[deppep]

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!

[deppep]

i'm sorry, misreplied  . I apologise for the mistake

[DevaDevil]

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:

Cr₂O₇^2- + 14H⁺ + 6e^- <--> 2Cr³⁺ + 7 H₂O;  E⁰ = 1.33V

(other half-cell will be Cu <--> Cu²⁺ + 2e^- )

[deppep]

Thanks for the reply  !


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?

[DevaDevil]

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)