Ammonia synthesis entropy change

Because the free-energy change for any process at constant temperature and pressure is AG = AH — TAS, we can calculate the standard free-energy change AG° for a reaction from the standard enthalpy change AH0 and the standard entropy change AS°. Consider again the Haber synthesis of ammonia ... 

For the ammonia synthesis reaction in Sample Exercise 19.5, is the enthalpy change for the reaction under standard conditions, AH°, so the changes in entropy will be standard entropy changes, AS°. Therefore, using the procedures described in Section 5.7, we have... 

Analyze and Plan We are asked to calculate the entropy change for the synthesis of ammonia from its constituent elements. Standard molar entropy values for the reactants and the product are given in Table 19.2. 

In the synthesis of ammonia, the combustion of propane, and many other spontaneous reactions, the entropy of the system decreases (AS < 0). Remember that the second law dictates that, for a spontaneous process, a decrease in the entropy of the system is outweighed by an increase in the entropy of the surroundings. In this section, we examine the influence of the surroundings—in particular, the addition (or removal) of heat and the temperature at which this heat flow occurs—on the total entropy change. 

With Equations 18.8 and 18.9, we ean ealculate the entropy change in both the system and surroundings for a chemical reaction, and we can determine whether the reaction is spontaneous. Consider the synthesis of ammonia at 25°C ... 

In an exothermic reaction, such as the synthesis of ammonia or a combustion reaction, the heat released by the reaction increases the disorder of the surroundings. In some cases, the entropy of the system may decrease, as when a gaseous reactant is converted into a solid or liquid. However, provided that AH is large and negative, the release of energy as heat into the surroundings increases their entropy so much that it dominates the overall change in entropy and the reaction is spontaneous (Fig. 7.18). 

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