Does anyone have any thoughts on the following?
The GCSE/O level book "Thinking Chemistry" by M Lewis and G
Waller takes an appropriately simple appoach to the description of
electrochemical cells. Current generation is treated essentially as
the reverse process of electrolysis. Three cells (two in a question
at the end of the appropriate chapter) are considered in the book.
The cells are
Zn | dil. H(sub2)S0(sub4) | Cu
Fe | dil. H(sub2)S0(sub4) | Cu
Mg | dil. H(sub2)S0(sub4) | Fe
In each case, there is a direct reaction between the left side metal
and the acid (for Mg very vigorous), but surely this cannot explain
why the cell should generate an emf. BUT they certainly do; I tried
it for myself. The temperature was near enough 25 deg C and the
acid concentration 1 M. I obtained measured emfs of +0.93 V,
+0.45 V and +1.46 V, respectively, and these were reasonably
steady; whether the cells would be good current generators is
another matter. I am aware that when you stick two electrodes of
different metals into an orange an emf is generated.
The question is how could you explain it to HE students, who are
completely happy with explaining what happens in a Daniell cell.
The only way I could do it was to say that, at the reactive left
electrodes, not only was there DIRECT reaction
M + 2H(super)+ ---> M + H(sub)2
but some M ----> M(super)2+ + 2e(super)-
with subsequent transfer of the electrons to the right where
2H(super)+ + 2e(super)- ----> H(sub)2
so we should have hydrogen evolved at both electrodes--I did not
see this on the right though. If then you write a Nernst relationship
for the emf, what do you do about the [Zn(super)2+], initially zero?
In some ways, there is resemblance to the lead acid battery--no
Pb(super)2+ or PbSO(sub)4 there initially.
I did in fact pose the same query on the Philosophical Chemistry
list, but since there has been no response so far, I decided to try
the Education channel.
Joe Lee
Dr Joe Lee
Department of Chemistry,
UMIST,
Manchester,
M60 1QD,
UK
Phone 0161 200 4513
Fax 0161 236 7677
Email [log in to unmask]
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