Chapter 14Solutions

“how things dissolve”Solutions – homogeneous mixtures of 2

or more substances◦Solvent – does the dissolving (H2O is the

universal solvent) ◦Solute – what gets dissolved

3 things happen when dissolving occurs:1. solvent molecules split up2. solute units split up3. Form solvent/solute attractions

Rules for solutions “likes dissolve like”

~ water is highly polar, dissolves polar solutes and ionic solutes fairly well

Polar – solvent/solute attraction  H2O

NaCl(s) Na+(aq) + Cl-(aq)

 Strong electrolytes dissociate into ions

nonpolar – molecules split up b/c of the weak attraction into smallest molecule unit

 H2O

C12H22O11 C12H22O11

Non-electrolytes undergo molecular solvation

 Ex. oil/water vs. oil/benzeneNonpolar/polarnonpolar/nonpolar

~ liquid in liquid – often volumes are not additive, called miscible.

Why? intermolecular attractions 50 mL H2O + 50 mL methanol ≈ 97 mL

total

gas in liquid – same attractiongases tend to be less soluble at higher

temps. The higher the pressure, the more

soluble, b/c the gas is forced into the solution more. If pressure decreases the gas is less soluble

Finely divided solids dissolve much more rapidly than large crystals◦Granulated sugar vs. sugar cubes

Unsaturated sol’n – a sol’n that is capable of dissolving more solute. (ex. 15.0g NaCl in 100g of H2O at room temp.) There are no un-dissolved solutes remaining.

Saturated sol’n – a sol’n has its maximum amount of solute dissolved in it. You always see un-dissolved solute. (ex. 40.0g NaCl in room temp. water) When a sol’n is saturated, the solute is still dissolving – it’s just that the solute is also re-crystallizing out of the sol’n at the same rate, so there is no apparent change.

Supersaturated sol’n – by heating a saturated sol’n and carefully cooling it, you can get more solute dissolved then theoretically possible. It is an unstable condition that is the basis for rock candy.

Spontaneity of the Dissolution ProcessAssume solvent is a liquidMajor factors that affect dissolution

of solutes (dissolving of solutes) ◦change of energy content, DHsolution

exothermic favors dissolution endothermic does not favor dissolution

◦change in disorder, or randomness, DSmixing

increase in disorder favors dissolution increase in order does not favor dissolution

Best conditions for dissolution◦exothermic & disordered

Molarity: M= n/Vn = moles of solute V = Liters of

solution

Molality: m = n/kgn = same kg = mass of solvent

Mole fraction (x) number of moles of one part per moles of all parts of the solution

xa = na xb = nb etc…

ntotal ntotal

Colligative propertiesproperties of solutions that

depend only on the number of particles dissolved and not the kinds of particles dissolved.

4 types◦Vapor Pressure◦Boiling Point Elevation◦Freezing Point Depression◦Osmotic Pressure

1. Vapor pressure-- Pvap goes down with increase in concentration.

Water vs. water + sugar *more sugar higher Pvap vs. lower Pvap

In solns, fewer molecules have the chance to turn into vapor

  Raoult’s Law – the vapor pressure of an ideal solution is directly proportional to the mole fraction of the solvent in the solutionPvap = (Pvap

o)(xsolvent)

 ~ ideal gases obey PV = nRT very well ~ Raoult’s Law is close to ideal when the solute and

solvent have very similar I.F.

Effect of Pressure on SolubilityPressure changes have little or no effect

on solubility of liquids and solids in liquids

Pressure changes have large effects on the solubility of gases in liquids◦ why carbonated drinks fizz when opened◦ cause of several scuba diving related

problems including the “bends”

2. & 3. Boiling Point Elevation & Freezing Point Depression

solutions exist as liquids over a wider temp. range

solutions move the triple point of a substance further down the curve.

  pg. 563 & next slide show a phase diagram comparison between pure solvent and a solution

Why? Higher B.P. – more attractions need to be overcome before boiling can occur. Lower F.P. – ions get in the way of freezing.

Phase Diagram

Dissociation of Electrolytes & Colligative Properties

Electrolytes have larger effects on boiling point elevation & freezing point depression than soluble nonelectrolytes

one mole of sugar dissolves in water to produce one mole of aqueous sugar molecules

one mole of NaCl dissolves in water to produce 1 mole of Na+ & 1 mole of Cl- ions

colligative properties depend on number of dissolved particles◦expect twice the effect for NaCl than for sugar

Ex. for F. P & B.P.

Salt in water for pasta raises the B.P.Which of the following raises the B.P. more?

◦0.1 M HF vs. 0.1 M HCl Weak electrolyte vs. strong electrolyte

(s.e. dissociates thus higher B.P.)

◦0.1 M CH3OH vs. 0.1 M CH3COOH Nonelectrolyte vs. weak electrolyte

(w.e. slightly soluble so higher B.P.)Salt on the road ~ often CaCl2 (3 particles

dissolved) is chosen over NaCl (2 particles dissolved) CaCl2 lowers F.P. more

Homemade ice-cream ~ Yummy!

B.P & F.P. EquationsBoiling point elevation∆Tb = imkb

∆T = change in temp. m = molalityk = constant

i = van’t hoff factor ~ measures the extent of ionization (number of dissolved particles)

Freezing point depression ∆Tf = imkf

pg 564. table 14-2

4. Osmotic pressurethe pressure produced on the surface of a

semipermeable membrane by osmosis. ◦Osmosis – net flow of solvent between two solns

separated by a semipermeable membrane◦Solvent passes from lower concentration soln.

into higher concentration soln.◦Ex. of semipermeable membranes ~ skin, cell

membranes, cellophane, and saran wrap

π = MRTπ = symbol for osmotic pressureM = molarity T = temp R = 0.082057 L atm/mol K

Example problemsEx. 1) What is the molality and mole fraction

of 50.0 g potassium chloride in 425 g water?

Ex. 2) 15.0 g of ethanol is dissolved in 750. g of formic acid. The freezing point of the solution is 7.20oC. The freezing point of pure formic acid is 8.40oC. What is kf for formic acid?

Ex. 3) What is the molar mass (grams/mole) of 30.0 mL of an unknown substance if 0.300 g of that substance has an osmotic pressure of 0.400kPa at room temperature?

ColloidsMixtures that have particle sizes

between those of true solutions and suspensions (suspensions have particles that are very large, they will settle out unless the mixture is constantly stirred).◦Fog, smoke, paint, milk

The Tyndall Effect – colloids scatter light when its shined on them.

Hydrophilic & Hydrophobic Colloids

Hydrophilic – water loving colloids◦Blood plasma, some biological proteins

Hydrophobic – water hating colloids, require emulsifying agents to stabilize in water (emulsion helps keep two things together that normally would not mix)◦Milk – emulsion of fat and proteins mixed in

water, casein is the emulsifier◦Mayonnaise – oil and eggs with water, lecithin

from egg yolk is the emulsifier◦Soaps and detergents are excellent emulsifying

agents ~ hard water reacts with soap anions and precipitates making bathroom scum

Hydrophilic & Hydrophobic Colloids

Medicines that are injected into humans such as shots must be at the same concentration as the existing chemical in our blood. These solutions are called isotonic. For example, if the medicine contains potassium ions, they must be the same concentration as the potassium ions in our blood. Why do medicines have to be made that way?