properties of solutions. 15.2 solvents and solutes – an aqueous solution is water that contains...

Properties of Solutions

15.2

Solvents and Solutes

–An aqueous solution is water that contains dissolved substances.

–In a solution, the dissolving medium is the solvent.

–In a solution, the dissolved particles are the solute.

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

Dissolution vs reaction

• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.

• If you can’t, the substance didn’t dissolve, it reacted.

Ni(s) + HCl(aq) NiCl2(aq) + H2(g) NiCl2(s)dry

Why does a solution form?

•LIKE DISSOLVES LIKE–Polar solvents such as water

dissolve ionic compounds and polar compounds.

–Nonpolar solvents such as gasoline dissolve nonpolar compounds.

15.2

• Vitamin A is soluble in nonpolar compounds (like fats).

• Vitamin C is soluble in water.

Which vitamin is water-soluble and which is fat-soluble?

Solution Formation

• The compositions of the solvent and the solute determine IF a substance will dissolve.

• The factors that determine the RATE which a substance dissolves are:

»stirring (agitation)»temperature»the surface area of the dissolving

particles

16.1

Solubility

• Some liquids combine in all proportions, while others don’t mix at all. –Two liquids are miscible if they

dissolve in each other in all proportions.

–Two liquids are immiscible if they are insoluble in each other.

16.1

Degree of Saturation

• A saturated solution contains the maximum amount of solute for a given quantity of solvent at a given temperature and pressure.

• An unsaturated solution contains less solute than a saturated solution at a given temperature and pressure.

16.1

Degree of saturation

• Supersaturated Solvent holds more solute than is normally possible at

that temperature.These solutions are unstable; crystallization can often

be stimulated by adding a “seed crystal” or scratching the side of the flask.

Factors Affecting Rate of Solubility

–Temperature –The solubility of most solid

substances increases as the temperature of the solvent increases.

–The solubilities of most gases are greater in cold water than in hot.

16.1

Factors Affecting Solubility

The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve.Example: ethanol in water

Ethanol = CH3CH2OH

Intermolecular forces = H-bonds; dipole-dipole; dispersion

Ions in water also have ion-dipole forces.

Factors Affecting Solubility

– Pressure• Changes in pressure have little effect on the solubility

of solids and liquids, but pressure strongly influences the solubility of gases.

• Gas solubility increases as the partial pressure of the gas above the solution increases.

16.1

Factors Affecting Solubility16.1

Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

Factors Affecting the Solubility of Gases

• Solubility of most gases decreases as temperature increases

• Gases tend to have weak intermolecular forces– Ex: N2 and O2 form weak dipole-induced dipole forces

and weak dispersion forces with water • As the kinetic energy of particles within a solution

increases the gas particles break free from weak attractions and re-enter the gas phase

Factors Affecting Solubility of Gases

• Real life:– Warm soda goes flat faster than cold soda– Fishing– Thermal pollution – The Bends

Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

Why?• Larger molecules have

stronger dispersion forces.

Gases in Solution

Temperature• The opposite is true of

gases. Higher temperature drives gases out of solution.

Carbonated soft drinks are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.

Concentration

mol of soluteL of solutionM =

Molarity (M)

• Because volume is temperature dependent, molarity can change with temperature.

Colligative Properties

• Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.

• Among colligative properties areVapor pressure lowering Boiling point elevationMelting point depressionOsmotic pressure

Vapor-Pressure Lowering

• Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate.

16.3

Vapor-Pressure Lowering

• Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions.

16.3

Vapor-Pressure Lowering

• Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl2 dissociates into three ions.

16.3

Boiling Point Elevation and Freezing Point Depression

Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling-Point Elevation

• The magnitude of the boiling-point elevation is proportional to the number of solute particles dissolved in the solvent.

• The boiling point of water increases by 0.512°C for every mole of particles that the solute forms when dissolved in 1000 g of water.

16.3

Boiling Point ElevationThe change in boiling point is proportional to the molality of the solution:

Tb = Kb m

where Kb is the molal boiling point elevation constant, a property of the solvent.

Tb is added to the normal boiling point of the solvent.

Freezing Point Depression• The change in freezing

point can be found similarly:

Tf = Kf m

• Here Kf is the molal freezing point depression constant of the solvent.

Tf is subtracted from the normal freezing point of the solvent.

Boiling Point Elevation and Freezing Point Depression

In both equations, T does not depend on what the solute is, but only on how many particles are dissolved.

Tb = Kb m

Tf = Kf m

Suspensions

• Suspensions are mixtures where the particles do not stay suspended indefinitely– Diameter is <1000nm

(solution particles are about 1 nm)

– Particles settle out (The Supernate is the water above the particles of a precipitate)

The Tyndall Effect

• Colloidal suspensions can scatter rays of light.

• This phenomenon is known as the Tyndall effect.

Colloids

• Colloids have particles bigger than in a solution but smaller than in a suspension.– Particles won’t settle out

over time– Examples: whipped cream,

marshmallow, milk, mayo, fog, aerosols, jellies, paint, blood, gelatin

Colloids in Biological Systems

Some molecules have a polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic (water-hating) end.

Colloids in Biological Systems

Sodium stearate is one example of such a molecule.

