Why following two reaction proceed differently?
`Pb_(3)O_(4)+8HClrarr3PbCl_(2)+Cl_(2)+4H_(2)O`
and
`Pb_(3)O_(4)+4HNO_(3)rarr2Pb(NO_(3))_(2)+PbO_(2)+2H_(2)O`

Explanation

`(i) Pb_(3)O_(4)` is a stoichiometric mixtrue of two oxides i.e 2 mole of PbO and 1 mole of `PbO_(2)` In phO, the oxidation state of Pb is + 2 while it is +4 in `PbO_(2)`.Both these oxides can react with HCI .Therefore, the reaction may be split in to two reactions.
`2overset(+2)Pb overset(-2)O+overset(+1)4Hoverset(-1)CIrarr2overset(+2)Pboverset(CI_(2))+2overset(+1)H_(2)O^(-2)`
`overset(+4)Pboverset(-2)O_(2)rarroverset(+2)Pboverset(CI_(2))rarroverset(+2)Pboverset(-1)CI_(2)+2overset(+1)H_(2)overset(-2)O`
The first reaction is not a redox reaction since none of the reactianty species undergoes schange in oxidation state as a result of the reaction .It is simply an acid base reaction.Thge second reaction is an expam[,le of redox reaction as there is na increse as well as decerase in oxidatiion ststes of the species involved in the reaciton
`(ii) HNO_(3)` is an oxidising agent (O.S of N= +5i.e maximum) Similarly `PbO_(2)` is also an oxidising agent (O.S of Pb =+4 i.emaximuim).Therefore , no chemical reaction is possible between `HNO_(3)` and `PbO_(2)` .Therefoe `PbO_(2)` becomes passive in nitric acid.However, chemical reaction can take place between PbO and `HNO_(3)` .This is a simple chemical reaction between a basic oxide and an acid and not a redox reaction
`2overset(+2)Pboverset(-2)O+4overset(+1)Hoverset(+5)Noverset(-2)O_(3)rarr2Pboverset(+5-2)NO_(3)^(2)+4overset(+1)H_(2)overset(-2)O`