Sciencemadness Discussion Board

Oxidation of metals by acids

guy - 24-11-2007 at 19:02

What is the 'mechanism' for the oxidation of metals by protons? I ask because I am trying to figure out some anomalies in the electrochemcial series. For example, why are cobalt and nickel so slow to react with acids? I don't think for these metals the answer is due to an oxide passivation because they both react readily in HNO3.

Tin on the other hand can react readily with HCl even though its reduction potential is greater.

[Edited on 11/24/2007 by guy]

12AX7 - 24-11-2007 at 20:36

The amount of energy released, and therefore to some extent the speed at which the reaction progresses, depends on the difference in energy, which is the amount of energy released in the reaction. Since hydrogen is defined as 0V, the metals with a negative reduction potential (like tin and nickel, but not like copper and silver) will react at a rate roughly proportional to their voltage.

Nitric acid is a stronger oxidizer than plain H+, so less reactive metals (like silver) can be dissolved in it. The nitrate part changes instead of the H part. It's obvious something different is going on, as NO2 gas is given off rather than H2.

Tim

The_Davster - 24-11-2007 at 21:18

Quote:
Originally posted by guy


Tin on the other hand can react readily with HCl even though its reduction potential is greater.

?
2H+ +2e- --->H2 0V
Sn2+ +2e----->Sn -0.14V
Tin and HCl do react, but it is nowhere approaching fast.


Ni2+ +2e- ----->Ni -0.26v
Co2+ +2e------>Co -0.28v
vs
Fe2+ +2e- --->Fe -0.45V
Zn2+ +2e- ---->Zn -0.76V

Look at the differences in potential! Nonreactive/slowly reacting metals all have only slightly negative reduction potentials when compared to SHE, wheras the more reactive metals all have greater relative potentials.

Nitric is completly different as Tim already said, look at its potential:
2NO3- + 4H+ + 2e- --->2NO2 +2H20 +0.80V

woelen - 25-11-2007 at 09:54

The_Davster is right, tin does not react quickly with HCl. Actually, its reaction is VERY slow. A granule with a diameter of 3 mm will take several days to dissolve in 25% HCl. I did this experiment some time ago with reagent grade HCl and very pure tin. Impurities in the tin might allow a faster reaction though.