Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: deppep on February 08, 2012, 10:21:25 AM
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Hi guys i'm studying about general chemistry in a physics class.
I've some exercices that i'm unable to solve, stuff like this:
Copper can be solubilizated in water, oxidating in Cu++ form, reacting with HCrO4- in an acid enviroment realized by H2SO4 Write down the balanced reaction.
my problem is to write (not to balance) an reduction/oxidation reaction.
The copper is oxidating, so cromium or hydrogen have to reduce. I've thought about 3 different reaction, but how to choose the correct one?
The unbalanced proposed reactions are:
Cu + HCrO4- -> Cu++ + Cr2O3 + H2O
Cu + HCrO4- -> Cu++ + CrO + H2O
Cu + HCrO4- -> CuCrO4 + H2
ciao!
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i'm sorry, misreplied :-\. I apologise for the mistake
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advise: look in standard reduction potential tables, that will give you a better idea of which reactions are possible.
in acid environment:
HCrO4- + 7H+ + 3e- <--> Cr3+ + 4 H2O; E0 = 1.2V
but also:
2 CrO42− + 2 H+ <--> Cr2O72− + H2O
which also makes the reduction of dichromate a possibility in the reaction with copper:
Cr2O72- + 14H+ + 6e- <--> 2Cr3+ + 7 H2O; E0 = 1.33V
(other half-cell will be Cu <--> Cu2+ + 2e- )
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Thanks for the reply :D!
So the total reaction is:
3Cu + Cr2O72- + 14H+ -->3Cu++ + 2Cr3+ + 7 H2O
And if i haven't data from std potential table?
Another question, i know that H2SO4 responsable of the acid environment, must it be added in the molecular form of total reaction?
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if you haven't got the data from the tables it gets hard. You will need to develop a sense of which ions / compounds are possible (for example, knowing that chrome prefers the 3+ state).
Your equation is good, but if your teacher wants to have the original chromic acid as reactant, use that half-cell reaction in stead of the dichromate one.
I do not know who taught you to use Cu++, but the standard way of representing the double charge is: Cu2+
sulfuric acid is usually added in concentrated form to a solution, in order to lower the pH (concentrated sulfuric acid is about 96% H2SO4 in water)