Transcript
Page 1: OFB Chapter 6 Condensed Phases and Phase Transitions

04/19/23 OFB Chapter 6 1

OFB Chapter 6

Condensed Phases and

Phase Transitions

6-1 Intermolecular Forces: Why Condensed Phases Exist

6-2 The Kinetic Theory of Liquids and Solids

6-3 Phase Equilibrium

6-4 Phase Transitions

6-5 Phase Diagrams

6-6 Colligative Properties of Solutions

6-7 Mixtures and Distillation

6-8 Colloidal Dispersions

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Intermolecular Forces: Why Condensed Phases Exist

• Intramolecular Forces– Chemical bonds

• Strong• Directional• Short Range (relative)

• Intermolecular Forces• Weaker than chemical bonds, usually much

weaker• Less directional than covalent bonds, more

directional than ionic bonds• Longer range than covalent bonds but at

shorter range than ionic bonds

• Condensed Phases– Solids and Liquids– Intermolecular forces: mutual

attractions hold the molecules closer together than gases

• Potential Energy Curves– Distinction between intramolecular and

intermolecular Forces

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Potential Energy

• At Rest

• Stretched, then relaxed

• Compressed, then relaxed

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Potential Energy Curves

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Potential Energy Curves

• Atoms or molecules approach at large distance (zero PE (energy of position)

• PE goes negative as intermolecular forces come into play

• PE minimum energy well is a balance of Intermolecular forces and repulsive forces

• PE goes positive as repulsive forces dominate

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Types of Non-Bonded (Intermolecular) Attractions

1. Dipole-Dipole Interactions – e.g., Hydrogen Bonding

2. Ion-Dipole Interactions

3. Induced Dipole Attractions

4. Dispersive Forces • temporary fluctuations in

electron distribution

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Types of Non-Bonded (Intermolecular) Attractions

1. Dipole-Dipole Interactions – e.g., Hydrogen Bonding

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Types of Non-Bonded (Intermolecular) Attractions

1. Dipole-Dipole Interactions – e.g., Hydrogen Bonding

2. Ion-Dipole Interactions

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Types of Non-Bonded (Intermolecular) Attractions

1. Dipole-Dipole Interactions – e.g., Hydrogen Bonding

2. Ion-Dipole Interactions

3. Induced Dipole Attractions

Argon Atom

Argon Atom

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Types of Non-Bonded (Intermolecular) Attractions

1. Dipole-Dipole Interactions – e.g., Hydrogen Bonding

2. Ion-Dipole Interactions

3. Induced Dipole Attractions

4. Dispersive Forces • temporary fluctuations in

electron distribution

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Types of Non-Bonded (Intermolecular) Attractions

1. Dipole-Dipole Interactions – e.g., Hydrogen Bonding– Between H2O to H2O or – R-OH to H2O or – R-OH to R-OH – (R is an organic unit)

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Kinetic Theory of Liquids and Solids

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6-3 Phase Equilibrium

• Coexisting phase equilibrium– Vapor pressure

Phase: A sample of matter that is uniform throughout, both in its chemical constitution and in its physical state.

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Correcting for the Vapor Pressure of Water in “Wet Gases”

• Many chemical reactions conducted in aqueous solution often generate gaseous products

• The gas collected will be “humid” or “wet” due to the coexistence of the gas and water vapor.

• A correction is made to account for the water vapor present

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Exercise page 6-2:

Passage of an electric current through a dilute aqueous solution of sodium sulfate produces a mixture of gaseous hydrogen and oxygen according to the following equation:

2 H2O(l) 2 H2(g) + O2(g)

One liter of the mixed gases is collected over water at 25oC and under a total pressure of 755.3 torr. Calculate the mass of oxygen that is present. The vapor pressure of water is 23.8 torr at 25oC.

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For ideal gas mixtures

P of a gas is proportional to the number of moles of gas

2 H2O(l) 2 H2(g) + O2(g)

Exercise page 6-2:

atm9625.0P

atm 0.0313atm 0.9938P

PPP

gases

gases

O2

Htotalgases

atm 0.32083 /atm 0.962

P 1/3H moles 2O 1mole

O 1moleP gases

22

2

2O

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atm9625.0P

atm 0.0313atm 0.9938P

PPP

gases

gases

O2

Htotalgases

atm 0.32083 /atm 0.962

P 1/3H moles 2O 1mole

O 1moleP gases

22

2

2O

gramsm

m

Mm

n

moles

KLatm

RT

VPn

RTnVP

O

O

O

O

OO

421.032

01316.0

01316.0

)298)(082.0()0.1)(32.0(

2

2

2

2

22

2

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6-6 Colligative Properties of Solutions

• For some properties, the amount of difference between a pure solvent and dilute solution depend only on the number of solute particles present and not on their chemical identify.

