To form a solution, there are three processes:

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

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

ΔH_{1} – for separation of solute molecules (ΔH_{1 }> 0, absorbs heat/energy)

ΔH_{2 }– for separation of solvent molecules (ΔH2_{ }> 0, absorbs heat/energy)

ΔH_{3} – for formation of interactions between solute molecules and solvent molecules (ΔH_{3 }< 0, releases heat/energy)

So the total enthalpy change is: ΔH_{ }= ΔH_{1 }+ ΔH_{2 }+ ΔH_{3}

_{ }

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

Answer is, when ΔH_{1} + ΔH_{2} + ΔH_{3} is > 0, because cooling means total enthalpy > 0. Therefore Absolute value of ΔH_{3 } < Absolute value of (ΔH_{1 }+ ΔH_{2)} (remember ΔH_{1 }+ ΔH_{2 } 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 (ΔH_{1 }+ ΔH_{2 }).

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

Answer is, when ΔH_{1} + ΔH_{2} + ΔH_{3} is < 0, because hotter means total enthalpy < 0. Therefore Absolute value of ΔH_{3 } > Absolute value of (ΔH_{1 }+ ΔH_{2)} (remember ΔH_{3 }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 ΔH_{3}.

**Subject :**Science**Topic :**Chemistry-
**Posted By :**Jason **Created on :**02-18-2011

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