Why do the following reactions proceed differently? Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O and Pb₃O₄ + 4HNO₃ → 2Pb(NO₃)₂ + PbO₂ + 2H₂O
Pb₃O₄ is a stoichiometric mixture of 2 mol of PbO and 1 mol of PbO₂. In PbO₂, lead is present in the +4 oxidation state, whereas the stable oxidation state of lead in PbO is +2. Because PbO₂ has lead in the +4 state, it acts as an oxidant (oxidising agent) and can oxidise the ion of HCl into chlorine. Meanwhile, PbO is a basic oxide. Therefore, the reaction with HCl splits into an acid-base reaction and a redox reaction: (acid-base) (redox)
On the other hand, HNO₃ itself is an oxidising agent. Therefore, it is unlikely that a redox reaction will occur between PbO₂ and HNO₃. PbO₂ shows a passive nature against HNO₃.
However, the acid-base reaction does occur between the basic oxide PbO and HNO₃:
It is this passive nature of PbO₂ against HNO₃ that makes the reaction different from the one that follows with HCl.
Explanation
The answer explains that Pb3O4 is a mixture of PbO and PbO2. PbO2 acts as an oxidising agent and reacts with HCl (a reducing acid) in a redox reaction, while PbO undergoes a simple acid-base reaction. With HNO3, which is already an oxidising agent, PbO2 remains passive and only PbO reacts via an acid-base pathway.
Solution Steps
Step 1: Identify the composition of Pb₃O₄ as 2 mol of PbO and 1 mol of PbO₂.
Step 2: Note that PbO₂ (Pb in +4 state) acts as an oxidant, while PbO (Pb in +2 state) is a basic oxide.
Step 3: For the HCl reaction, explain that PbO₂ oxidises Cl⁻ to Cl₂ (redox reaction) and PbO reacts via an acid-base reaction.
Step 4: For the HNO₃ reaction, explain that since HNO₃ is an oxidising agent, PbO₂ is passive towards it and only PbO undergoes an acid-base reaction.