Energetics of the Solution Process

When a solute dissolves in a solvent…

IMFs of solvent are disrupted IMFs of solute are disrupted IMFs between solute-solvent are created

Which steps are endothermic? exothermic?

Time for an experiment… (Investigate This 2.2)

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

ethanol,colorless liquid in vial

urea,white crystals in vial

H2O

in pipet

Investigate This 2.2

Add water to urea until the vial is ~1/2 full. Recap the vial and swirl to dissolve. Does the solution become warm or cool?

Add water to ethanol until the vial is ~full, cap, and mix. What happens?

Does this describe ethanol dissolving? Urea dissolving?

The diagram shows changes in the solute only.

Predicting Solubility (Investigate This 2.5)

methanolCH3OH

hexaneCH3CH2CH2CH2CH2CH3

1-butanolCH3CH2CH2CH2OH

Which solute is most soluble in water?

Polar Solute + Water

methanolCH3OH Type of Solute-

Solvent IMFs?

Non-Polar Solute + Water

hexaneCH3CH2CH2CH2CH2CH3 Type of Solute-

Solvent IMFs?

1-butanolCH3CH2CH2CH2OH

Water + Solute with Polar/Non-Polar Regions

Type of Solute-Solvent IMFs?

Dissolving of Ionic Compounds

Interactions Between Ions

Magnitude of attraction Ecoulomb q1q2/d

small ions of big charge have largest attractive forces

~400-4000 kJ/mol (huge!) accounts for high melting/boiling points

dion of charge q1 ion of charge q2

So,why do ionic compounds dissolve at all?

How can strong ion-ion forces be overcome?

Ion-Dipole Interactions

Ions can interact with polar molecules (dipoles)

Magnitude of attraction Ecoulomb q1q2/d

Increases with ion charge, dipole moment ~40-600 kJ/mole

ion dipole with partial charges

d

- +

Ion-Dipole Interactions Most important example is ion-water

Na+ + 6H2O [Na(OH2)6]+

E = -397 kJ/mole

Ion-Dipole Interactions Overcome Ion-Ion Forces

Energy Changes for NaCl Dissolving in Water

Like Dissolves Like

Type of Solute

Polar Molecules

Nonpolar Molecules

Ionic compounds

Soluble in

polar solvents (water, methanol, acetone)

nonpolar or slightly polar solvents (C6H6, CCl4, CHCCl3)

very polar solvents (water)

Solute-Solvent IMF

dipole-dipole or H-bonding

dispersion or weak dipole-dipole attractions

ion-dipole attractions