Question

Estimate the value of the equilibrium constant at 550 K for each of the following reactions.

a) 2CO(g)+O2(g)⇌2CO2(g)

b) 2H2S(g)⇌2H2(g)+S2(g)

Answer 1

a) 2CO(g) + O2(g) <-------> 2CO2(g)

∆G° = ∆G°f(Products) - ∆G°f(Reactants)

= (2×∆H°f(CO2) ) - ( 2×∆H°f(CO) + ∆H°f(O2))

= ( 2× ( -394.39kJ/mol) ) - ( 2 × (-137.6kJ/mol) + 0 )

= -788.78kJ/mol + 275.2kJ/mol

= -513.6kJ/mol

∆G° = - RTlnK

-513.6kJ/mol = - 8.314(J/K mol) × 550K× 2.303logK

logK = 513.6/10.53

= 48.77

K = 5.9 × 10^48

b) 2H2S(g) <-------> 2H2(g) + S2(g)

∆G°= (∆G°f(H2) + ∆G°(S2) ) - ( 2× H2S))

   = ( 0 + 0 ) - ( 2× (-21kJ/mol))

   = + 42kJ/mol

∆G° = - RTlnK

42kJ/mol = - ( 8.314 × 550K × 2.303 × logK)

   logK =-( 42/10.53)

= -3.99

   K = 1.02×10^-4