Question

Consider the followi ng chemical reaction: Pb 3 O 4 (s)=PbO 2 (s)+2PbO(s) The Smithsonian Institution has re ceived a sample of crystallin e minium, Pb 3 O 4 (s), and plans to display it in a seale d glass chamber maintained at 25 °C and 1.0 atm. (in contact with an inert atmosphere of argon gas.) Is minium thermodynamic ally stable under these conditions? If not, what do you think the most stable state of the system should be? Given: G f ° of Pb 3 O 4 (s), PbO 2 (s), and PbO(s) are -601.36, -215.31, and -189.20 kJ/mol, respectively.

Answer 1

Determine the standard free energy of the reaction, ΔG⁰_rxn.

ΔG⁰_rxn = [(1 mole PbO₂)*G_f⁰ (PbO₂, s) + (2 mole PbO)* G_f⁰ (PbO, s)] – [(1 mole Pb₃O₄)* G_f⁰ (Pb₃O₄, s)]

= [(1 mole)*(-215.31 kJ/mol) + (2 mole)*(-189.20 kJ/mol)] – [(1mole)*(-601.36 kJ/mol)]

= [(-215.31 kJ) +(-378.40 kJ)] – (-601.36 kJ)

= (-593.71 kJ) – (-601.36 kJ) = 7.65 kJ.

The free energy of the reaction has a positive value, i.e, ΔG⁰_rxn > 0. Consequently, the reaction is non-spontaneous. This means that the conversion of minium to PbO₂ and PbO is not feasible at room temperature and hence, minium is thermodynamically stable at room temperature (ans).