guy
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Oxidation of metals by acids
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]
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12AX7
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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
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The_Davster
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| Quote: | Originally posted by guy
Tin on the other hand can react readily with HCl even though its reduction potential is greater.
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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
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woelen
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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.
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