The formation of a solution from two components can be considered as: (i) Pure solvent → separated solvent molecules, ∆H₁ (ii) Pure solute → separated solute molecules, ∆H₂ (iii) Separated solvent and solute molecules → solution, ∆H₃ The solution so formed will be ideal if:
∆H_soln = ∆H₁ + ∆H₂ + ∆H₃
∆H_soln = ∆H₁ + ∆H₂ - ∆H₃
∆H_soln = ∆H₁ - ∆H₂ - ∆H₃
∆H_soln = ∆H₃ - ∆H₁ - ∆H₂
According to Hess's law, the total enthalpy of solution (∆H_soln) is the sum of the enthalpy changes of the individual steps involved in the formation of the solution:
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