Ask A Scientist Chemistry Archive

 
name         Christopher P.
status       educator
age          30s

Question -   I am a chemistry teacher.  In studying activity series, I
always do the old Zn + CuSO4 --> Cu + ZnSO4.  You can then weigh the
copper recovered and calculate percent yield, etc..  I have always
wondered, though, about the copper recovered at the end.  It is a reddish
solid that is very powdery when dry.  Of course, it looks nothing like
good old metallic copper, which bothers my students (and, I admit, me) to
no end.  I have tried to smelt it in a crucible (which should easily be
hot enough for the 1084degC. melting point - at least I think so), with
no luck.  I am guessing the copper is too impure to melt properly, but I
am not sure how to proceed with purifying it.  Can you give me any
hints?  Do I need to reduce it further?
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My guess is that in collecting and drying the copper that you are using an
oven. In this case, you are forming CuO on the outside. Trying to melt the
copper in a crucible (in air) will also form more CuO - and you are not going
to melt that. The rxn will always produce tiny pellets as every reaction site
is a formation and nucleation point of copper. I have no solution for you,
except maybe use the experience to teach your students that Cu + O2 will 
react.

Greg (Roberto Gregorius)
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The brown-red color of the precipitated Cu is due to a thin layer of CuO
(copper oxide). This is inevitable unless great care is taken to remove
oxygen from the system. If I recall also adding ammonia will produce a more
metallic-looking precipitate. This is exactly what happens if you leave a
new penny around for a few days -- it tarnishes. The amount of oxygen
absorbed is not so great as to affect the stoichiometry computation
significantly. Heating the copper in air is not going to help at all. In
fact it will make matters worse. High temperature promotes further
oxidation of the copper. To obtain a shiny copper product will require
exclusion of oxygen -- no easy feat in practice.

Vince Calder
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Solid metals in a condition of low physical integrity
(low density, small particles, high surface area, molecularly fluffy)
usually do look darker than their parent metal in good condition.
Dark reddish powder sounds about right to me for copper rapidly 
precipitated in a water solution.
Exchange reactions are especially prone to this.
If the Copper had high density, it would cover the metallic Zinc and stop 
the reaction with a thin shiny film.
For some reasons that isn't what happens.

Yes, the powder should be able to smelt to good shiny metallic Copper.
Some things to watch out for are copper oxides from water and air, 
leftover sulfur from sulfate.
Copper Oxides and Sulfides are dull, dark, and hard.  Not what you are 
trying to reach.
Zinc impurities might make slight problem, too, although they could 
benignly smelt with the copper to low brass,
  or evaporate out of the crucible due to the low boiling-point of zinc.
Zinc Oxide and sulfide would be white crusty precipitate dispersed in the 
copper, keeping the copper metal from being soft.

There will always be some percentage of Cu+ oxides, hydroxides, and 
sulfides and sulfates remaining in such powder.
They must be reduced to Cu(0) and the non-copper atoms evaporated away.

Heating Copper in air converts it to CuO and/or Cu2O, which coincidentally 
both melt within 10 degrees C of copper itself.
But they are dark gray.  What are you using to get/keep a reducing 
atmosphere inside your crucible?
A well-fitting lid and a modest lump of charcoal might be enough.
A deep blanket of charcoal lumps over the copper might be better.
It will evolve some carbon monoxide fumes when you heat it.
The well-fitting lid minimizes this, but ventilation is still essential 
for safety.
When you are done with the duration of heating you intend to do,
Perhaps a good way to cool it all down is to remove the lid and dump the 
contents into water.
That way the hot charcoal will not continue burning, and the copper will 
not be slowly blackening.
A small airtight metal container would be good, too.

Are you sure your crucible is getting hot enough?
1100C glows bright yellow-orange from any dark-colored substance.  Clear 
glass, white ceramic, and shiny metal glow less.
I am not sure a crucible over an average Bunsen-burner flame will average 
that hot inside.
I might try finding a ceramic tube, I.D. ~20% wider than the O.D. of the 
crucible,
and positioning it as a short chimney around the crucible with the flame 
rising from the bottom.
that way the entire crucible surface, even the lid, would be immersed in 
hot flame exhaust.
And then if you set the air/fuel ratio of your burner to be fuel-rich,
the atmosphere surrounding the leaks into your crucible will be mostly 
reducing rather than oxidizing.
If you cannot find a ceramic tube, merely boring a rough hole through some 
light-weight, heat-tolerant brick would do as well.

Try melting a 1/2-inch segment of 12-gauge Copper house wire inside your 
crucible.
See if you can melt that, or if perhaps it oxidizes (blackens).
If the copper is clean (dilute nitric acid) and the atmosphere is reducing,
it will melt and pull together into a somewhat liquid-like blob.
Maybe a shiny one if you do quite well.

Carbon powder could interfere with freely flowing and wetting behavior of 
shiny melted metal copper.
Carbon will not react with copper, but a fraction of a percent can 
dissolve in copper or other noble metals when melted,
making them slightly dark and giving them a stiff, non-liquid, micro-thin 
crust, presumably of precipitated graphite.
Try to keep your charcoal in the form of large non-dusty chunks not in 
contact with the copper.

A properly reducing atmosphere and 1100C almost cannot fail to smelt 
copper powder such as yours.
In my (semiconductors, etc) job we would use ceramic or quartz tube furnaces
with gently flowing 1% Hydrogen in Nitrogen to do this kind of thing.
No carbon needed.  Very clean, very uniform.  Perhaps that accounts for 
why it sounds easy to me.

Jim Swenson
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