Question

Under standard conditions, the free energy of formation (ΔGof) of CO2 ­(g) is -394 kJ/mol.

C (s) + O2 (g) → CO2 (g)

Assuming the temperature remains 25 oC and the partial pressure of O2 (g) remains 1 atm, what would be the value of ΔG for this reaction when the partial pressure of CO2 ­(g) is 7.88 x 10-3 atm?

Answer 1

C (s) + O2 (g) → CO2 (g)

Kp = (P_CO2) / (P_O2)
= (7.88 x 10^-3 atm) / (1 atm)
  = 7.88 x 10^-3

Now,

G = G⁰ + RT lnKp

Here,
G⁰ = - 394 kJ mol^-1 = - 394000 J mol^-1

R = 8.314 J K^-1 mol^-1

T = 25 ^oC = (273 + 25) K = 298 K

Kp = 7.88 x 10^-3

Substituting the above values, we get

G = G⁰ + RT lnKp

G = - 394000 J mol^-1 + (8.314 J K^-1 mol^-1) (298 K) ln (7.88 x 10^-3 )

G = - 394000 J mol^-1 + 2478 J mol^-1 (- 4.84)

G = - 394000 J mol^-1 - 11994 J mol^-1

G = - 405994 J mol^-1

G = - 406 kJ mol^-1