Changes of Entropy Associated with Transformations

In a chemical reaction, the substances involved produce new ones with changed entropy demands. Here, we are interested in the amount of entropy AS which is added or removed for compensation when a reaction takes place at constant pressure and constant temperature. Let us consider the reaction of 0.1 mol of iron and 0.1 mol of sulfur forming 0.1 mol of iron sulfide, at room conditions  

We see that exactly A5 = 0.1 Ct is lacking. This is what is needed to cover the entropy demands of the FeS produced by a conversion of A =0.1 mol. This amount of entropy AS must be introduced from outside if the iron sulfide is to be as warm at the end of the reaction as the iron and sulfur were before the process began. Without this added entropy, it would be colder. If the conversimi is multiplied, the required entropy multiplies correspondingly. 

This conversion-related quantity is called the molar reaction entropy ArS. In our example, the caveat for small A is unnecessary because only pure substances are participating in the reaction. However, if dissolved substances appear in the conversion formula, we can then only allow small additional conversions A for any arbitrary extent of the reaction. This is to ensure that the composition of the solution and, therefore, the entropy demands of the substances in it do not change noticeably. 

For an arbitrary reaction between pure and dissolved substances, 

The molar reaction entropy is the change of entropy—Abased upon the conversion— at constant p and T. It equals the sum of the molar entropies of the reaction partners 

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