Intermolecular Forces, Intermolecular Forces, Liquids & SolidsLiquids & Solids

Intermolecular Forces, Intermolecular Forces, Liquids & SolidsLiquids & Solids

Chapter 11Chapter 11

Overview

Liquids & Solids Intermolecular Forces Liquids Phase Changes Vapor Pressure Phase Diagrams Solids -- Structure Bonding Types in Solids

Liquids & Solids Solids

• particles close together• locked into relative positions

(crystalline)• strong interactions

(interparticle forces) Liquids

• particles farther apart• mobile relative to each other• weaker interactions between

particles

Remember

Gas Liquid

+ E

- Econdensation

vaporization

Hcondensation = - Hvaporization = +

Liquid Solid

+ E

- Efreezing

melting

Hfreezing = - Hmelting = +

Intermolecular Forces

Strength of IM Forces determine boiling points and melting points

Ion-Dipole Forces• occur between ions and dipoles• between charged particles and neutral, polar

covalent particles Dipole-Dipole Forces

• occur between two dipoles • between two, neutral, polar covalent particles

Ion - Dipole Force (40 - 600 kJ/mol)

+

O

H -

+

+

Dipole - Dipole Force(5 - 25 kJ/mol)

“Hydrogen Bonding” Force (4 - 25 kJ/mol)

“Hydrogen Bonding” Forces• special case of Dipole-Dipole forces• occur between two, neutral, polar covalent

particles which– have a H atom (which is bound to an O, F or N atom) for the

(+) dipole

– have an O, F, or N atom for the (-) dipole

• extra strength due to – small size and large EN of O, F or N

– and small size of H

London Dispersion Forces• occur between all particles even neutral, non-

polar covalent particles• occur between an instantaneous dipole and

an induced dipole • force is weak but strengthens with increasing

polarizability of the particles• polarizability of the particles increases with

increasing size or mass

Dipole - Induced Dipole(2 - 10 kJ/mol)

polar molecule non-polar molecule

Induced Dipole - Induced Dipole

Force(0.05 - 40 kJ/mol)

London Dispersion

Strength of Forces

• Ion - Dipole

• “Hydrogen Bonding”

• Dipole - Dipole

• Dipole - Induced Dipole

• London Dispersion

Increasing

He 4.6

Ne 27.3

Ar 87.5

Kr 120.9

Xe 166.1Boiling Points of Noble Gases

TempK

MM

F2 85.1

Cl2 238.6

Br2 332.0

I2 457.6

Boiling Points of Halogen Diatomics

TempK

MM

H2O 373

H2S 212

H2Se 231

H2Te 271

Boiling Points of Group 6 Dihydrides

TempK

MM

GeH4 184

CH4 109

SiH4 161

SnH4

What Does it Mean? “hydration” is very important in the solvation

process & compound formation

stronger the interaction, the more energy is released, more exothermic

the strength of the interaction determines the state of the substance

unusual properties of water are due to “hydrogen bonding”

What kind of forces are between:

O2 molecules

H2O and NH3 molecules

Ne atoms

HF and NH3 molecules

CH4 and Br2 molecules

London Dispersion

Hydrogen Bonding

London Dispersion

Hydrogen Bonding

London Dispersion

Properties of Liquids

Viscosity• resistance to flow• ease with which liquid particles move relative

to one another• related to attractive forces between particles• and structural properties of the particles

themselves• decreases with increasing energy

(temperature) of particles

Surface Tension• energy required to increase the surface area of a liquid

by a unit amount• a sphere produces the minimum surface area• competition between cohesive forces vs adhesive forces

– cohesive forces tend to minimize surface area– adhesive forces tend to maximize surface area

