Exercise 3 | Chemical Thermodynamics

What is the standard entropy change of the following reaction at 298K and standard pressure?

Comment the sign of the numerical answer.


N2 (g) + 3 H2 (g) → 2 NH3 (g)


Data: S0 [N2] = 191.5 J.mol-1.K-1

S0 [H2] = 130.6 J.mol-1.K-1

S0 [NH3] = 192.3 J.mol-1.K-1

ΔS0rxn = 2 S0 [NH3] – S0 [N2] – 3 S0 [H2]

ΔS0rxn = 2 x 192.3 – 191.5 – 3 x 130.6

ΔS0rxn = - 198.7 J.mol-1.K-1


ΔS0rxn < 0, which makes sense because 4 moles of gas are consumed to form only 2 moles of gas ⇒ the disorder decreases