Relative thermodynamic stability energetics

The relative thermodynamic stability of graphite versus diamond provides a classic illustration of the interplay between thermodynamics and kinetics. Graphite and diamond are both polymorphs (same composition but different phases) of carbon. At room temperature and pressure, thermodynamics tells us that diamond is less stable than graphite—in other words, there is an energetic driving force favoring the transformation of diamond into graphite. So, are diamonds forever Thermodynamics... 

Writing the overall equation, with the reactants on the left and the products on the right, is only the first step in our study of a reaction. If we truly want to understand a reaction, we must also know the mechanism, the step-by-step pathway from reactants to products. To know how well the reaction goes to products, we study its thermodynamics, the energetics of the reaction at equilibrium. The amounts of reactants and products present at equilibrium depend on their relative stabilities. 

The stability of liquid water is due in large part to the ability of water molecules to form hydrogen bonds with one another. Such bonds tend to stabilize the molecules in a pattern where the hydrogens of one water molecule are adjacent to oxygens of other water molecules. When chemical species dissolve, they must insert themselves into this matrix, and in the process break some of the bonds that exist between the water molecules. If a substance can form strong bonds with water, its dissolution will be thermodynamically favored, i.e., it will be highly soluble. Similarly, dissolution of a molecule that breaks water-to-water bonds and replaces these with weaker water-to-solute bonds will be energetically im-favorable, i.e., it will be relatively insoluble. These principles are presented schematically in Fig. 15-1. 

A summary of the electrochemical formulae developed above is provided in Table 7.3. AG, pe, E, and K contain virtually the same thermodynamic information. While is the quantity that is analytically measured, pe is preferred by marine chemists as it is temperature independent and numerically easier to work with. AG is often used to compare the relative stability of species because it provides a measure of energy yields in units of calories or joules. A comparison of the three electrochemical scales at 25°C is given in Figure 7.3. The merits of each thermodynamic parameter will become evident in the next section of the chapter where the energetics of some marine redox processes are considered. 

For any candidate molecule, one must consider the relative location of the highest occupied molecular orbital (HOMO) of D and the lowest unoccupied molecular orbital (LUMO) of the A molecule. The "parent" neutral molecule and the "daughter" radical anion radical A- or cation radical D+ must be thermodynamically and kinetically stable, as well as energetically accessible. Since most organic synthesis is done in solutions, the species should not oxidize or reduce the solvent. In addition, the second ionization state (D2+ or A-) must also be stable, since an additional electron or a hole must also fleetingly reside on D+ or A-. There are, at present, four methods useful for stabilizing these systems ... 

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