Lecture 8.4c- Intermolecular Forces

Bellwork- Polar ammonia

1)Draw a 3D sketch of ammonia (NH3)

2)What is the shape of this molecule?

3)Why does the molecule have this shape?

4)Show the bond and molecule polarity on the sketch.

5)Would this molecule be polar if it was planar?

Know your pet molecules!

CH4 NH3 H2O CO2 CH2OI may replace H’s with Cl or F to make polar bonds!

Intermolecular forces are forces between two or more molecules

Intermolecular forces

Intermolecular attractions are weaker than either ionic or covalent bonds.

These attractions determine whether a molecular compound is a gas, a liquid, or a solid at a given temperature.

Intermolecular Forces

dipole-dipole interactions occurbetween polar molecules

London Dispersion Forces occur between all molecules (polar and non-polar)

There are two types of intermolecular forces

The partial charges in polar molecules cause attractions and repulsions between separate polar molecules

A hydrogen bond is a specific type of dipole-dipole interaction that is extra

strong

DIPOLE-DIPOLE INTERACTIONS

HYDROGEN BOND

A molecule can H-bond if it contains hydrogen bound to an atom with high electronegativity (N, O, or F)

IN THIS CASE- The especially strong partial charge (δ+) on hydrogen makes it “stick” to anything negative.

Hydrogen bonding

N O F

No one in the corner grabs electrons like us! – N, O, & F

London Dispersion Forces are the weakest of all IMFs. They occur

between all molecules.

London Dispersion Forces

A covalent molecule consists of atoms bound by overlapping electron clouds.

Sometimes these electron clouds have temporary “thin or thick spots” causing temporary partial charges.

These temporary partial charges can cause temporary partial charges on other molecules or atoms.

London Dispersion Forces

Temporary partial charges experience attractions and repulsions just like permanent partial charges and ion charges.

London Dispersion Forces

The larger the molecule the larger the London Dispersion

Forces

What is the difference between the

partial charges on a polar molecule

(which cause dipole-dipole interactions)

and the partial charges

that cause London Dispersion attractions?

Getting weak

The stronger the intermolecular forces the more likely the compound

will be a solid at room temperature.

Compounds with weak intermolecular forces have low boiling and melting points.

Strong intermolecular forces require a lot of energy to overcome, so compounds with strong intermolecular forces have high

melting and boiling points.

Liquids with strong intermolecular forces will hold on to their molecules preventing

them from escaping as a vapor.

Molecular Solid- Strong intermolecular forces hold molecules in a rigid & regular pattern.

Molecular Liquid- Some intermolecular forces are overcome and molecules can slip past each other.

Molecular gas- Weak intermolecular forces cant hold molecules together, so each molecule is free and independent.

states of matter

Gases have weak intermolecular forces that were easy to break free of!

Intermolecular forces

Weakest lower melting point

Dispersion forces

Dipole-dipole interactions

Hydrogen bonds

STRONGEST higher melting point

Ionic compounds have even higher melting points because the ionic bond must be

overcome to free the ions. Ionic bonds are much stronger than any intermolecular force.

Intermolecular Forces

Network solid (Network crystal)- atoms are held together in a ordered pattern by strong covalent bonds.

–Melting a network solid would require breaking covalent bonds throughout the solid.

Diamond is a network solid.

Diamond does not melt.

It vaporizes to a gas at 3500°C or above.

TYPES OF SOLIDS

ionic solids – made of ions held together by ionic bonds

molecular solids- made of molecules held together by intermolecular forces

metallic solids- atoms held together by metallic bonds(sea of electrons model)

network solids- atoms held together by covalent bonds

ClassworkFill in the table

Intermolecular force

Molecules that experience it

Relative strength

dispersion forces

dipole-dipole interactions

hydrogen bonds

-N-H» F-H»

-O-H»

ClassworkFill in the table

Intermolecular force

Molecules that experience it

Relative strength

dispersion forces Non-polar molecules

weak

dipole-dipole interactions

Polar molecules stronger

hydrogen bonds

-N-H» F-H»

-O-H»

Polar molecules with N, O or F bound to H

Strongest of all intermolecular forces

Would you expect the following solids to have a relatively high or low melting point?

Metal

Ionic solid

Non-polar molecular solid

Polar molecular solid

Atomic solid of a noble gas

Network solid