Find Kc using deltaH and deltaG?

Question

Calculate the equilibrium constant at 800°C for the reaction of steam with hot coke to form hydrogen and carbon monoxide. The data refer to 800°C.

C(s) + H2O(g) CO(g) + H2(g)

Substance C(s) H2O(g) CO(g) H2(g)

H°f (kJ/mol) 0.0 -242 -111 0.0

G°f (kJ/mol) 0.0 -229 -137 0.0

S°(J/K·mol) 6.0 189 198 131

Answer 1

C(s) + H2O(g) --> CO(g) + H2(g)

dGo = prod - reactants

dGo = (-137 & zero) - ( zero & -229)

dGo = -137 + 229 = 92 kJ

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dH = prod - reactants

dH = (-111 & zero) - (zero & -242)

dH = -111 + 242 = 131 KJ

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dS = prod - reactants

dS = (198 & 131) - (6 & 189)

dS = 134 Joules

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normally we take the assumption that dH & dS are the same @ 800oC as they were @ 25C  @ standard conditions, but that dG changes:

dG = dH - TdS

dG = 131kJ  - (1073K)(0.134kJ)

dG = 131kJ - 143.8

dG = - 12.8 kJ

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now to your question, Find Kc:

dG = dGo & RTlnKc

-12,800 joules = 92,000 Joules - (8.314 Joules etc)(1073K)lnK

-12,800 joules - 92,000 Joules = - (8921)lnK

104,800 / 8921 = lnKc

ln Kc = 11.7477

Kc = 126458

your answer : K = 1.26 e5

(when we used the 92,000 Joules, it became a 2 sigfig problem, you might consider rounding off)