Energy Changes in the Solution Process

Heat (H)

Step 1: Solute separates into particles – Overcome the attractions present. Process is endothermic.

Solute (aggregated) + heat → Solute (separated) ; Hsolute > 0

Step 2: Solvent separates into particles. Overcome attractions. Endothermic.

Solvent (aggregated) + heat → Solvent (separated) ; Hsolvent > 0

Step 3: Solute and solvent particles mix. Particles attract each other. Exothermic.

Solute (separated) + solvent (separated) → Solution + heat ; Hmixing < 0

  • Total enthalpy change of the above process is called the Heat of Solution (Hsolution)

Hsolution = Hsolute + Hsolvent + Hmixing

  • If the sum of the endothermic terms is smaller than the exothermic term, the solution gets warmer as it forms.
  • If the endothermic terms exceed the exothermic terms, the solution will get cold.
  • If the endothermic terms are much greater than the exothermic terms, there will not be a solution that forms.

Examples Relating Heat and Solutions

  1. Oil and Water
  2. Hsolute for oil (long hydrocarbon chains) islarge
  3. Hsolvent for H2O (H-bonding) islarge
  4. Hmixingis small (negligible interactions)

Thus…

Hsolution= large positive + large positive + small negative

= large positive (solution does not occur)

  1. Ionic Solids in Water

Consider dissolving sodium chloride in water. We can describe the dissolving process quantitatively.

Step 1 – Separating the solute: NaCl(s) → Na+(g) + Cl-(g)

Hsolute = -Hlattice energy= + 786 kJ/mol

Step 1, the -Hlattice energy, is always endothermic for an ionic solid.

Step 2 and Step 3 are lumped together in this scenario:

Na+(g) + Cl-(g) → Na+(aq) + Cl-(aq)

Hsolvent + Hmixing = Hhydration= - 783 kJ/mol

Even though the Hsolvent is endothermic, for an ionic solid in water the Hhydration is always negative.

  • The magnitude of the Hhydration is related to the charge density of the ionic solid (charge of the ion/volume of the ion). Thus, higher charged, smaller ions will create heats of hydration that are larger negative (more exothermic) in value.

So, for one mole of NaCl dissolving in water (or for any ionic solid dissolving in water)…

Hsolution = -Hlattice energy + Hhydration

= 786 kJ + (-783 kJ)

= + 3 kJ

Dissolving NaCl in water is slightly endothermic (requires a small amount of energy to occur).

If the above case is true, why is NaCl in water so soluble in H2O? Entropy!

Entropy (S)

Nature’s tendency toward disorder.

  • The overall system becomes more disordered, with this favored process driving the solution to occur.
  • Solutions that need a small amount of energy to occur do so because of entropy.
  • Solutions that need a large amount of energy DO NOT OCCUR.

To Summarize all of the information from above…

The formation of a solution is dependent upon…

  1. Changes in heat

In general: Hsolution = Hsolute + Hsolvent + Hmixing

For an ionic solid in water: Hsolution = -Hlattice energy + Hhydration

  1. Changes in entropy

systems that show more disorder on the product side are favored and will have entropy as a driving force