9.3-(d)-2015, Lead, Group IV

Lead is a neurotoxin. Even studying about it can damage your brain :p !!!

oxides of lead, Pb3O4, PbO and PbO2 respectively

pic= three forms of lead. From left to right =
Pb3O4 (red lead), PbO (yellow), PbO2 (brown-ish)

There are some notes from a power point slide that says something like

“Pb3O2 does not react with nitric acid because PbO2 would be left behind.”

Having been asked about it by one of my super studious students, I didn’t find anything satisfactory about any possible explanation I could think of and indeed kind of found it to be in some contradiction with the “mental model” of how we are told the ‘think’ about Pb3O4 (i.e. treat it as if it’s a mix of 2PbO and 1PbO2). So I did a little digging… This is what I found out which help from a reference a book, I’ll give an extract here and punctuate it (unmarked) with my own comments.

Advanced Chemistry by Clugston and Fleming. Oxford Press
Chapter 19.6 page 340

there are three oxides of lead: Pb(II) oxide, PbO, a yellow solid used to make glass which gives ‘lead glass’ it’s sparkle. lead(IV)oxide, PbO2, which is a strongly oxidizing substance used in car batteries and is brown in colour. and “red lead” with formula Pb3O2 – often thought of (for ease of comprehension only) as containing (in effect) 2PbO and 1PbO2. It is said to consist of an array of ions, 2 Pb2+ ions and one Pb4+ ion per 4 O(2-) ions.

When “red lead” reacts with nitric acid [book does not say dilute of conc.], it turns into a brown suspension because PbO is more basic than PbO2, hence the PbO reacts, leaving the brown PbO2 solid behind.

So, there’s a contradiction here. I do believe that Pb2O3 will react with lead in the manner described in Clugston and Fleming, and that the slides contain an error.

Please note: the reaction with HNO3 is a pure acid base reaction only. HNO3 cannot undergo a redox reaction with Pb4+ as the N is already in the +V state so has no electrons to give to the Pb4_ which is a strong oxidising agent. If however Pb3O4 were reacted with HCl, then you would get a REDOX reaction in addition to the initial acid-base reaction, as the ‘remaining’ PbO2 would react with the oxidisable Cl- of HCl to produce Cl2(g) and PbCl2 (and 2 H2O). A similar phenomenon you may have come across is that you don’t use HCl(aq) to acidify KMnO4 solutions as KMnO4 will similarly oxidize the Cl- into Cl2, hence H2SO4 acid is used instead, KMnO4 isn’t able to oxidize the S in sulphate as S is already in the +VI state – it’s already ‘lost’ all its valence electrons, and has no more to give.

So Pb3O4(s) + 2 HNO3 –> 2Pb(NO3)2 + PbO2(s)

.is the reaction you can write, but I don’t know whether the acid can be dilute to has to be strong.

Update: Perhaps the BEST A-level/1st year degree level Inorganic book I’ve ever come across (Introduction to Inorganic Chemistry by G.I. Brown, ISBN: 0-582-35459-5, second edition, 1985)  says Pb2O4 deacts with dilute HNO3 (aq), so there you go 🙂



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