Intermolecular Forces

What is the difference between:INTER INTRA

Inter-molecular are forces between molecules and intra are forces within a molecule.

These are the forces that hold our world together!

What is the difference?

IMFs can be used to determine characteristics of a substance including the state of matter.

Intermolecular Forces (IMF)

1. Macro-covalent2. Metallic3. Hydrogen4. Ionic5. Dipole-Dipole6. Dispersion

Intermolecular Forces (IMF)

• The strongest of them all.• Examples:

Diamond (all Carbon atoms)Rocks (Silicon and Oxygen)

• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricity• Solids

1. Macro-Covalent

diamond graphite

carbonatoms

1. Macro-CovalentAllotropes:

Crystallinequartz (SiO2)

Non-crystallinequartz glass

Solids at room temperatureSTRONG Intermolecular ForcesMetal Atoms give up electronsThis creates many dipoles throughout the

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

2. Metallic

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.

Liquids

3. Hydrogen Bonding

3. Hydrogen Bonding

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

3. Hydrogen Bonding

Very strongPermanent strong charge interactionsUsually crystal in formation (solids)Very high melting points

4. Ionic

5. Dipole-DipoleCaused by attraction of positive end of one polar molecule to negative of another polar molecule.

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.

Molecular Polarity

Attractive forces between polar molecules

Liquids

Orientation of Polar Molecules

5. Dipole-Dipole

Get stronger with increase in mass

Gas

Only attractive force between non-polar molecules (symmetrical or those with only non-polar bonds) or single atoms.

6. London Dispersion Forces

6. London Dispersion Forces

Momentary Dipole

H2O

CCl4

CH2O

NH3

Hg

MgCl2

Practice – Write phase and IMF for each

H2O – Hydrogen Bonding - Liquid

CCl4 – London Dispersion Forces - Gas

CH2O – Dipole-dipole forces - liquid

NH3 – Hydrogen Bonding - Liquid

Hg – Metallic - Solid

MgCl2 – Ionic - Solid

Practice – Write phase and IMF for each

- WS 8.4

- Check webgrader - If I don’t have it by Friday it will not

go in your grade

Homework

Wednesday 4/11 and

Thursday 4/12

Surface Tension

Volatility

Capillarity

Solubility “Like dissolves like”

Boiling and Melting Point

Physical Properties Affected by IMFs

VolatilityAbility to evaporate• High when attractions are weak. Why?Compare volatility of alcohol and water.

CapillarityAbility to climb up a tube or surface• High when liquid molecules are attracted to

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

Affects of Intermolecular Forces

Capillary Action

Solubility

How does a snow globe work?

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!

Basilisk Lizard

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

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 Agents• Non-polar hydrophobic (water-hating) tails and

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

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

Ionic solid dissolving in water