11.1 NOTES Polarity and Intermolecular Bonding Review

The Kinetic Molecular Theory can also be applied to liquids and solids as well as gases. For gases, the KMT described the behavior of the gases in terms of particle motion. For solids and liquids, the KMT applies intermolecular forces and the impact on properties.

Forces of attraction – Inter vs. Intra Intermolecular forces are found between the molecules Intermolecular forces include London Dispersion forces, dipole- dipole forces and H-bonding Intramolecular forces are found within the molecule. Intramolecular forces are the bonds – ionic, metallic and covalent. Intramolecular forces are always stronger than intermolecular forces.

The stronger the attractive force, the closer the molecules will be with solids having the strongest forces and gases having the weakest forces.

A. Molecular Polarity – Polarity determines intermolecular forces A. Molecular Polarity – Polarity determines intermolecular forces. Let’s review… 1. Draw the Lewis structure 2. Determine bond polarity (EN difference) 3. Determine molecular polarity – if the bonds are nonpolar then the molecule must be nonpolar. If the bonds are polar, the molecule could be polar or nonpolar. To determine the polarity, consider the shape of the molecule. (If the molecule has polar bonds and the central atom has a lone pair then the molecule will be polar.)

Review Problem – Determine the molecular polarity of the following molecules: BF3 PCl3 H2O SiH4 SO2 CI4  

B. Intermolecular forces – this is a review of Chapter 9 material 1. Dispersion forces – Weak attractive forces that result from temporary dipoles. London Dispersion forces, commonly known as LDFs, are found in ALL molecules but are significant in nonpolar molecules. As the mass of a molecule increases (the number of electrons are increasing) the LDFs are also increasing in strength. Note – the (+) end of one molecule is attracted to the (-) end of the other.

Sample Problem – Using the states of the halogens, compare the strength of the LDFs present. F2 – lower mass - weak intermolecular forces – gas Br2 - liquid I2 – higher mass - stronger intermolecular forces - solid

2. Dipole-dipole forces – Attractive forces present between permanent dipoles. Since the dipole is a permanent dipole, dipole- dipole forces are stronger than LDFs and are found in polar molecules. The more polar the molecule, the stronger the dipole-dipole attractions.

3. Hydrogen bonds – A special dipole-dipole force that is much stronger than normal dipole forces. The only elements that form H- bonds are F, O, N and to form an H-bond, H must be bonded to one of these elements. These elements are all electronegative elements and the bonds with H are all very polar. The more polar the bond, the stronger the H-bond.

Consider the following molecules Consider the following molecules. How many of these molecules will have hydrogen bonding? CH3OH H2O HF NH3 CH3F CH3OCH3