MadSci Network: Chemistry

Re: Catalysts with the reaction of Zn and HCl to form H2 gas.

Date: Wed Apr 5 00:08:35 2000
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Chemistry
ID: 954248322.Ch


I am interested how you predicted catalytic effects and what specific 
materials you chose and what your results were.  This is an interesting 
subject.  A catalyst enters into a reaction and lowers the activation 
energy of the reaction and is itself essentially unchanged after the 
reaction is at equilibrium.  Of course close examination shows that the 
catalyst frequently does react with the material, usually in a cyclic 
manner, to change the reaction pathway or mechanism.  The catalyst usually 
can be recovered virtually unchanged.

If you add copperII ions to zinc metal there is a rapid reaction; copper 
II oxidizes the Zn forming  ZnII and a copper metal plate on some of the 
zinc surface.   This process is a single displacement reaction and is 
known as immersion plating.  Since the copper II becomes Cu 0 the copper 
II is strictly not the catalyst.  Neither it or the copper 0  release H2 
from acid yet the reaction is accelerated so the catalyst must be the 
plated copper metal which was formed and which can be recovered when the 
zinc is dissolved.   

How does it work?  What the copper metal does is separate in space the 
anode[oxidation] and cathode[reduction ]processes.  Both processes were 
taking place on the same Zn atom, so to speak,  and they were interfering 
with each other.  As oxidation occurred zinc ion concentrations increased 
reducing the oxidation potential of the zinc, causing a positive charge 
buildup near the metal and repelling hydrogen ions and effectively slowing 
the reaction.  This process is know as polarization and the reduction of 
the cell potential is known as the cell overvoltage.  There may be other 
factors involved because the reactions are a series of elementary steps 
involving single electron transfers which may differ on zinc and copper 
surfaces so the reaction would have to be thoroughly studied to be 
completely figured out.  The zinc metal is more electropositive than the 
copper metal so the zinc metal is depleted slightly of electrons while the 
copper is slightly enriched.  The hydrogen ions are now attracted to the 
copper and accept an electron from the copper to form hydrogen atoms which 
combine to give H2 molecules.  Notice there are no zinc ions to repel 
hydrogen ions or to lower the potential on the copper.  The zinc ions are 
being formed on the zinc metal and are being repelled by the more positive 
zinc metal.  In effect we have made a number of chemical cells with a zinc 
anode and a copper cathode and have separated the anode and cathode 
reactions.  Indeed if a copper and zinc strip are connected by a wire and 
both are immersed in  acid solution hydrogen gas will be evolved from the 
copper strip and the zinc strip will dissolve.  

This phenomenon is very common when dissimilar metals are in contact and 
are both exposed to the same electrolyte especially when the anode[more 
active metal] is exposed to oxygen .  It also can be used to protect metal 
by putting a more active metal [usually magnesium protecting steel] in 
contact with the structure.   In automobiles a variation is that the 
negative pole of the battery is connected to the car frame [ground].  This 
makes the frame electron rich and helps to slow rusting [but not 
completely  of course].  A problem related to this is found in the 
DeLorean automobiles which have a stainless steel body[cathode] and a 
steel frame[anode].  Thes cars have a severe rusting problem at the 
connection bolts if they are not carefully insulated to prevent electrical 
contact between the frame and body.
du/estir/dict.htm  gives definitions

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2000. All rights reserved.