Lecture 5.1 – Intermolecular Forces and the Dissolving Process

Today’s Learning Targets• LT 4.1 – I can discuss how ionic solids dissolve due to the

random molecular motion of the water molecules. • LT 4.2 – I can identify a compound as containing

hydrogen bonding, dipole – dipole forces, and Van der Waals forces.

• LT 4.3 – I can compare and contrast various intermolecular forces and relate them to the dissolving process of a solid.

What do solids, liquids, and gases look like?

I. Solids• Atoms in a solid hold a very fixed position. • They are said to be vibrating in place.

II. Liquids• Atoms in a liquid are further spread out from

one another.• They flow freely within the solution and are

moving in a random motion.

III. Gases• Atoms in a gas are in constant random

motion.

Most forces

Least forces

What are intermolecular forces?

I. Intermolecular Forces• Intermolecular forces are forces that attract molecules

to one another. • The three main types of intermolecular forces are:

(1) Hydrogen Bonding(2) Dipole-Dipole Forces(3) Dispersion Force (also known as Van der Waals

Forces)

II. Dipoles• A dipole is a separation of charge. • Molecules can have a separation of charge due to

electronegativity• This causes the electrons to be held closer to the

element with the higher electronegativity.

H F

FH

Pulls electrons towards itself because it has a higher electronegativity

Partial negative charge

δ-δ+

This is what a dipole looks like

III. Dipole-Dipole Forces• Dipole-dipole forces refer to the attraction

between molecules that have a permanent dipole.

What Dipole-Dipoles Look Like

Partial negative attracted to the partial positive

III. Dispersion Forces• Some molecules have an induced dipole. • Dispersion forces are the intermolecular

forces resulting from the uneven distribution of electrons and the creation of temporary dipoles.

• Weakest intermolecular force

There is no electronegativity difference between two hydrogens, so they are non-polar and do not have a dipole

But, when two H2 molecules are put next to one another, a dipole is induced because electrons rearrange themselves

What the Heck?

IV. Hydrogen Bonding• Hydrogen bonding is the attraction of one molecule

that contains hydrogen to another molecule. • When hydrogen is bound to N, O, or F, there is a

large difference in electronegativity • Creates a partial positive and negative charge• The partial positive charge on hydrogen is attracted

to N, O, or F of another molecule.

Hydrogen bond between H and O

V. Intermolecular Forces and Phase of Substance

• The degree of intermolecular forces determines the phase of a substance.

Least intermolecular forces

Most intermolecular forces