Quick Review. 1. Hybridization. 2. Delocalization. 3. We will not be talking about Molecular Orbital Model.

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1 Quick Review 1. ybridization. 2. Delocalization. 3. We will not be talking about Molecular Orbital Model. **OUR EXAM II IS TURSDAY April 2 nd at 7pm** *BQ5, BQ6 and BQM are in 66 Library *BQ1, BQ2, BQ3, BQ4, BQA, BQ9 are in 141 Wohlers *BQD is in 203 Noyes Lab

2 Summary What you need to be able to do: 1. Know how to predict hybridization around central atom. 2. Know difference between sigma and pi bonds. 3. Be able to count the number of sigma and pi bonds in a molecule. 4. Be able to predict which orbitals overlap to form sigma and pi bonds.

3 Intra vs Intermolecular Forces Intramolecular forces: 1. Forces that hold atoms together in molecules. 2. Are forces within the molecule (bonds). 3. Are very strong when compared to intermolecular forces. Intermolecular forces: 1. Forces that hold molecules together in the liquid and solid phases. 2. Are forces between molecules. 3. Are weak when compared to intramolecular forces yet account for many of the observed properties of matter.

4 Phases of Matter and IMF The strength of intermolecular forces at a certain temperature determines the phase of a substance. N 2 2 O NaCl All IMF broken 10-20% IMF broken The strength of IMF are related to some physical properties of a substance, all of which are related to phase changes: boiling point, melting point, vap, fus and sub.

5 Definitions Molar eat of Vaporization ( vap ): Amount of energy required to boil one mole of a substance. Molar eat of Fusion ( fus ): Amount of energy required to melt one mole of a substance. Molar eat of Sublimation ( sub ): Amount of energy required to sublime one mole of a substance.

6 Phase Changes and IMF Question: What is the relationship between boiling point, melting point, molar heat of vaporization, molar heat of fusion, molar heat of sublimation and the strength of intermolecular forces of a molecule?

7 Vapor Pressure Vapor Pressure The equilibrium pressure of a vapor (gas) over a liquid at a specific temperature. ow is vapor pressure related to temperature? ow is vapor pressure related to the strength of IMF?

8 Types of IMF 1. Electrostatic Forces *Due to attraction of oppositely charged species (ions). *Present in ionic molecules. *Extremely strong; no comparison in strength to the covalent IMF.

9 Types of IMF 2. Dipole-Dipole Forces *Due to permanent dipole-dipole interactions. *Strength increases with increasing polarity. *Present in covalent molecules with a permanent dipole moment, i.e. only present in polar molecules.

10 Types of IMF 3. ydrogen bonding *Due to attraction of N, O, or F atom in a molecule for an electron deficient hydrogen atom in a covalent molecule (special kind of dipole-dipole force). *ONLY occurs in molecules with a hydrogen atom bound directly to N, O or F atom (the three most electronegative atoms), i.e. molecule must have N-, O- or an F- covalent bond in order to form hydrogen bonding interactions. *Strongest covalent IMF; compounds which can form hydrogen bonding interactions have irregularly strong IMF for covalent compounds.

11 ydrogen Bonding -bonding is the strongest covalent IMF.

12 Types of IMF 4. London Dispersion Forces *Due to instantaneous dipole interactions. *Strength increases with increasing molar mass (size); shape is also a factor. *Present in all covalent molecules (atoms, polar and nonpolar covalent molecules). *Considered to be weakest IMF but can be very strong for very large molecules.

13 London Dispersion Forces London Dispersion Forces are generally considered to be the weakest type of IMF (can be strong for very large molecules).

14 IMF Summary Electrostatic forces > -bond > Dipole-Dipole > London Dispersion Increasing Strength of IMF

15 Example Problems Rank the halogens F 2, Cl 2, Br 2 and I 2 in order of increasing boiling point.

16 Example Problems Compare the boiling points of butane and acetone, can acetone hydrogen bond? C C C C Butane MM: g/mol C O C C Acetone MM: g/mol

17 Example Problems Why does the boiling point of non -bonding hydrides increase steadily from smaller to larger?

18 Example Problems Match the vapor pressure curves to the following compounds: C C OC C C C O Diethyl ether Ethanol O Water

19 Example Problems Order the following compounds in terms of increasing boiling point and vapor pressure: (a) C 4 (16.00 g/mol), C 3 -O-C 3 (46.00 g/mol), C 3 C 2 O (46.00 g/mol) and C 3 C 2 C 3 (44.00 g/mol) (b) Cl (36.45 g/mol), F (20.00 g/mol), LiF (26.00 g/mol), Ar (40.00 g/mol).

20 Other Caveats 1. Shape also affects the magnitude of London Dispersion Forces. The more elongated the molecule, the more surface area is in contact with neighboring molecules, the stronger the London Dispersion Forces. 2. I will not hold you responsible for surface tension and viscosity. 3. IMF are also related to solubility, like dissolves like. Polar substances and soluble ionic compounds dissolve in polar solvents like water and nonpolar substances dissolve in nonpolar solvent like hexane (C 6 14 ).