Spontaneity, Entropy and Gibbs Energy

When a volatile liquid (taken as the thermodynamic system) (Frames 1, 7) (say ether or alcohol) is applied to the skin (taken to be the surroundings) it rapidly and spontaneously evaporates (evidenced by a cooling sensation). Heat, q = AvapH° is absorbed from the skin by the ether during the evaporation process of the ether molecules  

Certainly this should be true if energy were the only driving force determining whether reactions take place (i.e. are spontaneous) and the energy curve reached a minimum at equilibrium. 

However, pursuing this idea further, this situation would further mean that only exothermic reactions (for which AH° 0 and enthalpy is lost from the system) would be expected to proceed naturally (i.e. be spontaneous). 

When we glance at the evidence below it appears that energy change - in the form of enthalpy change - is not always the only driving force contributing towards a spontaneous process or reaction. 

The word spontaneous in this context is meant to convey the sense of the dissolution of the crystals in water proceeding without further intervention. 

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Chapter 13 Spontaneous Change Entropy and Gibbs Energy... 

In this edition we retain the core organization of the previous edition with two notable exceptions. First, we have moved the chapter entitled Spontaneous Change Entropy and Gibbs Energy forward in the text. It is now Chapter 13. By moving the introduction of entropy and Gibbs energy forward in the text, we are able to use these concepts in subsequent chapters, cond, we have moved the chapter on chemical kinetics to Chapter 20. Consequently, the discussion of chemical kinetics now appears after the chapters that rely on equilibrium and thermodynamic concepts. 

FIGURE 8.33 On the left of the semipermeable membrane is the pure solvent with its characteristic molar enthalpy, entropy, and Gibbs free energy. On the right is the solution. The molar Gibbs free energy of the solvent is lower in the solution (an entropy effect), and so there is a spontaneous tendency for the solvent to flow into the solution. 

The Helmholtz and Gibbs energies on the other hand involve constant temperature and volume and constant temperature and pressure, respectively. Most experiments are done at constant Tandp, and most simulations at constant Tand V. Thus, we have now defined two functions of great practical use. In a spontaneous process at constant p and T or constant p and V, the Gibbs or Helmholtz energies, respectively, of the system decrease. These are, however, only other measures of the second law and imply that the total entropy of the system and the surroundings increases. 

Our goal in this chapter is to help you learn the laws of thermodynamics, especially the concepts of entropy and free energy. It might be helpful to review Chapter 6 on thermochemistry and the writing of thermochemical equations. The concept of Gibbs free energy (G) will be useful in predicting whether or not a reaction will occur spontaneously. Just like in all the previous chapters, in order to do well you must Practice, Practice, Practice. 

A new chapter (Chapter 23) introduces entropy and Gibbs free energy and shows how the sign and value of these terms enables us to predict whether or not a reaction is likely to be spontaneous... 

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