Condensed Phases and

Intermolecular Forces

Fundamentals

How do particle diagrams of liquids & solids compare to those of gases?

Describe the relative positions and motions of particles in each of the 3 phases.

The Question

Why do some substances exist as gases at room temperature, some as liquids, and some as solids?

2 kinds of forces

Part of the answer has to do with the forces between separate molecules.

There are 2 broad categories of forces you need to be aware of.

Intramolecular ForcesIntramolecular forces = attractive

forces that hold particles together in ionic, covalent, or metallic bonds.

Intra means “within.”

Intramolecular forces = bonding forces.

Intermolecular Forces

Inter means “between” or “among.”Intermolecular forces are forces

between molecules.Intermolecular forces are weaker than

intramolecular forces.

There are 3 types of intermolecular forces.

Intermolecular forces determine phase.Intermolecular forces determine phase.

“Competition” between strength of intermolecular forces and kinetic energy determines phase.

If intermolecular forces are strong, substance will be a solid or liquid at room temperature. Particles want to clump together.

If intermolecular forces are weak, substance will be a gas at room temperature. Particles spread apart.

It’s a balancing act!

Intermolecular Forces

Kinetic Energy

This substance = a gas at room temperature.

Intermolecular Forces vs. Kinetic Energy

Intermolecular Forces

Kinetic Energy

This substance = a condensed phase.

Why Temperature Changes Affect Phase

Since temperature is a measure of avg kinetic energy, changing temperature changes phase.

Changing temperature changes the average kinetic energy

Changing the temperature

Intermolecular Intermolecular ForcesForces

KineticEnergy

Intermolecular Forces

A few % the strength of the intramolecular or bonding forces

Account for phase at room temperature.Account for phase at room temperature.

Strong intermolecular forces condensed phase.

Weak intermolecular forces gas phase

Intermolecular Forces & Phase

Strong Intermolecular Forces = SolidsSolids have high melting points, high

boiling points.

Weak Intermolecular Forces = GasesGases have low melting points, low boiling

points.

3 Types of intermolecular forces

Dispersion ForcesDispersion Forces: These are the weakest. Occur between all molecules but are most important for nonpolar molecules.

Dipole-dipole forcesDipole-dipole forces: These are intermediate. Occur between polar molecules.

Hydrogen bondsHydrogen bonds: These are the strongest. Occur between molecules containing an H-F, H-O, or H-N bond.

Dispersion Forces

Instantaneous and momentary; fluctuating. Results from motion of electrons. Avg = fig. a.

If the charge cloud is not symmetrical it will induceinduce an asymmetry in its neighbor’s charge cloud!

Nonpolar molecules

NonNonpolar means nono poles. Can’t tell one end of the molecule from the other.

Symmetrical.

Examples of Nonpolar Molecules

Monatomic gas molecules: He, Ne, Ar, Kr, Xe, Rn

Diatomics where both atoms are the same element: H2, N2, O2, Cl2, F2, I2, Br2

Larger molecules if they are very symmetrical molecules: CH4, C2H6, C3H8

Dispersion Forces and Size

Dispersion forces increase with the size of the molecule.

The larger the electron cloud, the greater the fluctuations in charge can be.

Rn > Xe > Kr > Ar > Ne > HeI2 > Br2 > Cl2 > F2

C8H18 > C5H12 > C3H8 > CH4

Boiling point of N2 is 77 K. Intermolec-ular forces are very weak dispersion forces.

Dipole-dipole Forces & Polar Molecules

Molecule shows permanent separation of charge. It has poles – one end is somewhat negative & one end is somewhat positive.

Polar means the molecule has poles, +’ve & -’ve. The geometry and the charge distribution are not symmetrical.

Polar Molecules

Source: http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter10/Text_Images/FG10_01-01UN.JPG

What do you know about charge?

Opposites Attract!

This time, the situation is permanent!

Examples: HI, CH3Cl

Hydrogen Bonding

Occurs between molecules containing an H-F, H-O, or H-N bond. (FON!!!)

Hydrogen Bonding

Hydrogen bonding is the extreme case of dipole-dipole bonding.

F, O, and N are all small and electronegativeelectronegative. They really attract electrons. H has only 1 electron, so if it’s spending time somewhere else, the proton is almost “naked.”

The H end is always positive and the F, O, or N is always negative.

The bonding electrons spend more time by the oxygen atom than by the hydrogen atoms.

Oxygen end – bit negative

Hydrogens – bit positive Source: http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/notes/chpt2/water.gif

Hydrogen Bonding in Water

Source: http://images.google.com/imgres?imgurl=http://food.oregonstate.edu/images/learni/w6.jpg&imgrefurl=http://food.oregonstate.edu/learn/water.html&h=231&w=216&sz=19&tbnid=niAcgd-QCTMJ:&tbnh=103&tbnw=96&hl=en&start=7&prev=/images?q=hydrogen+bonding+in%

Source: http://images.google.com/imgres?imgurl=http://food.oregonstate.edu/images/learni/w6.jpg&imgrefurl=http://food.oregonstate.edu/learn/water.html&h=231&w=216&sz=19&tbnid=niAcgd-QCTMJ:&tbnh=103&tbnw=96&hl=en&start=7&prev=/images?q=hydrogen+bonding+in%

Hydrogen bonding is the strongest intermolecular force and influences the physical properties of the substance a great deal.

Result of Hydrogen Bonding

Increased Boiling Point over predicted value based on molecular size & comparison with similar compounds.

Strength of Hydrogen Bonding

Fluorine is the most electronegative element, so H-F bonds are the most polar and exhibit the strongest hydrogen bonding.

H-F > H-O > H-N

Intermolecular Forces vs.

Physical Properties If intermolecular forces increase, Boiling point Melting point Heat of Fusion Heat of Vaporization Evaporation Rate Vapor Pressure

Intermolecular Forces vs. Temperature

Intermolecular forces become more important as the temperature is lowered.

EX: Low temperature – low evaporation rate High temperature – high evaporation rate

Indicate the kind of intermolecular forces for each molecule below.NH3ArN2HClHFNeO2HBrCH3NH2

• Hydrogen bonding• Dispersion forces• Dispersion forces• Dipole-dipole forces• Hydrogen bonding• Dispersion• Dispersion• Dipole-dipole• Hydrogen bonding

ForcesForces

ForcesForces

INTERMOLECULARINTERMOLECULAR INTRAMOLECULARINTRAMOLECULAR

DispersionDispersion

Dipole-DipoleDipole-Dipole

Hydrogen BondingHydrogen Bonding

CovalentCovalent

IonicIonic

MetallicMetallic