Chapter 10Chapter 10Chapter 10Chapter 10

11

Liquids, Solids,Liquids, Solids,

and Phaseand Phase

ChangesChanges

I sometimes wonder (because I’m a nerd)….

• Why does more NaCl dissolve in 100 g of water than in 100 g of gasoline?

• Why does H2O have a higher boiling point than hexane (CH3CH2CH2CH2CH2CH3)?

• How do strands of DNA stay together without being covalently or ionically bonded to one another?

• Why do water and oil not mix (much)?

33

Polar Covalent Bonds and Dipole Moments

Bonds are polar if bonding electrons are shared unequally due to

differences in electronegativity

δ-δ+

H Cl

Partial negative and partial positive

δ+ δ-

Dipole points toward more

electronegative atom

Polar Covalent Bonds and Dipole Moments

Bond polarity is determined by differences in electronegativity. How do we know if a molecule

is polar overall?

Geometry!

δ+ δ-

HCl

Polar

BF3

Non-polar

δ-

δ-

δ-

δ+

Overall Polarity

No Overall Polarity

Polar Covalent Bonds and Dipole Moments

When bond dipoles are the same magnitude, certain geometries can allow them to cancel,

resulting in non-polar molecules

This is true as

long as all of the atoms attached

to central atom are the same.

Polar Covalent Bonds and Dipole Moments

BF3

No Overall Polarity

Trigonal Planar

δ-

δ-

δ-

δ+

CF4

No Overall Polarity

TetrahedralTrigonal

Bipyramidal

No Overall Polarity

δ-

δ-

δ-

δ- δ+

PF5

δ-

δ-

δ-

δ-

δ- δ+

Polar Covalent Bonds and Dipole MomentsPolar Covalent Bonds and Dipole MomentsPolar Covalent Bonds and Dipole MomentsPolar Covalent Bonds and Dipole Moments

Higher

electron density

Since CH3Cl has a carbon attached to two

different types of atoms, the dipole will not

cancel (they have different magnitudes)

In CH3Cl bonding electrons pulled toward

more electronegative chlorine

Overall dipole moment = polar molecule

Just when we thought we have it figured out…

Are there any molecules where all atoms have the same electronegativity, but the molecule is still

polar?

O OO

Ozone is polar, but why?

Shape!

All oxygens have the same electronegativity, so it shouldn’t be

polar…but…

δ- δ-

δ+

Intermolecular Forces (IMF)

Intermolecular Forces (IMF): attractive forces betweenmolecules

431 kJ/mol

16 kJ/mol

Intermolecular forces are weaker than bonds (intramolecular forces), but have profound effects on the properties of

liquids

Intermolecular Forces

Ion-dipole (10-50 kJ/mol)

Hydrogen bonds (10-40 kJ/mol)

Dipole-dipole (3-4 kJ/mol)

(Dipole-induced dipole)

London Dispersion (1-10 kJ/mol)

Increasing

strength

Dipole-Dipole Forces

1111

Dipole-dipole forces are between polar molecules.

δ+ δ- δ+ δ-

The partial positive (δ+) of one molecule is attracted to the partial

negative (δ-) of another molecule

δ+ δ-

Hydrogen Bonds (bridges)

1212

Hydrogen bond (bridge): special type of dipole-dipole force; attractive force between a hydrogen atom bonded to a very small, electronegative atom

(F, O, N) and lone e- pair

X = F, O, N

Covalent bond. This is NOT an

H-bond

This is the H-bond

X H XMolecule 1 Molecule 2

A hydrogen bond is NOT a covalent bond!!!

Hydrogen Bridges

1313

Molecules hydrogen bond to themselves or to other molecules

Hydrogen Bonding in H2O

H-bonds

O

H

HO

H

H

Double-stranded DNA

What keeps the

strands together?

H-bonding between T,A & G,C

Hydrogen Bridging

• Which of the following pure substances will experience hydrogen bonding?

H2O

H2Se

HBr

HF

NH3

PF3

1616

Ion-Dipole Forces

1717

Ions have full charges that are attracted to the partial charge on

polar molecules (dipoles)

+ -

Ion-Dipole Forces

Ion-dipole forces explain why many ionic compounds are able to dissolve in water

London Dispersion Forces

1919

London dispersion forces: attractive forces that result from temporary shift of electrons in atoms or

molecules; present in all molecules

This shift creates an imbalance in

the electron distribution.

