Homework

  13.115   13.122   13.130   13.133   13.139   13.143

  DUE MONDAY DECEMBER 15, 2008

House-keeping

  Make-up Class (WHEN?!)   Monday 4:30 – 6:00 PM Possibly C-114

Long test: December 17, 2008 Wednesday 4:30 – 6:00 PM or 6:00 – 7:30 PM

a thermodynamic approach

A few key terms

  Thermodynamics – the study of heat (“energy in transit”) and its interconversion.

  We talk of movement of energy in a system

The solution process

  Intermolecular Forces have to be broken  Solute-solute  Solvent-solvent

  Intermolecular Forces have to be created  Solute-solvent

  Just like phase changes, breaking and creating forces require an input and output of energy

The solution process

∆Hsol'n = ∆Hsolvent + ∆Hsolute + ∆Hmix

Breaking IMFs between solvent molecules: Endothermic, ΔHsolvent > 0

Breaking IMFs between solute molecules: Endothermic, ΔHsolute > 0

Forming IMFs between solute-solvent molecules: Exothermic, ΔHmix < 0

Energy Changes

Aqueous solutions salts

  ΔHlattice

  It is the energy needed to completely liberate ions from an ionic solid

  It is endothermic!

ΔHlattice = ΔHsolute

Aqueous solutions salts

  ΔHhydration

 Solvation (Hydration) is the process of surrounding a solute particle with solvent (water)

  It is the combined process of disturbing your solvent particles and mixing solute with solvent

  It is exothermic!

ΔHhydration = ΔHmix+ ΔHsolvent

Aqueous solutions salts

ΔHhydration = ΔHmix+ ΔHsolvent

ΔHlattice = ΔHsolute

∆Hsol'n = ∆Hsolvent + ∆Hsolute + ∆Hmix

∆Hsol'n = ∆Hlattice + ∆Hhydration

Aqueous solutions salts

The solution process

Statement 1: The solution process is accompanied by heat changes (enthalpy). Exothermic processes are favored.

However!

Entropy

Entropy (S) is directly related to the number of ways that a system can distribute its energy.

Entropy is directly related to the freedom of motion of particles and the number of ways they can be arranged

Sgas > Sliquid > Ssolid

Entropy of Solution

There are more ways you can distribute the energy in a solution than individual solute

Ssol’n > (Ssolute+Ssolvent) ∆Ssol’n> 0

The solution process

Statement 1: The solution process is accompanied by heat changes (enthalpy)

Statement 2: The solution process is also accompanied by the greatest tendency to distribute energy in as many ways possible (entropy)

∆Hsol’n ∆Ssol’n

So why does like dissolve like?

  Case 1: NaOH in water

So why does like dissolve like?

  Case 2: octane in hexane

 Enthalpy of solution is almost zero, the solution process is entropy driven

So why does like dissolve like?

  Case 3: NaCl in hexane

 A solution will not form because the entropy increase is not enough to compensate for the huge enthalpy change

So why does like dissolve like?

  Case 4: NH4NO3 in water

 The increase in entropy during crystal break down and the favorable mixing of ions in polar water solvent compensates for increase in enthalpy

Thermodynamics of solution process

  How is ∆G related to enthalpy (∆H) and entropy (∆S)?

  For solutions to occur, ∆Gsol’n<0

∆Hsol’n ∆Ssol’n Temp

Exercise

  Sketch a qualitative enthalpy diagram for the process of dissolving KCl (s) in H2O (endothermic)? NaI(s) in H2O (exothermic)?