Question

Which reaction occurs most rapidly at standard conditions?

H2(g) + I2(s) 2HI(g)
Cu(s) + S(s) CuS(s)
C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(g)
5C2O42-(aq) + 2MnO4-(aq) + 16H+(aq) 10CO2(g) + 2Mn2+(aq) + 2H2O(l)
Ag+(aq) + I-(aq) AgI2(s)

Answer 1

I don't know of any way to determine the relative rates of reactions other than pretty much just guessing. Even with guessing, I can't decide between the last two. Both occur in solution and (if I recall correctly) both occur quite rapidly. I would guess that the last one, just a simple precipitation, would probably be the fastest.

(NOTE: You have the wrong equation on the last one. It should be Ag+(aq) + I-(aq) --> AgI(s) There is no subscript on the iodide ion in AgI.)