What would be possible sources of error in this lab?

Question

http://www.temple.edu/chem-help/from_titus/911_480.htm

the lab starts with copper metal
then makes a series of reactions with other stuff
and finally getting copper metal back

i got a 62.6% yield, but i duno what happened to the rest.


thnx

Answer 1

It is not possible for me to say what you did wrong if you don't tell me all your observations during the process. that's why all observations during a lab job must be properly and methodically logged. Otherwise you will end up asking these things.

Well, I assume you added enough nitric acid and achieved complete dissolution of the copper sample.
First maybe you did not add enough hydroxide in the copper precipitation. You must add enough alkali in order to convert all copper ion into copper hydroxide, so the final solution must be colorless and clear. Beware if you use ammonium hydroxide as the neutralizer, you can complex copper into solution obtaining a deep blue solution, and thus not all copper hydroxide would be precipitated so preferably use sodium hydroxide. You may also use lime, but will obtain a bulky solid that may be difficult to handle and filtrate, and may keep some copper mecanically arrested, so it is not recommendable.

I don't think that conversion of copper hydroxide to copper oxide would be any problem. No copper coud be evaporated, and only check that no undissolved solid remains after the sulfuric acid dissolution in the next step. Add at least something like 10% excess of sulfuric acid (Free acid).

One of the problems that may have occured to you is that you did not add enough zinc in the final cementing (reducing)step. Did you add enough zinc to have the solution colorless? Or maybe you did not have enough free acid, in such case it would be difficult to cement all copper.

These might be some sources of error, but I can't tell anything else without more imput.

Cheers
.