) nitrate.
10.   Repeat steps 8-9 using nickel/nickel (II) nitrate in the beaker and silver/silver nitrate.

Analysis:
Prepare a table to record your observations. The table should include a column for the following:  the cell notation of the cell constructed, the voltmeter reading, and the calculated cell potential.  Include a column for expressing the accuracy of each result (in terms of a percentage difference).

The chart I have constructed

   Cell notation of the cell constructed   Voltmeter readings   Calculated cell potential   Percentage difference
Zinc nitrate/copper (II) nitrate   Zn│Zn2+(1 M)║ Cu2+(1 M)│Cu
1.05 V      
Nickel (II) nitrate/Zinc nitrate   Zn│Zn2+(1 M)║ Ni2+(1 M)│Ni
0.42 V      
Iron (II) nitrate/Zinc nitrate   Zn│Zn2+(1 M)║ Fe2+(1 M)│Fe
0.31 V      
Silver nitrate/Zinc nitrate   Zn│Zn2+(1 M)║ Ag+(1 M)│Ag
0.42 V      
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 05, 2013, 10:52:56 AM
Anyone please my assignment is due Thursday of this week???
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek on May 05, 2013, 01:51:27 PM
What are standard potentials of all half electrodes you constructed in the experiment?
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 05, 2013, 04:23:23 PM

Half electrodes                Standard Potential
Zn2+(aq) + 2e- -> Zn(s)            -0.76
Cu2+(aq) + e- -> Cu+(aq)            0.16
Fe2+(aq) + 2e- -> Fe(s)            -0.41
Ag+(aq) + e- -> Ag(s)                    0.80
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 06, 2013, 10:04:31 AM
What are standard potentials of all half electrodes you constructed in the experiment?
Half electrodes                Standard Potential
Zn2+(aq) + 2e- -> Zn(s)            -0.76
Cu2+(aq) + e- -> Cu+(aq)            0.16
Fe2+(aq) + 2e- -> Fe(s)            -0.41
Ag+(aq) + e- -> Ag(s)                    0.80
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek on May 06, 2013, 10:17:55 AM
Cu2+(aq) + e- -> Cu+(aq)            0.16

That's definitely not the right one.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 06, 2013, 01:42:41 PM
Cu2+(aq) + e- -> Cu+(aq)            0.16

That's definitely not the right one.
As you can see the Cu2+(aq) + e- -> Cu+(aq) is 0.16 on this website
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/electpot.html
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: kriggy on May 06, 2013, 01:46:24 PM
We have this one in our book at my UNI:
Cu2+(aq) + 2e- -> Cu(s) 0.337V
0,16 goes for Cu+ + e- -> Cu


ok basicaly you calculate standart cell potential for each of your cells and then compare it with value found in book and then calculate mistake or inaccuracy
The cell potential can be calculated by Nernst equation and inaccuracy is calculated like
[(potential calculated - potential in book)/potential in book]*100.
btw did you do more measurements? Or just one?
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 06, 2013, 03:21:50 PM
We have this one in our book at my UNI:
Cu2+(aq) + 2e- -> Cu(s) 0.337V
0,16 goes for Cu+ + e- -> Cu


ok basicaly you calculate standart cell potential for each of your cells and then compare it with value found in book and then calculate mistake or inaccuracy
The cell potential can be calculated by Nernst equation and inaccuracy is calculated like
[(potential calculated - potential in book)/potential in book]*100.
btw did you do more measurements? Or just one?
what do you mean by more measurements?
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: kriggy on May 06, 2013, 03:36:39 PM
I mean if you measure each cell only once or more.
For example when we did titrations at school, we did it 3-5 times and used average number. It gets more accurate results.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 07, 2013, 09:29:02 AM
I mean if you measure each cell only once or more.
For example when we did titrations at school, we did it 3-5 times and used average number. It gets more accurate results.
I only did it once for each reaction. And my chemistry class is online so the results will always show up the same.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 07, 2013, 07:06:55 PM
What would the anode and cathode of each reaction be?
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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


Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek 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%.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek 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
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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

 


Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek 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.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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 ...
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek 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.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: kriggy 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.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek 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.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: kriggy on May 09, 2013, 04:37:50 AM
Im sorry, I guess I understood the task wrong  ???
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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%
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek 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.

Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen 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
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 09, 2013, 04:19:56 PM
So the above is the theoretical potential and the measured potential would be the voltmeter reading I have from the lab? Which I got 1.05V.
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek on May 09, 2013, 04:33:45 PM
So the above is the theoretical potential and the measured potential would be the voltmeter reading I have from the lab? Which I got 1.05V.

Yes. And you are expected to compare 1.05V (experimental) with 1.1V (theoretical).
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 09, 2013, 04:54:21 PM
So the above is the theoretical potential and the measured potential would be the voltmeter reading I have from the lab? Which I got 1.05V.

Yes. And you are expected to compare 1.05V (experimental) with 1.1V (theoretical).
Which I did with this equation, which hopefully is formatted correctly

% 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%
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek on May 09, 2013, 05:07:17 PM
No, that's not what you did. What is 2.15?

[tex]difference~\% = \frac{theoretical - experimental}{theoretical}\times 100~\%[/tex]
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 09, 2013, 05:28:54 PM
No, that's not what you did. What is 2.15?

[tex]difference~\% = \frac{theoretical - experimental}{theoretical}\times 100~\%[/tex]

the 2.15 was me adding 1.1 and 1.05, which now I know is incorrect. Thanks for the equation, really
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: sallyhansen on May 09, 2013, 05:34:06 PM
No, that's not what you did. What is 2.15?

[tex]difference~\% = \frac{theoretical - experimental}{theoretical}\times 100~\%[/tex]

Also I ended up getting this. So would I subtract 100 by 10 to get 95.24% as the percent difference?
 
= 1.1 – 1.05/ 1.05 X 100
= 4.76%
Title: Re: Lab: Galvanic Cells - How to calculate standard cell potential
Post by: Borek on May 09, 2013, 05:58:54 PM
4.76% is OK.