Practice Problem 7

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

N2(g) + 3 H2(g) equilibr.gif (63 bytes) 2 NH3(g)


Solution

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

deltaH = -92.22 kJ    (favorable)

deltaS = -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:

TK = 25o C + 273.15 = 298.15 K

We then multiply the entropy of reaction by the absolute temperature and subtract the T deltaSo term from the deltaHo term:

deltaGo = deltaHo - T deltaSo

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

        = -92,220 J + 59,260 J

        = -32,960 J

deltaGo = -32.96 kJ

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

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