6
The Thermodynamics of Solutions
PRINCIPAL FACTS AND IDEAS
- A solution is a homogeneous mixture of two or more substances.
- An ideal solution is a model system in which every component has its
chemical potential given for all compositions by
μiμ∗i+RTln(xi)
whereμ∗iis the chemical potential of the pure substancei,Ris the ideal
gas constant,Tis the absolute temperature, andxiis the mole fraction of
the substance in the solution.
- Every component in an ideal solution at equilibrium with an ideal vapor
phase very nearly obeys Raoult’s law
PiPi∗xi
wherePiis the partial pressure of gaseous substanceiat equilibrium with
the solution andPi∗is the partial pressure of substanceiin the gas phase
at equilibrium with pure substancei(the vapor pressure of the pure
substance).
- Mixtures of substances with similar molecules form nearly ideal solutions.
- Nonelectrolyte solutes in dilute solutions very nearly obey Henry’s law:
Pikixi
wherekiis a function of temperature called the Henry’s law constant and
xiis the mole fraction of substanceiin the solution.
- Activities and activity coefficients describe deviations from ideal or dilute
behavior.
- The activities of strong electrolyte solutes require special treatment. The
Debye–Hückel theory provides an accurate limiting law for activity
coefficients of electrolyte solutes.
- Phase diagrams can be used to show the phase equilibria of multicom-
ponent systems and can be understood through the phase rule of Gibbs.
- Colligative properties depend on concentrations of solutes, but not on
their identities.
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