Question

The formation enthalpy for lead(IV) oxide from its elements can be determined by reaction of lead and lead(IV) oxide with nitric acid. Apply Hess's Law of Heat Summation to determine of the enthalpy of formation for lead(IV) oxide, given the following information and using the appropriate information from the discussion section (show your work as to how you determine the formation enthalpy presenting the appropriate equations):

Pb(s) + 2 HNO3(aq) ⟶ Pb(NO3)2(aq) + H2(g) ΔHrxn = -1.4 kJ

2 PbO2(s) + 4 HNO3(aq) ⟶ 2 Pb(NO3)2(aq) + 2 H2O(l) + O2(g) ΔHrxn = -21.3kJ

How does the value determine above compared to the accepted value? (show your work and properly cite the source of the accepted value).

Answer 1

Pb(s) + 2 HNO3(aq) ⟶ Pb(NO3)2(aq) + H2(g)   ; ΔH1 = -1.4 kJ   --------(1)

2 PbO2(s) + 4 HNO3(aq) ⟶ 2 Pb(NO3)2(aq) + 2 H2O(l) + O2(g) ; ΔH2 = -21.3 kJ    ----(2)

Also H2(g) + 0.5 O2(g) ------> H2O(l) ;ΔH3 = -285.83 kJ ---(3)

Pb(s) + O2(g) -----> PbO2(s) ; ΔH = ? ---(4)

(4) = (1) + [0.5 x reverse of (2)] + (3)

So ΔH = ΔH1 + (0.5x (- ΔH2)) + ΔH3

           = -1.4 + (0.5x(-(-21.3)))+ (-285.83) kJ

           = -276.58 kJ