Solvent Effects in Chemistry
Solvent Effects in Chemistry
The alchemists' adage, " Corpora non agunt nisi fluida ," "Substances do not react unless fluid," is not strictly accurate, for crystals can be transformed by processes of nucleation and growth. There is growing interest in "mechanochemical" processes, which are carried out by grinding solid reagents together (and which no doubt involve a degree of local melting). Nevertheless, it is still generally true enough to be worthy of attention. Seltzer tablets, for instance, must be dissolved in water before they react to evolve carbon dioxide. The "fluid" state may be gaseous or liquid, and the reaction may be a homogeneous one occurring throughout a single gas or liquid phase, or a heterogeneous one occurring only at an interface between a solid and a fluid, or at the interface between two immiscible fluids. As the title suggests, this book is concerned mainly with homogeneous reactions, and will emphasize reactions of substances dissolved in liquids of various kinds.
The word "solvent" implies that the component of the solution so described is present in excess; one definition is "the component of a solution that is present in the largest amount." In most of what follows it will be assumed that the solution is dilute. We will not attempt to define how dilute is "dilute," except to note that we will routinely use most physicochemical laws in their simplest available forms, and then require that all solute concentrations be low enough that the laws are valid, at least approximately.
Of all solvents, water is of course the cheapest and closest to hand. Because of this alone it will be the solvent of choice for many applications. In fact, it has dominated our thinking for so long that any other solvent tends to be tagged nonaqueous , as if water were in some essential way unique. It is true that it has an unusual combination of properties (see, e.g., Marcus, 1998, pp. 230-232). One property in which it is nearly unique is a consequence of its ability to act both as an acid and as a base. That is the enhanced apparent mobility of the H3O+ and HO- ions, explained by the Grotthuss mechanism (Cukierman, 2006; de Grotthuss, 1806):
in which protons hop from one molecule or ion to the next following the electric field, without actual motion of the larger ion through the liquid. This property is shared (in part) with very few solvents, including methanol and liquid hydrogen fluoride, but not liquid ammonia, as may be seen from the ionic equivalent conductances (see Table 1.1 ). It is apparent that in water, both the positive and negative ions are anomalously mobile. In ammonia neither is, in hydrogen fluoride only the negative ion is, and in methanol only the positive ion is.
Table 1.1 Limiting equivalent conductances of ions in amphiprotic solvents
In H2O at 25°C In NH3 at -33.5°C a In HF at 20°C b In MeOH at 25°C c H3O+ 349.8 NH4+ 131 H2F+ 102 MeOH2+ 141.8 HO- 198.5 NH2- 133 HF2- 350 MeO- 53.02 Na+ 50.11 Na+ 130 Na+ 150 Na+ 45.5 K+ 73.52 K+ 168 K+ 150 K+ 53.6
a Kraus and Brey (1913).
b Kilpatrick and Lewis (1956).
c Ogston (1936), Conway (1952, pp. 155, 162).
As aqueous solution of an acid is diluted by addition of a solvent that does not contribute to the hydroge