[happydude]

Hi,

I'm having a problem understanding this one particular question.  

Complete and Balance:
Cr + H₂SO₄

2Cr + 3H₂SO₄ -----> Cr₂(SO₄)₃ + 3H₂

What I don't understand is how they figured out the Cr had a valance number of 3 instead of two from the original equation.

Also, if someone could outline the basic rules for predicting chemical reactions, it would be very much appreciated.  
Thanks

Earl

[Alberto_Kravina]

Also, if someone could outline the basic rules for predicting chemical reactions, it would be very much appreciated.

Well, for redox reactions you can easily predict if the reaction happens. Just calculate Delta E⁰.

DeltaE⁰= E⁰(Reduction) - E⁰ oxidation

If DeltaE⁰ is postive the reaction takes place, if it is negative the reaction doesn't take place

[Mitch]

There is no way to tell at the highschool level what the product of Cr and H2SO4 would of been. I would of quessed Cr would of picked a +4 state. But, I guess since, Sulfur isn't a hard lewis base it would of formed a +3. But, there is not fast and hard rule, that I know of, that could of told you that.

[happydude]

Thanks Mitch.  I feel a bit better now...I thought I was missing something.

Alberto, I haven't started doing redox reactions yet but I'm sure that will come in handy when the time comes.