exp 23 questions, calculations, % deviation, rubric, "template"

[Zellmer, Robert]

I always get several questions about exp 23.  I realize we haven't had
a chance to cover this material yet but will get through some of it before
the reports are due.  The following should help lead you in the right
directions.

There's a Excel file for the Data-to-Collect tables (page 162 in the manual)
and report table for Part C (page 163 in the manual) on Carmen.  There's
also a template for the report questions.

For Part A:

In the lab you were using the Cu electrode as your cathode and it's
acting as the standard electrode (the electrode in the beaker).  Each of
your other electrodes are the anodes.  Since the other electrodes are
the anodes (oxidation) in the reaction, your E^o values for the cell were
for those electrodes being the half-cell where oxidation takes place (the
oxidation reactions).  For the table in your report you need to write the
reactions as reduction reactions (which means if you've written them
in the notebook or report as oxidation reactions you will need to reverse
them so they are written as reduction reactions).  You should get a table
which looks like Table 20.1 in the textbook (p. 863 in the 14th ed., p. 871
in the 13th ed., p. 841 in the 12th ed., p. 857 in the 11th ed. and p. 863 in
the 10th ed.) and Table 23.1 in the manual (page 151).

Essentially what you were doing is using the Cu electrode as the
standard electrode rather then the normal standard hydrogen electrode (SHE).
If you look at Table 20.1 you will see the SHE has a value of zero and the Cu
electrode as an E^o of 0.34 V.  You could redo Table 20.1 by making the Cu
electrode have a value of 0, which means you would be subtracting 0.34 from
the values in Table 20.1. So if you based the table on a standard Cu electrode
all the values in Table 20.1 would change by 0.34 (decrease by 0.34).  In this
"new" table the value for the hydrogen electrode would be -0.34 V.

Rather than do all this, we are having you use the eqn used in class, the
textbook and manual which relates the E^o for the cell to the E^o values of
the cathode and anode,

E^o(cell) = E^o_red (cathode) - E^o_red(anode),

as given on page 152 of the manual.  Using this eqn with the E^o_cell
which you got in the lab you should get E^o_red (anode) for your anodes which
should agree with those given in Table 20.1 (and Appendix E) of the book.  The
E^o_red for Cu should wind up being 0.34, as given in Table 20.1.  Your value for
the Fe electrode should wind up being -0.44.  You will likely find some of your
values don't agree with those in Table 20.1 (or Appendix E).

For Part B:

This is like Part A.  The more conc. soln. of Cu^2+ is the cathode (the dilute soln
is the anode).  Compare the values you get with that for a standard electrode
when the conc. of both solns would be 1 M.  What's the differences?  Why?
Look at the Nernst Eqn. in section 20.6 of the textbook.  In this eqn for a conc.
cell the conc. of the metal cation for the anode (oxidation) is in the numerator and
the conc. of the metal cation for the cathode (reduction) is in the denominator in
the reaction quotient, Q.  Calculate what the E for these conc.cells should be and
how do your experimental values compare with the calc. ones.


For Part C:

Your table should look like that on page 163 of the manual (and is in the Excel
file).  The 2nd column is "moles of metal calculated from the current".  It might
make things easier if you add another column for the moles of electrons
transferred but it's not necessary.

The calculated AWs for the two electrodes should be the same since you
are using the same electrodes.  The mass gained at the cathode should equal
mass lost by the anode.  Since the moles lost by the anode equals the moles
gained at the cathode the AWs should be the same.  However, you will find
you may not get the same AWs.

Your calculation for the moles of electrons will look similar to that on page
155 of the manual.  Use what ever time period you actual had (30.0 min,
30.5 min, 31.0 min etc.)  Then you need to calculate the moles of metal
lost at the anode and gained at the cathode.  That's part of the calculations
shown on page 156, EXCEPT it's only the first multiplication shown and you
do NOT use the AW to calculate mass.  You are just getting the moles of
metal.  Since all your metals involve a 2-electron transfer it will look like the
first step for Cu on page 156 (hint:  all of the ions you used had charges,
ox. states, of +2).  You are stopping with moles of metal and not going all
the way to mass.

Then you take the mass of metal gained by the cathode (which you measured
in the lab) and divide by the moles of metal you calculated from the moles of
electrons transferred to get the molar mass (atomic wt.).  You do the same for
the anode, using the mass lost by the anode, which you measured in the lab.
The Actual AW is from the periodic table.

In the table for part C you need to calculate % error.  You can find this on
page F-5 of the manual and at the following link,

Treatment of Numerical Data (Error Analysis, sig. fig., slope from a graph)<http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/App_F_1250_lab_manual.pdf>

You true value in this calculation is the known AW for your metal from the
periodic table.


This is a "short" report.  What you need to include is on page 161 of the manual
and below.

Report:

* Cover Page
* Page with Purpose and Procedure
* Sample Calculations
    o Calculations for the reduction potential (emf) for one electrode other than Cu/Cu
        for Part A
    o Calculations for the cell potential for one set of concentrations from Part B (Nernst
        Equation)
    o Calculations for the table in Part C for either anode or cathode
* Tables
        Part A - make a table similar to Table 23.1 on page 151.  Make sure you write the
        reactions as reductions with the proper electrode potentials (calculated as described
        above).
        Part B - Make a table with the three Cu/Cu reactions, including conc., and the E for
        the different concentrations.  For example,

            Cu^2+ (aq, conc. of concentrated) --->  Cu^2+ (aq, conc. of dilute)        E value

            Cu^2+ (aq, 1.0 M) ----> Cu^2+ (aq, 1.0 M)          E value
            Cu^2+ (aq, 1.0 M) ----> Cu^2+ (aq, 0.1 M)          E value
            Cu^2+ (aq, 1.0 M) ----> Cu^2+ (aq, 0.01 M)        E value

        Part C - table like that on page 163 (in Excel file)

* Answers to Questions (explain your answers)  - this is the "template" on Carmen.
    That's provided so you wouldn't have to type out the questions.

There is NO Discussion or Conclusion sections for this report.

The report MUST be typed.  NO scans.


Rubric and Sample Calculations:

Look at the PDF rubric given in the module.  It makes it a little easier to determine
what's required for the sample calcs.

The table in part C lists "Moles of Metal".  You should show a calculation
for moles of e- transferred and then moles of metal, as shown on the rubric.
If you do it all as one step that's fine (using dimensional analysis you could
do it all in one long step).  Remember, you are supposed to show a sample
for every calculation.

Post Lab:

There's a post-lab on Carmen.

Dr. Zellmer
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osu.edu/pipermail/cbc-chem1250/attachments/20220413/1b6f9c82/attachment-0001.html>