Practice Problem 7

Use the values of  H° and S calculated in Practice Problem 5 to predict whether the following reaction is spontaneous at 25°C:

N₂(g) + 3 H₂(g) 2 NH₃(g)

Solution

According to Practie Problem 5, this reaction is favored by enthalpy but not by entropy:

H° = -92.22 kJ    (favorable)

S° = -198.75 J/K    (unfavorable)

Before we can compare these terms to see which is larger, we have to incorporate inot our calculation the tmeperature at which the reaction is run:

T_K = 25^o C + 273.15 = 298.15 K

We then multiply the entropy of reaction by the absolute temperature and subtract the T S^o term from the H^o term:

G^o = H^o - T S^o

            = -92,220 J - (298.15 K x -198.75 J/K)

        = -92,220 J + 59,260 J

        = -32,960 J

G^o = -32.96 kJ

According to this calculation, the reaction should be spontaneous at 25°C.