Chapter 14: Solutions

• Many of the forces we’ve talked about occur between ions/molecules in solutions

• Definition: A homogeneous mixture (only one phase)

• Examples: saltwater, tap water, gemstones, brass, air

• Made up of a solvent and a solute:Solvent: the substance present in the larger amount.

Solute: the other substance

• Dissolving depends on attractive forces and entropy

Solutions

• There is a maximum amount of any solute that will dissolve in a given solvent

• If less than the maximum has been added, solution is unsaturatedIf the max or more than the max has been added, solution is saturated

• The concentration of a solution is the amount of solute that has been dissolved in a solvent.

• Many units of concentration:molarity (mol/L), weight % (g/g), ppm (mg/L)

SolubilityIf a solute will dissolve in a solvent, it is soluble.

Some solutes have limits,

some are infinitely soluble in a solvent.

Sugar: 200 g in 100 mL water at 20 ºCEthanol: infinitely soluble in waterGases are infinitely soluble in one another

Trends and Control of Solubility

General rules: 1. polar solutes dissolve in polar solvents2. nonpolar solutes dissolve in nonpolar solvents

“Like dissolves Like”

Oil and water don’t mix: is oil polar or nonpolar?

You try: which of these will dissolve in water?

CH3OH CH3CH3 NH3

Trends and Control of Solubility

What controls Solubility:

Enthalpy (enthalpy of solution)

negative if new forces are stronger than original forces

Entropy

more complicated

“Forcing” Solubility

• How do you make something dissolve if it is normally insoluble in a solvent?

• Grease and grime are made up of nonpolar compounds. Will they dissolve in water?

• They will dissolve in nonpolar solvents, like turpentine, gasoline. Good for cleaning hands?

• Soap: the substance that makes nonpolar compounds dissolve in water. How would you design a soap molecule? What chemical properties should it have?

Soap and Detergent• Soap is both polar and nonpolar

Attracted to water (polar) and grime (nonpolar)Soap is the salt of a fatty acid, it is a surfactant molecule.

hydrophobic end hydrophilic end

Soap• Soap is both polar and nonpolar

Attracted to water (polar) and grime (nonpolar)

• Made from fats, which contain 3 fatty acids and glycerol

Soap• Nonpolar end is attracted

to grimePolar end is attracted to waterDirt is picked up off clothes, skin, and rinsed away

See: Soap movie

Soap

• Hard water and soap• Hard water contains high concentration of

Ca2+ and Mg2+ ions• Ion-ion forces between Ca2+, Mg2+ and soap

anion results in soap scum (soap forms a solid compound on your skin, bathtub, etc.)

• Water softeners replace Ca2+, Mg2+ with Na+

• How does dry-cleaning work? Previously used CCl4, now less carcinogenic solvents

Fabric Softener: Cationic Surfactants

What effect will softener use have on absorptivity?

without with

without with

Introduction to PolymersPolymers are long molecules made of repeating units, called monomers.

In general:

Specific example:

Forces between polymer chains: Crosslinks

Weak: Intermolecular force crosslinks

Strong: Colvalentbond crosslinks

Cotton

Cellulose polymer

Absorbs water. Cellulose chains crosslinked by H-bonding.

Wrinkling

Chemical crosslinkingagent.

Permanent Press Cotton