Formation of 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 Solution so formed will be ideal if
- Hsoln = H1 + H2 + H3
- Hsoln = H1 + H2 - H3
- Hsoln = H1 - H2 - H3
- Hsoln = H3 - H1 - H2
Answer
(1) Hsoln = H1 + H2 + H3
Solution
Steps (i) and (ii) break solvent–solvent and solute–solute attractions (energy absorbed, H1 and H2 positive); step (iii) forms solute–solvent attractions (energy released, H3 negative).
By Hess's law the enthalpy of solution is the sum of the three steps:
Hsoln = H1 + H2 + H3
As per NCERT, for an ideal solution the solute–solvent interactions are nearly equal to the solute–solute and solvent–solvent interactions, so mix H = 0.
∴ The solution is ideal when Hsoln = H1 + H2 + H3 with the energy released in step (iii) exactly balancing that absorbed in steps (i) and (ii), i.e. Hsoln = 0.
Answer: (1)