1. Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia

2. 2 Introduction The concept of ideal solution cam now be extended to solutions of several volatile components. In an ideal solution of two volatile liquids we have: The total vapor pressure of mixture is then: This clearly show that the total vapor pressure (at constant T) changes linearly with the composition of mixture. Some mixtures with resemblance in molecular structure

3. 3 Vapor pressure diagrams Coexistence curve or “bubble line”

4. 4 The composition of vapor The composition of liquid and vapor is not necessarily the same. It seems obvious that the vapor should be more richer in the more volatile component. Using Dalton`s law: It follows that: Inverting this expression we obtain: Now, combining this two results:

5. 5 The composition of vapor p yAyA pB*pB* pA*pA* Coexistence curve or “dew line”

6. 6 Total phase diagram Combining both diagrams into one plot allow us to see the composition of both liquid and gas phase: Above the liquid line the liquid phase is more stable. Bellow the vapor line the gas phase is more stable. In the middle region of the diagram we have the liquid vapor equilibrium, with two phases coexisting. At a given pressure we have a liquid with composition a in equilibrium with vapor of composition b.

7. 7 Interpretation of the diagram If we know the composition of one phase at a given temperature we can determine the composition of another phase from the diagram. tie line The line a 2 a 2 ’’a 2 ’, for instance, is a “tie line”. It describes a state at constant pressure, and since T is already fixed, the Gibbs phase rule allow us to conclude that the compositions of both phases are completely defined! Remember: g+F = C+2

8. 8 The lever rule lever rule To calculate the amounts of both phases in equlibria we use the lever rule: we measure the distances l and l’ along the horizontal tie line.

9. 9 T-x Diagrams distillation Instead of T being fixed as in previous diagrams, we can keep constant pressure, and generate T-x diagrams. These diagrams have great importance in practical separation processes like distillation. TB*TB* TA*TA* heat vaporization condensation

10. 10 Real Solutions azeotrope Some mixture behave almost ideally. But, in many other cases, real solutions display marked deviations. In some systems with strong non- ideality, the mixtures form an azeotrope (coming from the Greek word for “boiling without changing”)

11. 11 Azeotropic Mixtures Component A: t b /°CComponent B: t/°CAzeotrope: %A and t b /°C H2OH2O100HCl-8020.2108.6 C 6 H 5 OH182.2C 6 H 5 NH 2 184.342186.2 CHCl 3 61.2CH 3 COCH 3 56.178.564.4 H2OH2O100C 2 H 5 OH78.34.078.2 CCl 4 76.8CH 3 OH64.779.455.7 CHCl 3 61.2CH 3 OH64.787.453.4 Suppose we try to distillate an ethanol/water mixture in a fractionating column. The fractionation shifts the vapor towards the azeotropic composition and vapor emerges on the top of the column. Then it boils unchanged when water content is 4% and temperature around 78 °C (commercial ethanol)

12. 12 Fractionating column (oil)