Intermolecular Forces

Forces Between Molecules

Why are intermolecular forces important?

They determine the phase of a substance at room temperature. (Competition with kinetic energy.)

Solids

Strong Intermolecular Forces

Gases

Weak Intermolecular Forces

3 Types of Intermolecular Forces

Dispersion or Van der Waals Dipole-dipole

Hydrogen Bonds

Strongest Intermolecular Force

Hydrogen Bonds

Hydrogen Bonds

When do Hydrogen bonds occur?

Between molecules containing F, O, N bonded to a

H atom

Strongest hydrogen bonds?

Between molecules containing F-H bonds

Weakest hydrogen bonds?

Between molecules containing N-H bonds

What effect does H-bonding have on the properties of the

substance?

Hydrogen bonding leads to substantial increases in the

expected boiling point.

Weakest Intermolecular Force

Dispersion or Van der Waals forces

When do dispersion forces occur?

Occur between all molecules. Most important between

nonpolar molecules

Nonpolar

No Poles!The molecule is symmetric!

Cannot tell 1 end from the other.

2 Easy categories of Nonpolar Molecules

Monatomic Gases (Column 18)(kickballs)

&Diatomic Elements

(H2, N2, O2, F2, Cl2, Br2, I2)(footballs)

2 More Easy categories of Nonpolar Molecules

Hydrocarbons(CxHy)

&Small symmetric molecules(CO2, CS2, CF4, CBr4, etc.)

What are the diatomic elements?

H2, N2, O2, F2, Br2, Cl2, & I2

What can you say about dispersion forces as a

function of molecular size?

The larger the molecules the stronger the dispersion

forces!

When do dipole-dipole forces occur?

Between polar molecules.Permanent separation of charge.

1 side - (electron rich) vs. 1 side + (electron poor)

Polar

Has Poles!The molecule is NOT

symmetric!Can tell 1 end from the other.

Vapor

Gas phase of a substance that is normally a liquid at room

temperature.

Vapor Pressure

The partial pressure of a vapor above its liquid.

What does vapor pressure depend on? (Hint: see Table

H)

Vapor pressure depends on the temperature of the liquid.

What happens to the vapor pressure as the temperature of

the liquid is increased?

The vapor pressure always increases with temperature.

Does the vapor pressure depend on the amount of

liquid present?

No. A thimbleful is as good as an ocean.

What are some differences between evaporation &

boiling?

Evaporation occurs at all temperatures. Boiling occurs at a

definite temperature. Evaporation occurs at the surface.

Boiling occurs throughout the liquid.

Heat of fusion

Amount of energy required to convert 1 gram of a pure solid to

the liquid phase at its melting point.

Heat of vaporization

Amount of energy required to convert 1 gram of a pure liquid to the gas phase at its boiling

point.

What happens to the boiling point as the intermolecular

forces increase?

The stronger the intermolecular forces, the higher the boiling point.

What happens to the melting point as the intermolecular

forces increase?

The stronger the intermolecular forces, the higher the melting point.

What happens to the heat of fusion as the intermolecular

forces increase?

The stronger the intermolecular forces, the higher the heat of fusion.

What happens to the heat of vaporization as the

intermolecular forces increase?

The stronger the intermolecular forces, the

higher the heat of vaporization.

What happens to the evaporation rate as the intermolecular forces

increase?

The stronger the intermolecular forces, the

lower the evaporation rate.

What happens to the vapor pressure as the intermolecular

forces increase?

The stronger the intermolecular forces, the lower the vapor pressure.

Pre

ssu

re,

atm

Temperature, C

Relationship between IMF and physical properties?

IMF BP MP Hf HvEv.Rt V.P.

IMF BP MP Hf Hv EvRt VP

Boiling Point

Temperature at which the vapor pressure of a liquid is

equal to the external pressure.

Normal Boiling Point

Temperature at which the vapor pressure of a liquid is

equal to 1 atm or 101.3 kPa or 760 torr.

What happens to the boiling point when the external

pressure is reduced?

The boiling point is reduced.

Sublimation

Solid to gas

Deposition

Gas to solid

Vaporization

Liquid to Gas

Condensation

Gas to liquid

Fusion

Solid to liquidOr

Melting

Freezing

Liquid to solidCrystallizationSolidification

Which phase changes are endothermic?

Solid to liquidLiquid to GasSolid to Gas

G

L

S

Which phase changes are exothermic?

Gas to LiquidLiquid to SolidGas to Solid

G

L

S

Given 3 substances at room temperature: 1 solid, 1 liquid, & 1 gas, which has the strongest intermolecular forces?

Solid > Liquid > Gas

Describe the mp & bp of gases.

Low!

What happens to the temperature as heat is added to a substance at its melting or boiling point?

Nothing, until the phase change is complete. The heat

energy is going into the potential energy of the

system.

HF

HF

--------

Hydrogen Bonding!

Ne

Ne

--------

Van der Waals or dispersion forces

H-Cl

H-Cl

--------

Dipole-dipole forces

H-N

H-F

--------

Hydrogen Bonding

H H--

OH

H

-

-----------

----

---

Time

Tem

pera

tur

e

I II III IV V

Solid

Solid & Liquid

Liquid

Liquid & Gas

Gas

K.E. K.E. K.E.

K.E.↔K.E.↔P.E.↔

P.E.↔P.E.↔

P.E.P.E.Melt pt.

Boil pt.

Equation for a pure substance in a single phase being warmed up or cooled down.

Q = mCTQ = energy in Joulesm = mass in grams

C = specific heat in J/gT = temperature change

Equation for a substance melting or freezing at the freezing point.

Q = mHf

Q = energy in Joulesm = mass in gramsHf = heat of fusion

Equation for a substance boiling or condensing at the boiling point.

Q = mHv

Q = energy in Joulesm = mass in grams

Hv = heat of vaporization

Temperature of the ice-water equilibrium.

0C or 273KFreezing pointMelting point

Temperature of the steam-water equilibrium.

100C or 373KBoiling point

Condensation point