2.4 RECAPITULATION
Thermodynamics describes the (changes in) energy (or enthalpyH)andin
entropyðSÞof a system; entropy is a measure of disorder. These parameters
are combined in the free orGibbs energyG¼HTS. Absolute values of
these parameters cannot be given, but the magnitude of changes in them can
often be established. Every system tends to change in the direction of the
lowest free energy, for instance by evening out of concentration (increase in
entropy) or by reaction between components (decrease in enthalpy). If it has
attained such a state, it is stable; if not, it is unstable. However,
thermodynamics tells us nothing about rates of change, and some systems
can be metastable or change extremely slowly.
A substance in solution has achemical potential, which is the partial
molar free energy of the substance, which determines its reactivity. At
constant pressure and temperature, reactivity is given by the thermodynamic
activity of the substance; for a so-called ideal system, this equals the mole
fraction. Most food systems are nonideal, and then activity equals mole
fraction times anactivity coefficient, which may markedly deviate from
unity. In many dilute solutions, the solute behaves as if the system were
ideal. For such ideally dilute systems, simple relations exist for the solubility
of substances, partitioning over phases, and the so-called colligative
properties (lowering of vapor pressure, boiling point elevation, freezing
point depression, osmotic pressure).
At high concentrations of a (neutral) solute, the activity coefficient is
generally greater than unity, often appreciably. The activity coefficient can
be markedly below unity if the substance is subject to self-association or to
association with (adsorption onto) other substances.
Forionizable substances, the activity coefficient is generally smaller
than unity, the more so for a higher total ionic strength, due to screening of
positive charges by negative ones and vice versa; the coefficient is also
smaller for ions of higher valence. For fairly small ionic strength (up to
about 0.1 molar), a simple theory predicts the value of the activity
coefficients. The smaller the activity coefficient, the higher the solubility of
the substance and the stronger its degree of dissociation. This means that
addition of a different salt (e.g., NaCl) to a solution (e.g., of calcium
phosphate) will increase the degree of dissociation and the solubility of the
latter. It should be realized that salts of multivalent ions are not nearly
completely dissociated unless the ionic strength is very small. The relations
(especially the state of association) of multicomponent salt solutions are
intricate.