I would like to add some information regarding 'direct action' by zinc,
since it might be assumed from comments that this is not electrochemical.
If fresh bright zinc foil is placed in 2 mol/L sulphuric acid, then almost
no reaction takes place. If the zinc foil is then touched to a piece of
copper in the acid, then rapid effervescence takes place at the copper! If
granulated zinc is placed in the acid, then it effervesces readily.
Granulated zinc contains carbon as a result of the production process for
extraction of zinc, i.e. carbon reduction of zinc oxide. No doubt, an
electrochemical cell involving zinc as one electrode and carbon as the
other is set up. Since they are in electrical contact, the cell is
shortcircuited and zinc forms zinc ions, and hydrogen is liberated at the
particles of carbon.
In the first set of observations, then over a period of time, the zinc
foil will be eaten (corroded) away when it touches the copper. In this
case, the two observations have been separated, that is, corrosion of the
zinc and production of hydrogen. In this sense, the explanation of zinc
reacting with sulphuric acid in terms of two half cell reactions seems
very satisfying. It does seem likely that a similar explanation works for
the granulated zinc, even though we can not see the separate elements.
Since very few metals are produced as pure materials, and many have carbon
in them when produced, most reactions are likely to be electrochemical in
nature. One piece of evidence for this is an iron post in India, made of
very pure iron, which is hardly corroded after many hundreds of years
standing in all weathers.
On another tack, I gave a talk at an RSC meeting some years ago, when I
demonstrated the production of electricity from two different metals and a
grape. I asked one chemist in the audience if I could borrow a piece of
gold jewellery to act as the positive electrode, against magnesium as the
other electrode, and the reason for refusal was that the jewellery might
be damaged! So much for electrochemical understanding for that chemist.
John Oversby
_____________________________________________________________________________
From: John Oversby
School of Education
The University of Reading
Bulmershe Court
Earley
Reading RG6 1HY
UK
Tel + 44 1189 875123 x 4906
Fax + 44 1189 318650
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|