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?

Which one has the most forces between it? Which one has the least?

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.

Least forces Most forces Which one has the most forces between it? Which one has the least? Most 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.

Pulls electrons towards itself because it has a higher electronegativity δ- δ+ Partial negative charge H F 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

But, when two H2 molecules are put next to one another, a dipole is induced because electrons rearrange themselves There is no electronegativity difference between two hydrogens, so they are non-polar and do not have a dipole

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