To form a solution, there are three processes:

1. Separation of solute molecules, overcoming the attraction forces between solvent molecules, this requires energy input. (Enthalpy greater than zero)
2. Separation of solvent molecules, overcoming the attraction forces between solvent molecules, this also requires energy input. (Therefore, enthalpy greater than zero.)
3. Formation of nteractions between solute molecules and solvent molecules

Let’s give symbols for the energy changes of the each step mention above:

ΔH₁ – for separation of solute molecules (ΔH₁ > 0, absorbs heat/energy)

ΔH₂ – for separation of solvent molecules (ΔH2 > 0, absorbs heat/energy)

ΔH₃ – for formation of interactions between solute molecules and solvent molecules (ΔH₃ < 0, releases heat/energy)

So the total enthalpy change is: ΔH = ΔH₁ + ΔH₂ + ΔH₃

 

When will addition of solute to solvent be accompanied by cooling then?

Answer is, when ΔH₁ + ΔH₂ + ΔH₃ is > 0, because cooling means total enthalpy > 0. Therefore Absolute value of ΔH₃   < Absolute value of (ΔH₁ + ΔH₂₎ (remember ΔH₁ + ΔH₂    is always positive, so if the solution formation is ended up with cooling, total enthalpy is greater than zero for the reason of having greater absolute value of (ΔH₁ + ΔH₂ ).

When will addition of solute to solvent be accompanied by getting hotter then?

Answer is, when ΔH₁ + ΔH₂ + ΔH₃ is < 0, because hotter means total enthalpy < 0. Therefore Absolute value of ΔH₃   > Absolute value of (ΔH₁ + ΔH₂₎ (remember ΔH₃ is always negative, so if the solution formation is ended up with getting hotter, total enthalpy is less than zero for the reason of having greater absolute value of ΔH₃.