Real Behavior of Liquid Mixtures

In the previous section some criteria were established which determine whether Raoult s law can be apphed for a binaiy mixture over the whole range of concentration. The lesser the extent to which these criteria are true the greater the deviations of the partial pressures from their linear dependency on the mole fractions x,. These deviations can be positive as well as negative. In the first case the partial pressure is higher and in the second case it is lower than the partial pressure calculated with Raoult s law. If the attractive interactions between the two mixture components are much smaller than the ones between identical molecules, positive deviations from Raoult s law result The stronger the interactions between heterogeneous molecules the smaller the partial pressures compared to Raoult s law. 

A summary of different binaiy mixtures is shown in Fig. 2.1-21. The upper row contains partial pressures and the total pressure of different mixtures plotted vs. the mole fraction of the liquid phase. Below, the boiling curve and the dew-point curve are shown for p = 1 bar. The lower row contains the respective selectivity diagrams. 

For the mixture isopropyl ether/isopropyl alcohol, positive deviations from Raoult s law occur. They are so strong that the total pressure reaches a maximum for a certain concentration. The vapor pressure maximum corresponds to a boiling temperature minimum as it is depicted in the middle row. Boiling curve and dewpoint curve contact each other in the so-called azeotropic points. Here the mole fractions of vapor y and liquid have the same value which also follows from the equilibrium diagram below. In the azeotropic point A the equilibrium curve intersects the diagonal Pa = To the left of point H the mole fraction of compo- 

In the right column the mixture nitric acid/water is shown. For this the explanations given for the mixture acetone/chloroform hold as principal. Taking into account that there are extremely strong interaction energies between nitric acid and water molecules the described effects are very distinct in this case. There are, for example, very negative deviations from Raoult s law and the distance of the equilibrium curve to the diagonal is considerable. Solutions of small amounts of nitric acid in water show very small partial pressures of the nitric acid. This effect is even more 

More general statements can be obtained from mixed phase thermodynamics. The dimension p may be an extensive state variable, e.g., volume V, enthalpy H, free enthalpy G = H-(T-S), and entropy S. The value of the state variable dimension is a function of pressure p, temperature T, and the amount of substances otc. of its components. The total differential di of the state variable y/ is 

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