Question

a) What is ΔG_rxn^o (in kJ) at 2715 K for the following reaction?

2POCl₃(g) → 2PCl₃(g) + O₂(g)

POCl₃(g): ΔH_f^o = -592.7 kJ/mol and Sº = 324.6 J/K mol)
PCl₃(g): ΔH_f^o = -287.0 kJ/mol and Sº = 311.7 J/K mol)
O₂(g): ΔH_f^o = ? kJ/mol and Sº = 205.0 J/K mol)

b) At what temperature (IN KELVIN) does the above reaction become spontaneous?

c) What is ΔG_rxn^o (in kJ) at 488 K for the following reaction?

PbO(g) + CO₂(g) → PbCO₃(s)

PbO: ΔH_f^o = -219.0 kJ/mol and S^o = 66.5 J/K mol)
PbCO₃(s): ΔH_f^o = -699.1 kJ/mol and S^o = 131.0 J/K mol)
CO₂: ΔH_f^o = -393.5 kJ/mol and S^o = 213.6 J/K mol)

d) At what temperautre (in K) does the above reaction become spontaneous?

Answer 1

a)

dGrxn = Hrxn - T*Srxn

Hrxn = 2*-287.0 + 0 - -592.7 = 18.7 kJ/mol

Srxn = 2*311.7 + 205.0 - (2*324.6 ) =179.2

G = 18700 - 298*179.2 = - 34701.6 J/mol = -34.7 kJ/mol

b)

G = H - TS

H - TS < 0

H < TS

H/S < T

18700/179.2 < T

T > 104.352 K

c)

Grxn = H - TS = 18700 - 488*179.2 = -68749.6 J/mol = 68.749 kJ/mol

d)

G < 0

H - TS <0

H/S < T

H = (-699.1 ) - (-393.5 + -219.0 ) = -149.1

S = 131.0 - (213.6 +66.5 ) = -86.6

H/S < T

149100 / 86.6 < T

T > 1721.7K