• Called Colligative Properties• Examples

1. Vapor Pressure

2. Melting Point

3. Boiling Point

4. Osmotic Pressure

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Colligative Properties of Solutions

Mass percentage (weight percentage):

mass percentage of the component =

100%mass of component total mass of mixture

Mole fraction: The chemical amount (in moles) divided by the total amount (in moles)

X1 = n1/(n1 + n2)

X2 = n2/(n1 + n2) = 1 - X1

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Molality

msolute =

moles solute per kilograms solvent

= mol kg-1

Molarity

csolute =

moles solute per volume solution

= mol L-1

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Exercise 6-7:

Suppose that 32.6 g of acetic acid, CH3COOH, is dissolved in 83.8 g of water, giving a total solution volume of 112.1 mL. Calculate the molality and molarity of acetic acid (molar mass 60.05 g mol-1) in this solution.

1solute

1solute

kg mol 6.48kg 0.0838

mol 0.543

kgg

1000

g 83.8

molg

60.05

g 32.6

m

kg molsolvent kg

solute molesmMolality

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Exercise 6-7:

Suppose that 32.6 g of acetic acid, CH3COOH, is dissolved in 83.8 g of water, giving a total solution volume of 112.1 mL. Calculate the molality and molarity of acetic acid (molar mass 60.05 g mol-1) in this solution.

1solute

1solute

L mol 84.40.1121

mol 0.543

Lml

1000

ml 112.1

molg

60.05

g 32.6

c

L molsolvent volumesolute moles

c Molarity

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Ideal Solutions and Raoult’s Law

• Consider a non-volatile solute in a solvent

• X1 = mole fraction of solvent

P1=X1 P°1

Negative deviation

= solute-solvent attractions > solvent-solvent attractions

Positive deviation

= solute-solvent attractions < solvent-solvent attractions

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1. Lowering of Vapor Pressure– Vapor Pressure of a solvent above a

dilute solution is always less than the vapor pressure above the pure solvent.

2. Elevation of Boiling Point– The boiling point of a solution of a

non-volatile solute in a volatile solvent always exceeds the boiling point of a pure solvent

Ideal Solutions and Raoult’s Law

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Elevation of Boiling Point

Where m = molality

(moles of solute per kilogram of solvent)

The Effect of Dissociation

i = the number of particles released into the

solution per formula unit of solute

e.g., NaCl dissociates into i =

e.g., Na2SO4 dissociates into i =

(2 Na+ + 1 SO4-2)

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3. Depression of Freezing Point

ΔTf = − m Kf

ΔTf = − i m Kf

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Osmotic Pressure• Fourth Colligative Property• Important for transport of molecules

across cell membranes

Osmotic Pressure = Π = g d h

Π = c RT

ΠV = n RT

ΠV = i c RT

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Exercise 6-17

A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37°C. Compute the molar mass of the compound

cGiven

Strategy

lmole

lg

moleg

M Rearrange .)3

massMolar g

molethat Recall 2.)

mol/L in c the find to cRT Π use 1.)

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Exercise 6-17

A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37°C. Compute the molar mass of the compound

g/mol1.05x10mol/l 1.132x10

lg1.19

lmole

lg

moleg

M Rearrange 3.)

mol/l 1.132x10c

273.15K))(37Kmol atm L (0.0820

atm 0.0288RTΠ

c

RTΠ

cor cRT Π use 1.)

olution

33

3

11

M

S

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6-8 Colloidal Dispersions• Colloids are large particles dispersed

in solution– 1nm to 1000 nm in size– E.g., Globular proteins 500nm

• Examples– Opal (water in solid SiO2)– Aerosols (liquids in Gas)– Smoke (solids in Air)– Milk (fat droplets & solids in water)– Mayonnaise (water droplets in oil)– Paint (solid pigments in liquid)– Biological fluids (proteins & fats in

water)• Characteristics

– Large particle size colloids: translucent, cloudy, milky)

– Small particle size colloids: can be clear

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6-8 Colloidal Dispersions– Tyndall Effect

• Light Scattering

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Chapter 6Condensed Phases and Phase

Transitions

Examples / Exercises– All (6-1 thru 6-17, skip 6-18)

HW Problems– 16, 18, 22, 28, 32, 40


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