Predictions entropy changes, qualitative

Stated as an abstraction and generalization of engineering observations on the efficiency of heat engines. We start the discussion by presenting a nonmathematical qualitative summary of the arguments on efficiency. Then we define entropy and state the second law. Section 13.5 applies the definition to calculate entropy changes and to predict spontaneity of processes. 

We can usually make qualitative predictions about entropy changes by focusing on these factors. For example, when water vaporizes, the molecules spread out into a larger volume. Because they occupy a larger volume, there is an increase in their freedom of motion, giving rise to a greater number of possible microstates, and hence an increase in entropy. 

As we said earlier, entropy often is described as a measure of randomness or disorder. Although this can be a useful description, it should be used with caution and not taken too literally. It is generally preferable to view the change in entropy of a system in terms of the change in the number of microstates of the system. Nevertheless, we can use the concept of disorder to make some qualitative predictions about the entropy changes that accompany certain processes. 

Sample Problem 18.1 lets you practice the qualitative prediction of entropy change for a process. 

Entropy change for a process can be calculated using standard entropy values or can be predicted qualitatively based on factors such as temperature, phase, and number of molecules. 

Making Qualitative Predictions of Entropy Changes in Physical and Chemical Processes... 

Slightly greater variations in the activation entropy are noted in ester than the halide thermolyses, but the A values again approximate to 10 and indicate the cyclic unimolecular nature of the transition states. With changing ester, the rate of pyrolysis and strength of the liberated acid qualitatively increase in the same direction" . The dichloroacetate, chloroacetate and acetate of t-butyl alcohol exemplify this relationship and at 250°C their rates of pyrolysis follow the order 18.6 4.4 1 (ref. 406). Nucleophilicity of the carbonyl function cannot be the dominating influence as a reverse order of reactivity would be predicted . 

It is useful to be able to predict the sign of AS°. You gain some understanding of the reaction, and you can use the prediction for qualitative work. For quantitative work, however, you need to find the value of AS°. You can find the standard change of entropy, AS°, for a reaction by subtracting the standard entropies of reactants from the standard entropies of products, similar to the way you obtained AH°. 

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