• high surface tension reflects strong cohesive forces• capillary action -- low surface tension, strong adhesive

forces

Phase Changes Energy Changes

• melting, solid to liquid – endothermic -- Hfusion

• vaporization, liquid to gas – endothermic -- Hvaporization

• condensation, gas to liquid – exothermic -- Hcondensation = - Hvaporization

• freezing, liquid to solid – exothermic -- Hfreezing = - Hfusion

Phase Changes, cont’d

• sublimation, solid to gas – endothermic -- Hsublimation

• deposition, gas to solid – exothermic -- Hdeposition = - Hsublimation

Properties of Liquids

liquid solidfreezing -- exothermic

melting -- endothermic

liquid gasvaporization -- endothermiccondensation -- exothermic

Temp

Time

solid

liquid

gas

solid + liquid in equilibrium

liquid + gas in equilibrium

constant temp

Critical Temperature & Pressure• critical temp.

– highest temperature at which a substance can exist as a liquid

• critical pressure– pressure required to cause liquefaction at the critical temperature

• a gas cannot be liquefied above the critical temperature• critical point

– corresponds to Tc and Pc

– the point above which a supercritical gas exists– the substance cannot be liquefied by increasing the pressure

Pressure

Liquid

H2O

Vapor

Tc = 647.6 K

Critical Point

217.7 atm = Pc

Temperature

Vapor Pressure Molecular Description

• liquids have a distribution of energies for the liquid molecules

• at any temperature, some molecules have sufficient energy for vaporization

• the higher the temperature, the greater number of molecules with energy of vaporization

• at constant temperature, average energy of molecules is constant but in dynamic equilibrium

• vapor pressure is the pressure exerted by vaporized molecules when liquid and vapor states are in dynamic equilibrium

Evaporation

Equilibrium Vapor Pressure

liquid

gas

Eq. Vapor Pressure -- partial pressure of gas over a liquid at equilibrium

Volatility, Vapor Pressure & Temp.• substances with high vapor pressure are volatile • in a open container, dynamic equilibrium cannot

be established -- complete evaporation • higher the temperature, greater vapor pressure,

greater volatility Vapor Pressure & Boiling Point

• boiling point -- temperature at which the vapor pressure = atmospheric pressure

– boiling point increases with increasing external pressure, vice versa

• normal boiling point -- temp. at which the vapor pressure = 1 atm

Normal Boiling Point: temperature at which the vapor pressure is equal to 1.00 atmosphere

Boiling Point: temperature at which the vapor pressure is equal to the external or atmospheric pressure

Which has highest boiling point:

H2O or H2S

BrCl or Cl2

BrCl or HCl

CH4 or C2H6

H2O

BrCl

HCl

C2H6

molar enthalpy of vaporization(kJ/mol) x amount of substance(moles)

Hvap water = +40.7 kJ/mol @ 100º

Clausius Clapeyron Equation: ln P = - Hvap / RT + C

Calculate the Energy of Vaporization:

Phase Diagramsnormal freezing pt.

solid

liquid

normal boiling pt.

vapor

Triple Point

Pressure

Temperature

H2O

solid vapor

solid liquid

liquid vapor

Structures of Solids

Unit Cells• characteristic 3-dimensional repeating unit

of a crystalline substance • primitive cubic (sc) 1 atom/u.c.

• body-centered cubic (bcc) 2 atoms/u.c.

• face-centered cubic (fcc) 4 atoms/u.c.

primitive cubic

body-centered cubic

face-centered cubic

Close Packing of Spheres

Layered as ABABABAB or

ABCABC

Physical Properties of Solids:

solid liquidmelting -- endothermicfreezing -- exothermic

solid gas

sublimation -- endothermicdeposition -- exothermic

Melting Point: Temperature at which the crystal lattice breaks down

Hfusion Hfreezing

melting freezing

Which one of each pair will have the higher melting point?

NaF or NaI

HCl or HI

O2 or Br2

NaCl or BrCl

H2S or H2O

NaF

HCl

Br2

NaCl

H2O

•Ionic cations & anions

•Metallic metal atoms

•Molecular molecules

•Network atoms

•Amorphous irregular networks

Types of Solids:

Types Units

Network Solid

Diamond

Network Solid

Graphite

NaCl NaCl