Enthalpy and Entropy Changes on Mixing

The enthalpy change on mixing, A7/m. is the difference in energy between the mixture F012 and the pure components oi and 02 and is given by 

Equation (8.19) can be rewritten using the above equations in the form  

The enthalpy interaction parameter xh is obtained from the partial molar heat of mixing. For component 1 this has the form 

Similarly the entropic interaction parameter, xs- is obtained from the partial molar excess entropy changes on mixing and is given by 

Combining eqn (8.22) and (8.23) gives the excess chemical potential of component 1 in the mixture  

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Also of importance is the effect of temperature on the gas solubility. From this information it is possible to determine the enthalpy and entropy change experienced by the gas when it changes from the ideal gas state (/z and ) to the mixed liquid state ( andT,). 

AHyn is the enthalpy change on mixing, ASm is the entropy change on mixing, and T is the absolute temperature. If AGm < 0, mixing will occur spontaneously. 

Since the mixing is the reverse process of separation, the changes in enthalpy and entropy are usually negative and positive, respectively. Therefore the vector appears in the second quadrant on the thermodynamic compass. 

The normal isotopic abundances for Li are 92.48 mole % for 7Li and 7.52 mole % for 6Li. Making reasonable approximations, determine the entropy, enthalpy, and Gibbs free energy changes on mixing the pure isotopes. Discuss your results in terms of the statements made in Section 1.21 in conjunction with the Third Law of Thermodynamics. 

A so-called regular solution is obtained when the enthalpy change (AHmix) is nonideal (i.e., non-zero, either positive or negative) but the entropy change (A mix) is still ideal. So on the molecular level, while an ideal solution is one in which the different types of molecules (A and B, for example) behave exactly as if they are surrounded by molecules of their own kind (that is, all intermolecular interactions are equivalent), a regular solution can form only if the random distribution of molecules persists even in the presence of A-B interactions that differ from the purely A-A and B-B interactions of the original components A and B. This concept has proved to be very useful in the development of an understanding of miscibility criteria. 

The theory predicts the magnitude and sign of volume change on mixing and the enthalpy of mixing however in certain instances it yields too small values of the excess entropy. In some cases the dependence of the parameter x oii polymer concentration is described correctly [85, 86] in other cases it is under-evaluated [87, 88]. 

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