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.