Question #ea17c

1 Answer
Nov 8, 2015

That actually depends on how concentrated the acid is.


The most important thing to take note of here is the concentration of the acid because that will actually influence what reaction, if any, takes place.

Now, depending on the concentration of the acid (and its temperature), in the reaction between iron and nitric acid you will usually see iron being oxidized to either #"Fe"^(2+)# or #"Fe"^(3+)#, and the nitric acid being reduced to various nitrogen oxides.

For example, iron will react with dilute nitric acid to form iron(II) nitrate and nitrous oxide, #"N"_2"O"#

#4"Fe"_ ((s)) + 10"HNO"_ (3(aq)) -> 4"Fe"("NO"_ 3)_ (2(aq)) + "N"_ 2"O"_ ((g)) + 5"H"_ 2"O"_ ((l))#

Now here comes the tricky part. Iron will react with concentrated nitric acid, but not with highly concentrated nitric acid.

That happens because very concentrated nitric acid will actually form a protective metal oxide layer on the surface of the metal, which essentially stops the reaction by preventing the further oxidation of the metal.

This process is known as passivation #->#

So if your nitric acid is highly concentrated, i.e. at concentrations that exceed #85%#, in which case the solution is actually fuming, then you can say that no reaction takes place between iron and nitric acid

#"Fe"_ ((s)) + "HNO"_ (3(aq)) -> color(red)("no reaction")#

Now, relatively concentrated nitric acid can react with iron and be reduced to either #"NO"_2# or #"NO"#, depending on the temperature of the acid.

For example, cold, relatively concentrated nitric acid will react with iron to form iron(III) nitrate and nitrogen dioxide

#"Fe"_ ((s)) + 6"HNO"_ (3(aq)) -> "Fe"("NO"_ 3)_ (3(aq)) + 3"NO"_ (2(g)) + 3"H"_ 2"O"_ ((l))#

If the sample is hot, it will produce #"NO"# instead (and I think iron(II) nitrate again).

So, to sum this up, I think it's safe to say that the term "concentrated nitric acid" doesn't actually provide enough information as to what reaction, if any, you can expect to see.

However, I've seen this term, "concentrated", used in the context of this reaction, iron with nitric acid, to refer to the case when no reaction takes place, so without additional information, you can say that, too.