Sulfuric acid and gold

haiha

Hi everybody
I know that concentrated sulfuric acid doesn't react gold. But I am puzzled because the oxidation potential of sulfure in concentrated H2SO4 is even higher than that of gold. Could anyone help me please.
Thanks

symbolipoint

haiha,
Recheck the standard reduction potentials for acidic sulfuric acid and of metallic gold. Which one would more tend to want to be in reduced form?

1 sulfate + 4 H+ + 2e -----> sulfurous acid + 1 water, E = +1.5 v

1 auric(+3) + 3e -----> 1 gold in reduced metal form, E = +.17 v

Now, I forgot how the rest of this works, but you probably want to balance the two half reactions and find the resulting new "E" value; and you will find that it is negative, meaning the reaction is not spontaneous. You must use more energy in order to dissolve the gold than would be provided just from sulfuric acid.

haiha

Thanks symbolipoint for answering me. But when i check in the manual, the potential of S2O7 (which is in oleum) is about 1.9-2.0 v (i don't remember exactly), and that is higher than that of metalic gold.

symbolipoint

Your comments there aren't exactly so clear. Are you giving the "oleum" reduction potential or are you giving it as an oxidation potential? What is the expected other form starting from "oleum"? Be aware, oleum is a solution of sulfur trioxide in sulfuric acid; no water is present. Would you give the exact electrochemical potential involving oleum? That might help see the situation more clearly. (I could not find that in an online search and it's not in my old textbook).