Lecture 4.2 – 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.

Watch Mr. Astor Cook Pepperonis• Why do pepperoni pizzas taste so

delicious?• What substance is creating the taste?

Explanation for Deliciousness• Fats are basically long carbon chains, and how well they

pack at low temperatures is determined by how long they are and how many kinks they have in the chain from C=C double bonds. Liquid fats (like oils) have lots of kinks (also called unsaturations) that prevent it from packing tightly when cold, so they stay liquid.

• When you cook pepperoni, these fats are effectively rendered - they melt, dissolving out some of the colour and flavour compounds from the meat and spices. The fat acts as a nonpolar organic solvent, drawing out some nonpolar compounds in the meat you may find delicious. Lots of flavorful substances dissolve in fat better than in water, like capcaisin (from chili peppers) and cinnamic acid (from cinnamon).

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)

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

SUMMARIZE

White Board Races

White Board Race Questions• Draw what atoms in a solid state look like.

What type of movement do they have?• What state has the least intermolecular

forces?• What type of bonds does solution of HF

have?• C12H24 has no permanent dipoles, what

type of intermolecular forces does it have?

White Board Races• HCl looks like this:

• What type of intermolecular forces exist in a solution of HCl?

• What is the strongest intermolecular force?

ClHδ-

δ+

Let’s talk Wikispace

• Units 1, 2, and 3 need to be finished to completion.• This means every lesson from Units 1, 2, and 3

must have a video, script, and picture.• Unit 3 must have all of the Kool-Aid Molarity Lab

including the excel graph portion!• Need help with microsoft excel? Check out this “

Microsoft Excel and Chemistry” document.

Microsoft Excel and Chemistry

Lab Report!

Lab Report!You will be writing a formal lab report on the lab “Lab 6 – What is the Concentration of Kool Aid”. This lab report will be divided into four sections:• Introduction• Materials and Procedures• Results• Discussion

Introduction Section

• Contains:– 1 paragraph providing background information for

the lab (what are the LARGE chemistry concepts covered?)

– 1 paragraph stating the purpose of the lab and how it ties into the background

– 1 sentence hypothesis in an “If ______, then ______ because ___________” format

• Make sure to write in 3rd person

Discussion Section

• Contains:– 1 paragraph regarding the conclusions you drew

from the lab– 1 paragraph per post-lab question– 1 paragraph discussing sources of error:• Human error: Error from your inaccuracy• Experimental error: Problems with lab design

• Make sure to write in 3rd person

Closing Time• Homework 4.1• Wikispaces• Formal Lab Report