#### Enthalpy of Mixing and Deviation from Raoult's Law: Summary

The answers to the ConcepTests are given below and will open in a separate window.
##### Key points from this module:
• When components mix to form an ideal solution, the heat of mixing is zero so the temperature does not change.
• When two components initially at the same temperature are mixed adiabatically, the final temperature is greater than the initial temperature for an exothermic mixture and less than the initial temperature for an endothermic mixture.
• Binary mixtures with an endothermic heat of mixing exhibit positive deviations from Raoult’s law (activity coefficients > 1).
• Binary mixtures with an exothermic heat of mixing exhibit negative deviations from Raoult’s law (activity coefficients < 1).
• An endothermic heat of mixing means the attractive forces between unlike molecules are less than the average attractive forces of the pure components.
• An exothermic heat of mixing means the attractive forces between unlike molecules are greater than the average attractive forces of the pure components.
##### From studying this module, you should now be able to:
• Calculate the heat of mixing from an enthalpy versus mole fraction diagram when two pure components are mixed.
• Calculate the adiabatic temperature from an enthalpy versus mole fraction diagram when two pure components are mixed.
• Calculate the adiabatic temperature from an enthalpy-mole fraction diagram when two mixtures of different mole fractions (but containing the same two components) are mixed.
• Determine from an enthalpy-mole fraction diagram whether mixing of two pure components is endothermic, exothermic, or ideal.
• Predict positive or negative deviations from Raoult’s law from the heat of mixing and predict whether mixing is endothermic or exothermic from deviations from Raoult’s law.

Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder