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Question:

The following reaction is performed at 298K: 2NO(g) + O2(g) <=> 2NO2(g). The standard free energy of formation of NO(g) is 86.6 kJ/mol at 298K. What is the standard free energy of formation of NO2(g) at 298 K?

86600 + R(298)ln(1.6×1012)J/mol

0.5[2×86,600−R(298)ln(1.6×1012)]J/mol

R(298) ln(1.6×1012)J/mol

8660−(1.6×1012)R(298)J/mol

Solution:

The relationship between the standard Gibbs free energy change for reaction and the equilibrium constant is as shown below.
ΔG0rxn = −RTlnKp. (1)
The standard Gibbs free energy change for the reaction is given by the expression
ΔG0rxn = 2ΔG0f(NO2) − ΔG0f(NO). (2).
From equations (1) and (2).
2ΔG0f(NO2) − ΔG0f(NO) = −RTlnKp
ΔG0f(NO2) = 0.5 × [2ΔG0f(NO) − RTlnKp]
Substitute values in the above expression.
ΔG0f(NO2) = 0.5 × [2 × 86600 − R(298)ln1.6 × 10^12] J/mol