The Nature of Solutions

Chapter 18 – Solutions

Chapter 17.3 Aqueous Solutions

Definitions

•• Solution Solution -- homogeneous mixture

Solvent Solvent - present in

greater amount, doing the dissolving

Solute Solute - substance being dissolved

Types of Solutions-17.4

Phase Solute Solvent Example

Gas

Liquid

Liquid

Solid alloys

marshmallow

Kool-aid

antifreeze

soda

airgas

liquid

liquid

liquid

solid

solidsolid

gas

solid

liquid

gas

gas

Liquid

Solid

B. Solvation

•• Solvation Solvation –– the process of dissolving

solute particles are separated and pulled into solution, forming new

IMF’s between solute and solvent

solute particles are surrounded by

solvent particles, where the IMF’s

between solute molecules are broken

B. Solvation

•• DissociationDissociation

– separation

of an ionic

solid into

aqueous

ions

NaCl(s) →→→→ Na+(aq) + Cl–(aq)

B. Solvation

•• Molecular Molecular SolvationSolvation– molecules stay

intact, but are pulled apart thanks to partial charges

C6H12O6(s) →→→→ C6H12O6(aq)

B. Solvation

NONPOLAR

NONPOLAR

POLAR

POLAR

“Like Dissolves Like”““Like Dissolves LikeLike Dissolves Like””

B. Solvation

•• Soap/DetergentSoap/Detergent– Long nonpolar “tail” with polar “head”

– dissolves nonpolar grease in polar water

C. Solubility

SATURATED SOLUTION

no more solute dissolves

UNSATURATED SOLUTIONmore solute dissolves

SUPERSATURATED SOLUTION

Too much dissolved*unstable*

increasing concentration

Solubility Table

LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World , 1996, page 517

shows the dependence

of solubility on temperature

Solubility of Sodium Acetate

Temperature (oC)

025 50 75 100

So

lub

ility

(g/1

00

g H

2O

)

50

100

150Supersaturated

solution

Unsaturated solution

Saturated

Charles H.Corwin, Introductory Chemistry 2005, page 378

Solubility and Solvation Factors

Factors that affect Solvation

• Solvation-the process of dissolving

• We are looking for factors that affect

rate…not amount

• Solvation only occurs when and where the

solute and solvent particles come into contact

with one another

• So how do we increase the contact?

Would it dissolve faster if

• the mixture was shaken?

– Agitation increases solvation because

– ________________________

– ________________________

Would it dissolve faster if I

• Tried to dissolve a sugar cube or powdered

sugar?

– Surface area increases solvation because

– ________________________

Would it dissolve faster if I…

• Heated the mixture, or cooled the mixture?

– Heat increases solvation because

– ________________________

– ________________________

– ________________________

Solubility Factors

• Factors that affect the amount of solute

dissolved

Pressure

• When we open a soda bottle, what happens?

• What really happens….

– _________________________

– _________________________

• Henry’s Law

– ________________ proportional

Temperature

0 10 20 30 40 50 60 70 80 90 100

Solubility vs. Temperature

So

lub

ility

(g

ram

s o

f so

lute

/10

0 g

H2O

)

KI

KCl

20

10

30

40

50

60

70

80

90

110

120

130

140

100

NaNO3

KNO3

HCl NH4Cl

NH3

NaCl KClO3

SO2

•Generally, higher temperature

leads to higher solubility for

liquids and solids

•However, higher temperature

leads to lower solubility for gases

Why?-As liquids/solids warm,

they are easier to break-apart

and dissolve. When gases warm

up, it is easier to escape and not

dissolve.

gases

solids

18.2 + 18.4- Math

• Percent Mass/volume (m/v %)

• Percent by Volume (v/v %)

• Molarity (M)

• Molality (m)

• Dilution

• Stoichiometry

Mass Percent

• Percent (Mass/volume)=

Mass of solute (g) x100%

Solution volume (mL)

Percent by Volume

• Percent (volume/volume)=

volume of solute x100%

Solution volume

Be sure that both volume units are the same.

Molarity (M)

• Molarity=

moles of solute

Liters of Solution

• No need to multiply by 100 to get a %

• Convert grams to moles if needed

• 1 L = 1000mL, so divide mL by 1000 to get to Liters

• You know when moles are mentioned, stoichiometry is not far behind

Molality (m)

• Molality=

moles of solute

kg of solvent

• No need to multiply by 100 to get a %

• Multiply kilograms by 1000 to get grams

• Divide grams by 1000 to get kilograms

• We’ll need this when we work with colligative properties.

Colligative Properties

18.3 + 18.4

Definition

• Physical properties of liquids affected by the

number of solute particles dissolved, not the

identity of the solute

• “depending on the collection”

Vapor Pressure

• _____________________________________

• The more solute particles, the less the vapor

pressure from the solvent

Vapor Pressure

Boiling Point Elevation

and

Freezing Point Depression

Boiling Point Elevation

• When is this used?

– _________

– _________

• Why do solute particles raise the boiling

point?

Freezing Point Depression

• When is this used?

– _________

– _________

• Why do solute particles lower the freezing

point?

The math

• Freezing Point Depression=ΔTf

ΔTf=iKfm

i= number of aqueous solute particles

Kf=freezing point constant for solvents, water = 1.86

m= molality of solution

The math

• Boiling Point Elevation=ΔTb

ΔTb=iKbm

i= number of aqueous solute particles

Kb=boiling point constant for solvents, water = 0.512

m= molality of solution

What is the freezing point depression of a solution containing 478 g of sodium chloride

in 3202 g of water?

Why is CaCl2 used as rock salt?