[CHM 112] Electrochemistry Lab Report Help

[Andrew J. Pounds]

So your electrochemistry lab report is due this week.  I promised some 
help so here it is.

*(1)*  Remember that when the voltage reads positive we assume that the 
black electrode is connected to the ANODE (where oxidation is taking 
place - that is electrons are being lost). So for the first reaction on 
the report form,  Zinc is the anode and copper is the cathode.  The 
balanced cell reaction would be


Zn + Cu^2+ (0.1) ---> Zn^2+ (0.1) + Cu


*(2)*  In the second reaction  Copper would be the anode and Zinc would 
be the cathode and the reaction would be:

Cu + Zn^2+ (0.1) ---> Cu^2+ (0.1) + Zn

*(3) *Where this gets a little tricky is when you write the equation for 
the concentration cells. Let's imagine we are look at the reaction 
between Cu(0.01) on the black lead and Cu(0.1) on the red lead.   Here 
is the balanced chemical equation:

Cu + Cu^2+ (0.1) ---> Cu + Cu^2+ (0.01)

The concentration of the cell that is the anode is going to be on the 
product side of the reaction.

Using your observed cell voltage (or really just the sign of the cell 
voltage) you should be able to pick out which four reactions are 
spontaneous.


*(4)* For the concentration cells involving only copper, the value of 
E^0 _cell will be zero - but the value of E_cell observed and calculated 
will not be zero.  To calculate E_cell you need to use the Nernst 
equation.  Calculate the reaction quotient Q like we have done before - 
and make sure to put the product and reactant concentrations in their 
proper place. Also, make sure that you use the proper value of "n" the 
number of moles of electrons transferred.

I tested this out with several sets of student data and everything 
seemed to work as expected.  You will most likely not get the exact same 
values for your measured and calculated E_cell values - but they should 
in general have the same sign and follow a trend.

As always -- please e-mail me with any questions!


-- 
*/Andrew J. Pounds, Ph.D./*
/Professor of Chemistry and Computer Science/
/Director of the Computational Science Program/
/Mercer University, Macon, GA 31207 (478) 301-5627/
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