Solution.1-1 nm

Not able to filter

No Tyndall Effect

Colloid1 – 1000nmNot able to

filter

Sometimes Tyndall effect

Suspension< 1000 nm

Will filter

Sometimes Tyndall effect

Acids and Bases…. The basics lol

Bases have a

pH greater than 7

Acids – Base Theories

• Arrhenius • Bronsted- Lowry• Lewis

1. Arrhenius Definition - 1887

• Acids produce hydrogen ions (H1+) in aqueous solution (HCl → H1+ + Cl1-)

• Bases produce hydroxide ions (OH1-) when dissolved in water.

(NaOH → Na1+ + OH1-)• Limited to aqueous solutions.• Only one kind of base (hydroxides)• NH3 (ammonia) could not be an

Arrhenius base: no OH1- produced.

Polyprotic Acids?

• Some compounds have more than one ionizable hydrogen to release

• HNO3 nitric acid - monoprotic

• H2SO4 sulfuric acid - diprotic - 2 H+

• H3PO4 phosphoric acid - triprotic - 3 H+

• Having more than one ionizable hydrogen does not mean stronger!

Arrehnius Acids • Not all compounds that have

hydrogen are acids. Water?• Also, not all the hydrogen in an

acid may be released as ions–only those that have very polar

bonds are ionizable - this is when the hydrogen is joined to a very electronegative element

2. Brønsted-Lowry - 1923• A broader definition than Arrhenius• Acid is hydrogen-ion donor (H+ or

proton); base is hydrogen-ion acceptor.• Acids and bases always come in pairs.• HCl is an acid

– When it dissolves in water, it gives it’s proton to water.

HCl(g) + H2O(l) ↔ H3O+(aq) + Cl-(aq)

• Water is a base; makes hydronium ion.

Why Ammonia is a Base

• Ammonia can be explained as a base by using Brønsted-Lowry:

NH3(aq) + H2O(l) ↔ NH41+

(aq) + OH1-(aq)

Ammonia is the hydrogen ion acceptor (base), and water is the hydrogen ion donor (acid).

This causes the OH1- concentration to be greater than in pure water, and the ammonia solution is basic

3. Lewis Acids and Bases• Gilbert Lewis focused on the

donation or acceptance of a pair of electrons during a reaction

• Lewis Acid - electron pair acceptor• Lewis Base - electron pair donor• Most general of all 3 definitions;

acids don’t even need hydrogen!

• Summary: Table 19.4, page 592

Hydrogen Ions from Water• Water ionizes, or falls apart into ions:

H2O ↔ H1+ + OH1-

• Called the “self ionization” of water• Occurs to a very small extent:

[H1+ ] = [OH1-] = 1 x 10-7 M• Since they are equal, a neutral solution

results from water

Kw = [H1+ ] x [OH1-] = 1 x 10-14 M2

• Kw is called the “ion product constant” for water

Ion Product Constant• H2O ↔ H1+ + OH1-

• Kw is constant in every aqueous solution:

[H+] x [OH-] = 1 x 10-14 M2

• If [H+] > 10-7 then [OH-] < 10-7

• If [H+] < 10-7 then [OH-] > 10-7

• If we know one, other can be determined

• If [H+] > 10-7 , it is acidic and [OH-] < 10-7

• If [H+] < 10-7 , it is basic and [OH-] > 10-7

– Basic solutions also called “alkaline”

The pH concept – from 0 to 14

• pH = pouvoir hydrogene (Fr.) “hydrogen power”

• definition: pH = -log[H+]• in neutral pH = -log(1 x 10-7) = 7• in acidic solution [H+] > 10-7

• pH < -log(10-7)– pH < 7 (from 0 to 7 is the acid range)– in base, pH > 7 (7 to 14 is base range)

Calculating pOH

• pOH = -log [OH-] • [H+] x [OH-] = 1 x 10-14 M2

• pH + pOH = 14• Thus, a solution with a pOH less

than 7 is basic; with a pOH greater than 7 is an acid

• Not greatly used like pH is.

pH and Significant Figures

For pH calculations, the hydrogen ion concentration is usually expressed in scientific notation

[H1+] = 0.0010 M = 1.0 x 10-3 M, and 0.0010 has 2 significant figures

the pH = 3.00, with the two numbers to the right of the decimal corresponding to the two significant figures

Ex:

• What is the pH of a solution with a hydrogen ion concentration of 1 x 10 -5

• What is the pOH of a solution with a hydrogen ion concentration of 1 x 10 -10

• What is the pH of a solution with a hydrogen ion concentration of 4.2 x 10-10

• The pH of an unknown solution is 6.35. What is the hydrogen – ion concentration?

Acid-Base Reactions

Acid + Base Water + Salt Properties related to every day:

–antacids depend on neutralization– farmers adjust the soil pH– formation of cave stalactites–human body kidney stones from

insoluble salts

Acid-Base Reactions• Neutralization Reaction - a reaction

in which an acid and a base react in an aqueous solution to produce a salt and water:

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

H2SO4(aq) + 2KOH(aq) K2SO4(aq) + 2 H2O(l)

– Table 19.9, page 613 lists some salts