Dissolution of salts in water

The dissolution of salt in water (2) is endothermic (AH > 0)—i. e., the liquid cools. Nevertheless, the process still occurs spontaneously, since the degree of order in the system decreases. The Na"" and Cl ions are initially rigidly fixed in a crystal lattice. In solution, they move about independently and in random directions through the fluid. The decrease in order (AS > 0) leads to a negative -T AS term, which compensates for the positive AH term and results in a negative AG term overall. Processes of this type are described as being entropy-driven. The folding of proteins (see p. 74) and the formation of ordered lipid structures in water (see p. 28) are also mainly entropy-driven. 

So Watt s engine work and De Luc s meteorology certainly had plenty of opportunity to intersect. In fact, substantially similar chemical ideas were deployed in each domain.14 This intersection is apparent retrospectively in De Luc s Idees sur la Meteorologie. A centrepiece of the Idees was De Luc s critique of the solution theory of evaporation as put forward by Senebier, Franklin, Hutton and others. According to that theory, water vapour was produced - that is, evaporation occurred - by the dissolution of water in air in a way precisely analogous to the dissolution of salt in water. This theory faced the difficulty that evaporation occurred in vacuo. De Luc s alternative theory was that water vapour was in fact a chemical product of water and heat. In recounting how he became confident of that alternative theory De Luc gave a prominent place to Watt s steam experiments. 

We will not be concerned here with diffusion per se instead we will concentrate on the issue of mass transfer between phases and how that is handled in the context of our analysis tools. The examples begin with an analysis of the dissolution of salt in water and move to more complex systems including the permeation of hydrogen through a palladium membrane. 

Still another kind of subsurface water contamination is that produced by salt, either by the incursion of salt water from the sea when freshwater is pumped too intensively from wells in coastal areas or by the dissolution of salt in water that is in contact underground with deposits of rock salt. Salt can also become troublesome in the subsurface water of arid regions, where repeated use of irrigation water leads to slow concentration by evaporation during each use. 

Ionic hydration is crucial to the dissolution of salts in water since the energy released by hydration is used to offset that required to break up the crystal— the lattice energy. 

The dissolution of electrolytes in water has a strong effect on the internal pressure of the solvent, a phenomenon known as the salt effect. Almost all electrolytes (perchloric acid is the exception) increase the internal pressure of water by elec-trostriction, a term used to describe the polarization and attraction of water molecules. The effect of this internal pressure is to squeeze out the organic... 

Many of the equations in this book are written with the reversible reaction sign (two-way half-arrows e.g. eqn. 2.5). This shows that the reaction can proceed in either direction and this is fundamental to equilibrium-based chemistry (see Box 3.2). Reactions depicting dissolution of substances in water may or may not show the water molecule involved, but dissolution is implied by the (aq) status symbol. Equation 2.7, read from left to right, shows dissolution of rock salt (halite). 

Some attention must be paid if one of the reactants is in the solid phase. Equilibrium calculations are simplified in this case because the activity of the solid does not appear in the equilibrium constant (recall from eq. [14.24 that activity of the solid is practically 1 unless pressure is high). This creates the interesting situation that a reversible reaction involving a solid may proceed to completion. This is very similar to the familiar dissolution of solids in water and other solvents. If an amount of a soluble solid (e.g., salt or sugar) is mixed with a liquid, some of the solid dissolves until the solubility limit is reached, at which point we have equilibrium between the undissolved and dissolved fractions of the solid. If, however, the amount of solid is below the solubility limit, the entire amount dissolves. This is an example of a reversible reaction that goes to completion. Such process is possible only if it involves a solid, because the concentration of the solid does not appear in the equilibrium constant. With components in any other phase, the mol fraction of a reactant appears in the denominator of the equilibrium constant, and this prevents a reversible reaction from reaching completion unless the equilibrium constant is a very large number. 

As has already been shown, an important parameter that controls water solubilization significantly is addition of electrolytes to the system. In the present case, this essentially means dissolution of salts in the water pool, thus changing its composition. 

A few of these are given. Here, as elsewhere, Lavoisier correctly distinguished betw een solution when no chemical change occurs (solution of salts in water) and dissolution when it does (metals in acids). 

Practice Problem A Which of the following processes is a physical change (a) evaporation of water (b) combination of hydrogen and oxygen gas to produce water (c) dissolution of sugar in water (d) separation of sodium chloride (table salt) into its constituent elements, sodium and chlorine (e) combustion of sugar to produce carbon dioxide and water. 

Table 5.1 Thermodynamics of dissolution processes of ionic salts in water at 25 °C.

AHs may be either positive or negative. That principally depends on the relative magnitudes of the terms that figure on the right-hand side of the equation. In some cases heat is evolved on the dissolution of a salt in water. It is mainly due to the fact that heat evolved when the gaseous ions are hydrated (AHX) is more than the heat absorbed in rupturing the crystal lattice. In the majority of cases, however, there is an absorption ofheat when a salt dissolves in water. 

An instant ice pack for first-aid treatment uses the endothermic nature of the dissolution of an ionic salt in water to provide cold therapy. Two typical materials that absorb heat as they dissolve in water are ammonium nitrate and ammonium chloride ... 

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