Q. 19

PbO and PbO2 react with HCl according to following chemical equations:

2PbO + 4HCl 2PbCl2 + 2H2O

PbO2 + 4HCl PbCl2 + Cl2 + 2H2O

Why do these compounds differ in their reactivity?

Answer :

say first reaction is (i) ad second as (ii)

Primary reason (1)


The electronic configuration of central atom plays an important role in showing multiple oxidation states. Pb has an EC as: [Xe] 4f14 5d10 6s2 6p2. The outermost orbital can give 4 electrons, so can exhibit multiple oxidation states. In PbO the Oxidation number is +2 and that in PbO2 is +4.


There is a difference in energy required and steps in both reactions (i) and (ii). For reaction



The ionization of PbO will result as


This gives easy substitution by Cl atoms and the reaction proceeds at faster rate. Although for PbO2,



Dissociates as,



This ion is unstable and needs to first get reduced to Pb2+, also thus resulting in oxidation of Cl- ion produced to Cl2 due availability of enough energy. Thus the difference in reactivity is justified.


Reason (2)


In order to substitute oxygen in both the compounds, the bond between Pb and O needs to be broken. The driving force needed to break these bonds is the condition that it can attain a lower energy state and a more stable compound by electron transfer or by substitution. The one bond can be broken easily in PbO as compared to PbO2 which has two bonds to replace with Cl. Thus the substitution reaction in (ii) moves slowly as compared to reaction (i).


Also when in an aqueous solution, the more number of moles of PbO further allows more of substitution reaction to occur, thus giving more amount of substituted product. Thus even with same reactant a given compound can give different amounts of product based on the extensive nature of reactant.


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