This is an instantaneous dipole.

Electron clouds can be

shifted around a molecule or atom through

interactions with other molecules and atoms.

Boiling Points and Dispersion forces

Boiling point increases with the size of molecules because ofincreases in London forces with larger electron clouds

2020

As the mass of an atom or a

nonpolar

molecule increases, so

does the size of

the electron cloud, which

gives more area with which to

interact with

other electron clouds.

solid

liquid

gas

Dispersion Forces

2121

Which member of each pair has the stronger London Dispersion

forces?

Ne or KrCl2 or F2SiCl4 or CH4

CH4 or CHCl3

Intermolecular Forces

• For each substance below, indicate the strongest type of intermolecular force observed.

H2O HF

HBr NH3

PF3 CH3OH

F2 CO

CO2

N22222

Trends in Intermolecular Forces

2323

Which member of each pair has stronger intermolecular forces (and higher boiling point)?

CH3OH or CH3SHCH4 or CH3CH2CH3

CO or F2CO or HF CO2 or NH3

NH3 or N2

Like dissolves like“Like dissolves like” is a good qualitative rule of thumb to determine if

one substance will dissolve in another.

Basically, it states that two substances with

similar intermolecular forces should be able to

dissolve in each other

Polar substances Polar substancesdissolve

Nonpolar substances Nonpolar substancesdissolve

Polar substances dissolve Ionic substances

In general…

Structure and Solubility

CH3CH2CH2OH

CH3CH2CH2CH2OH

CH3CH2CH2CH2CH2OH

CH3CH2CH2CH2CH2CH2OH

∞

Solubility

(mL/100mL H2O)Alcohol

0.11

0.030

0.0058

CH3CH2OH ∞

What is the trend?

Structure and Solubility

6 carbons2 carbons

The larger the carbon tail, the less soluble the alcohol in H2O

∞ 0.0058

ml/100 mLH2O

Fats/oils and water

Corn Oil

Water

Why don’t oil and

water mix very well?

What are the intermolecular forces between:

•Oil/oil

•Water/water

Notice the C/H tails

Soaps / Detergents and Intermolecular Forces

Soaps and detergents have both a polar (and ionic) and a non-polar area within

the molecule

Nonpolar

(hydrophobic)

Polar and Ionic

(hydrophilic)

Soaps / Detergents and Intermolecular Forces

Grease / Oil

Particle

Ion – Dipole

Dipole - Dipole

London Dispersion

Small spheres of soap are called micelles

Water

surrounding the micelle

Cholesterol

Heating Curve of Water

Adding energy to water usually

increases the temperature. Except during melting and boiling.

Why?To boil or melt, water must absorb a

certain amount of energy

Phase Diagram

A – B: Vaporization curve

Vaporization, condensation

A – C: Melting curve

Melting (fusion), freezing

A – D: Sublimation curve

Sublimation, deposition

solid

gas

liquid

B: Critical point

Supercritical fluid

A: Triple point

Critical Pressure

Critical Temperature

Phase Diagrams: CO2 and Water

Adding pressure will melt the solid

Adding pressure will freeze liquid

Phase Diagram

Features of a phase diagram:

Triple point: temperature and pressure at which all three phases (s, l, and g) are in equilibrium

Vaporization curve: generally, as pressure increases, temperature increases

Melting curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid

Normal melting point: melting point at 1 atm

Normal boiling point: boiling point at 1 atm

3434

Critical Temperature and Pressure

Critical point: liquid and gas phases are indistinguishable (point marking Tc and Tp)

Critical temperature, Tc : highest temperature at which a substance can exist as a liquid (cannot be liquefied) no matter how much pressure is applied

Critical pressure, Tp : minimum pressure that must be applied to bring about liquefaction at the critical temperature

I sometimes wonder (because I’m a nerd)….

• Why does more NaCl dissolve in water than in gasoline?

• Why does H2O have a higher boiling point than hexane?

• How do strands of DNA stay together without being covalently or ionically bonded to one another?

• Why do water and oil not mix (much)?