[rleung]

Hi,

I am confused about when to use the half-reaction method or oxidation number method for balancing equations.  I always thought they were interchangeable, but they do not appear to always be.  For example, here is an equation I cannot seem to balance via both methods:

Cu(s) + HNO3(aq) ---> Cu(NO3)2 (aq) 2NO2(g) + H2O(l)

I cannot seem to balance this via the half-reaction method.  I split up the equations into the following:

Cu(s) ---> Cu(2+) + 2e-

2e- + 6HNO3 ---> 4NO3(-) + 2NO2 + 2H2O + 2H(+)
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Cu(s) + 6HNO3(-) ---> Cu(2+) + 4NO3(-) + 2NO2 + 2H2O + 2H(+)

However, if I balance it via the oxidation number method, I get the following:

Cu(s) + 4HNO3 (aq) ---> Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)

Am I doing something wrong?  Thanks.

Ryan

[mike]

They are the same you just haven't cancelled everything out in the first one.

2 H⁺ and 4 NO₃^- is the same as 2 HNO₃ and 2 H⁺

you can then get rid of 2 HNO₃ from both sides of the equation.

[Borek]

Adding to mike's comment: also note that one equation is "normal" while other is ionic (not yet "net" ionic ) - that makes it more difficult for you to spot that they are the same.