[zephyrblows]

ex1:
NO3-(aq) + 2H+(aq) +e- ? NO2(g) + H2O(l) +0.80
NO3-(aq) + 4H+(aq) + 3e- ? NO(g) + 2H2O(l) +0.96
There are more H+ in the second one, but it happen in dilute HNO3 solution???

ex2:
MnO4-(aq) + e- ? MnO42-(aq) +0.56
Not an OH- at all. However, this happens in SB solutions?

ex3:
MnO4-(aq) + 2H2O(l) + 3e- ? MnO2(s) + 4 OH-(aq) +0.59
MnO4?(aq) + 4H+ + 3e? ? MnO2(s) + 2H2O +1.70
I feel dizzy now.

ex4:
Given MnO₄- + SO₃2- + H+     -->    ??
I found this : H2SO3(aq) + 4H+ + 4e? ? S(s) + 3H2O..+0.16V
But the correct product is SO₄2- ? ? ? ? ?

(actually there will be no tables or lists during the test, so I have to memerize one,
but I just don't know how to choose the most common half reactions)

THANKS A BILLION....

[Borek]

Rules of thumb:

nitric acid when contrated gets reduced to NO₂, when diluted to NO.

permanganate in basic solutions gets reduced to manganate, in neutral - to MnO₂, in acidic - to Mn₂₊.

SO₃^2- most likely gets oxidized to SO₄^2-.

Remember these are only approximations. They will work for most simple cases, don't start to believe they are whole truth about permanganate and nitric acid chemistry

[DevaDevil]

indeed good rules of thumb

especially inorganic nitrogen electrochemistry is a mess when looked into it in detail (some possible products in oxidation/reduction: nitrate, nitrite , nitric oxide, nitrous oxide, ammonia, molecular nitrogen and hydroxylamine) but for simple reactions those Borek mentioned are the most important.