Sulfur can be separated from lead in the mineral galena, PbS(s), by “roasting” t

Question

Sulfur can be separated from lead in the mineral galena, PbS(s), by “roasting” the ore in the presence of oxygen as shown in the following reaction:

2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g) The following thermodynamic data pertain to this reaction at 25 °C.

H°f (kJ/mol) S°(J/K·mol) PbS(s) –94.3 91.2 O2(g) 0 205.0 PbO(s) –217.86 69.45 SO2(g) –296.4 248.5

a) Determine H°rxn and S°rxn for this reaction.

b) Using your answers to part (a), determine G° for this reaction at 25 °C.

c) The "roasting" process requires heat. Determine G° for this reaction if we raise T to 850°C.

Explanation / Answer

a)

HRxn = Hproducts - Hreactnants = 2*-217.86 + 2*-296.4 - (2*-94.3 + 3*0) = -839.92 kJ/mol

SRxn = Sproducts - Sreactnants = 2*69.45 + 2*248.5 - (2*91.2 + 3*205) = -161.5 J/K

b)

deterimne dG

dG = dH - T*dS = -839.92 -298 *(-161.5 /1000) = -791.793 kJ

c)

if T = 850°C = 850+273 = 1123 K

dG = dH - T*dS = -839.92 -1123 *(-161.5 /1000) = -658.5555 kJ

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