Intermolecular Attractions

What is the difference?

What is the difference between:What is the difference between: Inter-molecular?Inter-molecular? Intra-molecular?Intra-molecular?

More solidMore solid Moves lessMoves less These are the forces that hold our These are the forces that hold our

world together!world together!

Intermolecular Forces

Macro-covalentMacro-covalent IonicIonic MetallicMetallic HydrogenHydrogen Dipole-DipoleDipole-Dipole DispersionDispersion

Macrocovalent

The strongest of them all.The strongest of them all. Diamond (all Carbon atoms)Diamond (all Carbon atoms) Rocks (Silicon and Oxygen)Rocks (Silicon and Oxygen) See Toy ExamplesSee Toy Examples

Covalent• Lattice points occupied by atoms• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricity• Examples

diamond graphite

carbonatoms

Crystallinequartz (SiO2)

Non-crystallinequartz glass

Ionic

Very strongVery strong Permanent strong charge interactionsPermanent strong charge interactions Usually crystal in formationUsually crystal in formation Very high melting pointsVery high melting points See toy example.See toy example. Example of Latticeof Lattice

Metallic Bonding

Solids at room temperatureSolids at room temperature This means STRONG Intermolecular ForcesThis means STRONG Intermolecular Forces Metal Atoms give up electronsMetal Atoms give up electrons This creates many dipoles throughout the This creates many dipoles throughout the

atoms, they SHIFT but are always thereatoms, they SHIFT but are always there ““SEA of electrons”SEA of electrons”

Hydrogen Bonding

A strong intermolecular attractionsCaused by very strong dipole-dipole

attraction between molecules with N-H, O-H, and F-H bonds.

Responsible for many of water’s special properties.

Hydrogen Bonding

Hydrogen Bond

The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. IT IS NOT A BOND.

A H…B A H…Aor

A & B are N, O, or F

Dipole-Dipole (can you think of an example of this?)

Dipole-Dipole Attraction

Caused by attraction of end of one polar molecule to of another polar molecule.

For a molecule to have dipoles it must be polar:A. Asymmetrical in shape

and B. With polar bonds

Molecular Polarity

In order for a molecule to be polar it must have polar bonds and be asymmetrical in shape. Lone pairs always lead to asymmetry.

Linear, trigonal planar, and tetrahedral are symmetrical shapes and dipoles will cancel if all bonds are equal.

Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

Dispersion Forces (non polar molecules)

““Moving the Fat Around”Moving the Fat Around” Caused by momentary (temporary) Caused by momentary (temporary)

dipoles (pg. 444)dipoles (pg. 444) Get stronger with increase in massGet stronger with increase in mass Only attractive force between Only attractive force between non-polar non-polar

moleculesmolecules (symmetrical or those with (symmetrical or those with only non-polar bonds) or single atoms.only non-polar bonds) or single atoms.

London Dispersion Forces

Force that exits among noble gas atoms and nonpolar molecules

Can create a temporary dipole moment

Momentary Dipoles

Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights

Substance FW (g/mol) mp (°C) bp (°C)

F2 38 -220 -188

Cl2 71 -100.98 -34.6

Br2 160 -7.2 58.78

I2 254 113.5 184.35

Boiling point is a true measure of the strength of intermolecular attractions. Why?

Let’s look at the effect of size on nonpolar molecules.

Conclusion?

Boiling Point of Different Hydrocarbons

Melting Points and Boiling Points of Substances with Similar Formula Weights

Substance FW (g/mol) mp (°C) bp (°C)

F2 38 -220 -188

NO 30 -164 -152

CH3OH 32 -94 65

Ca 40 893 1484

NaF 42 993 1695

Let’s compare strength of intermolecular attractions in molecules of the same size.

F2 = nonpolar, NO = dipole-dipole, CH3OH = hydrogen bonding, Ca = metal, NaF = ionic.

What does that say about the relative strength of these attractions?

State of Matter-(at room temp)• Ionic Compounds = Always Solid

• Covalent Compounds-Solid, liquid or gas

Depends on size and polarity.

Gas small and non-polar or dipole-dipole

Liquid small & H-bonding, or larger with non-polar or d/d

Solid large and polar, or very large and non-polar.

Practice – Label each with Phase and type of IMF

SiO2

FeCCl4

NOSO2

C2H6

C10H22

Hg

O3

CO2

N2

C(diamond)

C2H5OH

C2H21OH

SO3

CI4

Practice – Label each with Phase and type of IMF Answers

SiO2 Solid, macrocovalent

Fe Solid, metallicCCl4 liquid, nonpolarNO gas, dipole-dipoleSO2 gas, dipole-dipole

C2H6 gas, non polar

C10H22 liquid, non-polarHg liquid, metallic

O3 gas, polar (assymetrical design)

CO2 gas, non polar (bent)

N2 gas, non-polar

C(diamond) solid, macrocovalent

C2H5OH liquid, H-bonding

C2H21OH liquid, H-bonding + dispersion

SO3 Gas, non polar (symmetrical)

CI4 gas, non polar

Boiling Point of Different Hydrides

Boiling Points of Halogen Hydrides

Boiling Point Differences

VolatilityAbility to evaporateHigh when attractions are weak. Why?Compare volatility of alcohol and water.

CapillarityAbility to climb up a tube or surfaceHigh when liquid molecules are attracted to

the tube or surface.Ex. Meniscus, paper towels, blood test

Capillary Action

Incredibly Tacky Household Products

Surface Tension

A “skin” develops on the surface of liquids.

Water’s is very strong. High when molecules

are attracted to each other cohesion.

Ex. Paper clip “floating” Water striders

Basilisk Lizard

More Surface Tension Examples

“Don’t touch the tent”Rainx

•SurfactantsSurfactants Soaps and Soaps and detergentsdetergents

Surfactants

Molecules that act to disrupt a liquid’s surface tension “wetting agent”

Structure-long non-polar hydrocarbon tail and a polar or ionic head

How do they work?Disrupt surface tension by acting like a

“wedge”

Surfactants as Cleaning AgentsNon-polar hydrophobic (water-hating) tails

and polar or ionic hydrophilic (water-loving) heads form micelles.

Soap dissolves grease by taking grease molecules into the non-polar interior of the micelle.

Micelles are carried away during Micelles are carried away during rinsing.rinsing.

Shampoo/Conditioners

Changes of State and Attractive Forces

Heating Curves

Water

Why is the plateau for boiling so much longer than for melting?

SolubilityWhat dissolves in what?I. Ionic substances dissolve in waterII. Covalent compounds:

A. Non-polar dissolves in non-polar solvents.B. Polar solutes dissolve in polar solvents.C. Partially polar (only polar in a small part of the molecule) solutes dissolve in partially polar solvents.

Like Dissolves Like!

The solute and the solvent have to be attracted to each other in order to dissolve.

Ionic solid dissolving in water