Lecture 37Chapter 12 Sections 2-4• Solutions

• Colligative Properties

Announcements

• Free points! Go to ‘first post’ area of chemboard and sayhello. Also, post your keypad serial number if you haven’talready emailed that to me.

• Seminar today 4:00 in VDW 102

Solubility of Solids

Remember there are 4 basic solids?1. Network solids (diamond, graphite) cannot dissolve

without breaking covalent bonds.2. Molecular solids – “like dissolve like”3. Metals – do not dissolve in water (some will react,

but not dissolve)4. Ionic Solids – remember the solubility rules?

Which of the following vitamins are fat-soluble(dissolve fatty hydrocarbons) and which are watersoluble?

Example

Vitamins are organic moleculesthat are required for properfunction but are notsynthesized by the humanbody. Thus, vitamins mustbe present in the foodspeople eat. Vitamins fallinto two categories: fat-soluble, which dissolve infatty hydrocarbon-liketissues and water-soluble.

Alloys

• Alloy: a mixture of substances with metallic properties. (asolid solution)

• Some are true solutions homogeneous (brass)

• Others are heterogeneous (solder)

Water is Extremely Unique

• Undoubtedly the most important solvent• Solvent for all biological reactions• The only small molecule that is not a gas under normal

(surface of the earth) conditions• Highly polar bonds, bent geometry• Essentially every characteristic of water is non-standard

Energetics of Solubility

• Molar heat of solution (ΔHsoln): - net energy flow resultingfrom the dissolution of a solid in water.

• Dependent on1. Energy input to separate molecules of solid2. Energy input to create a cavity in the solvent

– For water this involves breaking H-bonds3. Energy released when solvent molecules form

favorable interactions with solute.

Most solutes have negative ΔH because they formstabilizing interactions with the solvent. If they didn’t,they probably wouldn’t dissolve in the first place.

Solubility of Salts

Solubility Equilibrium

• When a solute concentrationreaches the solubility of thatsubstance, no further net changesoccur.

• If more solute molecules aredissolved, others are precipitated

• A saturated solution in contactwith excess solute is in a state ofdynamic equilibrium.

• A dynamic equilibrium isrepresented by double arrows

Solute (pure state) ↔ Solute (aq)

Effect of Temperature

• In general, solids aremore soluble at highertemperatures

• True for most organicsolids and ionic salts

• Opposite is true forgaseous solutes

Gas-Solution Equilibria

• The amount of a dissolved gas increases linearly with thepartial pressure of the gas– Increased pressure ‘pushes’ more gas into solution

• Henry’s Law

• Where KH is Henry’s Law Constant, different for eachcombination of solvent and solute and varies (decreases)with temperature

[ ] ( )eqgasHeqaq pKgas =

Colligative Properties

• Solute particles can affect some physical properties of asolvent.

• The physical properties are referred to as colligativeproperties.– Vapor Pressure– Freezing Point– Boiling Point– Osmotic Pressure

• Colligative properties depend only on the amount of solutedissolved, not on its identity.

Vapor Pressure Reduction

• A pure solvent in a closedsystem will reach a dynamicequilibrium between theliquid and vapor phases.

• The addition of a solute willdecrease the vapor pressurebecause the solute moleculesstabilize liquid solvent butnot vapor.

• Raoult’s Law

Boiling and Freezing Points

• Boiling-point elevation – the increase in boiling point of asolvent by adding a solute.– This is one effect of vapor pressure reduction

• Higher T is now required to reach 1 atm of vaporpressure

• Freezing-point depression – the lowering of the freezing-point of a solvent by adding solute.– Again, liquid solvent molecules are stabilized by

presence of solute.– Solute is not present in solid, so solid is not stabilized

Freezing Point Depression

• Where cm is molality (moles of solute/kg of solvent)• Kf is the constant of freezing point depression for the

solvent.• Similar equation for boiling point elevation.

Osmotic Pressure

Objects flow from high to low concentration

Solute (big blue guys) want to flow from right to left.

Concentrations want to be equal

If solute can’t flow through membrane, then solvent mustflow to try to make concentrations equal

Solvent flows mainly left to right

• Solvent wants to flow from the more dilute to the more concentrated.• This flow can be balanced by applying extra pressure

– This is called osmotic pressure (Π)• The solution of higher concentration always has a higher osmotic

pressure than the one of lower concentration.

Osmotic Pressure Equation

M is the total concentration (molarity) of all solutesR is the gas constantT is the temperature

Measurements of osmotic pressure (as with all colligativeproperties) can be used to measure the molecular weight of anunknown material.

You will use freezing point depression in lab next week todetermine unknown molecular weights.

MRT=!

Remember: You are done with thehomework when you understand it!

Today• Finish reading Chapt 12• Go to seminar

Before Monday• Finish CAPA