At 6:47 PM +0100 5/22/99, Dr Joe Lee asked about cells of the type
M | H2SO4 | Cu
in which M is an active metal such as Zn, for which an EMF of 0.93 V
was obtained with emission of hydrogen observed at the Zn electrode only.
The cell reaction here is amost certainly
Zn + Cu++ --> Cu + Zn++
The hydrogen bubbles are due to direct attack of the acid on
the zinc and have nothing to do with the electrochemistry other
than to help raise the zinc ion concentration.
The Cu++ ions are also formed by direct attack of H+, but of course
build up to only a very small concentration owing to the greater
stability of reduced copper. I presume that the small extent of this
reaction accounts for the lack of gas bubbles at the copper electrode.
(Recall that even in neutral solution, enough Cu++ dissolves to initiate
corrosion at the point where copper and steel pipes are joined.)
Taking [Zn++] = 0.1M and [Cu++] = 1E-8 M as guesstimates, the potential
seems to work out to about what was observed. I doubt that this cell
would be capable of delivering much current, owing to the small value of
[Cu++] and to the slow kinetics of H2 discharge on copper.
Joe also poses a question that I always ask my students: what happens
when a term in the denominator of the Nernst equation is zero?
Does the infinite voltage this implies mean that you are likely to get
electrocuted when you stir your tea with a metallic spoon?
The answer to this has two parts.
1) Assume that only one atom of M dissolves in a litre of water,
making [M++] roughly 1E-24 M. Substituting this into the Nernst
equation and taking the log should relieve any fear.
2) Any electrons liberated in such small numbers would get lost in
the plumbing; you would not have a true equilibrium situation and
the Nernst Equation would not have much meaning. Electrochemists
say that such an electrode is "unpoised". I'm not sure what the
practical lower limit for poising is, but my guess would be
around 1E-10 to 1E-8M.
For a slghtly unconventional view of electrode potentials and their use,
and a view that in my opinion is far clearer than that presented in
conventional textbooks, you might wish to look at
http://www.sfu.ca/chemcai/AQCHEM/FallElect.html. A more in-depth treatment
of elementary electrochemistry will be found in the Acrobat document at
http://www.sfu.ca/chemcai/pdf/c1xElchem.pdf .
--
Steve Lower - Simon Fraser University - [log in to unmask]
ChemCAI Teacher resources Web page: http://www.sfu.ca/chemcai/
Chem1 Instructional Software for General Chemistry: http://www.chem1.com
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