In ideal solution, we take enthalpy of mixing as zero in equation of gibbs free energy. Why is it?
Mathematical solution for why enthalpy of mixing is equal to 0 for ideal gas: https://chemistry.stackexchange.com/questions/6943/mathematical-basis-of-why-enthalpy-of-mixing-is-0-for-ideal-gas
A shorter qualitative answer: https://socratic.org/questions/why-is-enthalpy-of-mixing-zero
Hello.
We take the enthalpy of mixing as zero as the definition of an ideal solution. There is no reason other than the simplification of assigning properties to the arrangement of "particles" (such as solute molecules in a solution), ideal solutions essentially track arrangement effects with no "interaction" effects.
This works for ideal gases, which are also assumed to neglect all interactions between particles (again, molecules). It also gives rise to the simple equations for colligative properties that are taught to undergrads, in which properties such as osmotic pressure, vapor pressure depression, freezing/melting point changes, are related solely to the concentrations of solutes.
Of course, the ideal solution fails a lot in the real world... sucrose does not interact the same with water it does with water, but this is what ideal solution theory predicts. Ideal solutions predict that ALL solutes have the same solubility (molar) in a given solvent, for instance. In real solutions, solubility differences arise because of the solute-solvent interactions, which are the exact source of the nonzero enthalpy of mixing.
There is lots more, but hopefully this gives some idea of the argument.
Best regards!
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