Q.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?

The key idea is the stability of oxidation states of lead. Lead exhibits +2 and +4 oxidation states, but the +2 state is far more stable due to the inert pair effect.

Reasoning:

1. In PbO, lead is already in its stable +2 state. It simply acts as a basic oxide, undergoing a neutralisation reaction with HCl to form a salt and water — no redox occurs.

2. In PbO2, lead is in the +4 state, which is strongly oxidising. It tends to revert to the more stable +2 state by accepting electrons. When it reacts with HCl, it oxidises chloride ions ($\text{Cl}^-$) to chlorine gas ($\text{Cl}_2$), while itself being reduced to $\text{Pb}^{2+}$. …

Why the difference? The concept of oxidation state stability

Lead is a heavy element in Group 14. Down the group, the inert pair effect becomes pronounced — the two s-electrons in the outermost shell become increasingly reluctant to participate in bonding. For lead, the +2 oxidation state is far more stable than the +4 state. PbO₂ is therefore a strong oxidising agent: it wants to gain two electrons and drop to the more stable Pb²⁺.

This is the key. PbO already has lead in its stable +2 state, so it behaves as a normal basic oxide. PbO₂, with lead in the unstable +4 state, will oxidise anything it can — including chloride ions from HCl.

Let's walk through each reaction.

1. PbO + HCl — a simple acid-base reaction

PbO is a basic oxide. It contains Pb²⁺, which is already in its most stable oxidation state. When treated with HCl, the oxide ion (O²⁻) simply accepts protons:

The balanced equation given is just twice this:

No change in oxidation state occurs. Pb remains +2, Cl remains –1, H remains +1, O remains –2. This is a straightforward neutralisation — no redox.

2. PbO₂ + HCl — a redox reaction driven by the instability of Pb⁴⁺

Now look at PbO₂. Lead is in the +4 state here — much less stable than +2. The Pb⁴⁺ ion is a strong oxidising agent. It wants to gain two electrons and become Pb²⁺.

Where can it get those electrons? From the chloride ions (Cl⁻) in HCl. Chloride can be oxidised to chlorine gas (Cl₂).

Let's track the changes:

• Reduction half: Pb⁴⁺ + 2e⁻ → Pb²⁺
• Oxidation half: 2Cl⁻ → Cl₂ + 2e⁻

Combine them:

Notice that only two of the four chloride ions get oxidised to Cl₂. The other two remain as Cl⁻ in PbCl₂. The water comes from the oxide ions combining with H⁺.

3. Why doesn't PbO also produce Cl₂?

Because PbO already has lead in its stable +2 state. There is no driving force for it to accept electrons. It has no tendency to oxidise Cl⁻. So the reaction stops at simple salt formation. …