[chewietee]

I'm having trouble analyzing the results of a qualitative test I did that tests the reducing power of Sn2+ and Pb2+ solutions.  I had a test tube of each solution and added a few drops of acidified KMnO4 to each.  The Sn2+ solution was initially cloudy and colourless and took a while for a slight black precipitate to form.  The Pb2+ solution was initially clear, and then a purplish precipitate formed.  What exactly is going on in these reactions??

I assume that the cations are being oxidized to the +4 state, and that MnO4- is being reduced to Mn2+ (which is pink in colour?), but the E value of reduction of MnO4- is 1.51 V, while the oxidation of Sn2+ is -0.15 V, which means that the Sn2+ can reduce MnO4-, so shouldn't that reaction have yielded a pinkish coloured solution from Mn2+ instead of the observed black precipitate??

Also, the oxidation potential of Pb2+ is -1.69 V, so it can't reduce MnO4+, yet a pinkish precipitate was observed.  How is this possible??

Sorry for the long post!!!  Any help would be awesome!!!!!

[chewietee]

Ok...So I guess my question is, do Sn2+ or Pb2+ react with acidified KMnO4??

[Borek]

First of all - just comparing standard potentials is only a pointer, you should calculate formal potentials to be sure.

Are you sure black precipitate was not brown?

What is color of PbO₂?

[chewietee]

Ok..when you calculate the standard potentials.  Wouldn't the redox reaction where MnO4- is reduced and Pb2+ is oxidized yield a negative overall potential, therefore the reaction would'nt occur???  Whereas the reaction where Sn2+ is oxidixed yields a positive overall potential, so the reaction would occur. 
Also, PbO2 is a black solid.

[chewietee]

Is it possible I had the test tubes mixed up??