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Offline deppep

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write a redox reaction
« on: February 08, 2012, 10:21:25 AM »
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!

Offline deppep

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Re: write a redox reaction
« Reply #1 on: February 08, 2012, 10:23:51 AM »
i'm sorry, misreplied  :-\. I apologise for the mistake

Offline DevaDevil

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Re: write a redox reaction
« Reply #2 on: February 08, 2012, 11:55:24 AM »
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- )

Offline deppep

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Re: write a redox reaction
« Reply #3 on: February 08, 2012, 12:33:36 PM »
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?



Offline DevaDevil

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Re: write a redox reaction
« Reply #4 on: February 08, 2012, 04:02:51 PM »
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)

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