LECTURE 9 Intermolecular Forces Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

INTERMOLECULAR FORCES: Bonding forces are relatively strong because they involve larger charges that are closer together. Intermolecular forces are relatively weak because they typically involve smaller charges that are farther apart.

Figure 12.9Covalent and van der Waals radii.

Figure Periodic trends in covalent and van der Waals radii (in pm).

Figure Polar molecules and dipole-dipole forces.

THE HYDROGEN BOND A dipole-dipole intermolecular force The atoms which are small and highly electronegative are N, O, and F. A hydrogen bond may occur when an H atom in a molecule, bound to small highly electronegative atom with lone pairs of electrons, is attracted to the lone pairs in another molecule... F H O N FH O NH hydrogen bond donor hydrogen bond acceptor hydrogen bond acceptor hydrogen bond donor hydrogen bond donor hydrogen bond acceptor The Hydrogen Bond

Figure Dipole moment and boiling point.

SAMPLE PROBLEM 12.3 Drawing Hydrogen Bonds Between Molecules of a Substance SOLUTION: PROBLEM:Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bond(s) between them. (a)(b) (c) PLAN: (a) C 2 H 6 has no H bonding sites. Find molecules in which H is bonded to N, O or F. Draw H bonds in the format B: H A (c)(b) CH 3 CN O H H CCH 3 N O H H CH 3 C N O H H CCH3CH3 N O H H

Figure Hydrogen bonding and boiling point.

Polarizability and Charge-Induced Dipole Forces Distortion of an electron cloud Polarizability increases down a group. Size increases and the larger electron clouds are further from the nucleus. Polarizability decreases left to right across a period. Increasing Z eff shrinks atomic size and holds the electrons more tightly. Cations are less polarizable than their parent atom because they are smaller. Anions are more polarizable than their parent atom because they are larger.

Figure Dispersion forces among nonpolar particles. instantaneous dipoles separated Cl2 molecules

Figure Molar mass and boiling point.

Figure Molecular shape and boiling point.

SAMPLE PROBLEM 12.4 Predicting the Types Intermolecular Force PROBLEM:For each pair of substances, identify the key intermolecular force(s) in each substance, and select the substance with the higher boiling point: (a) MgCl 2 or PCl 3 (b) CH 3 NH 2 or CH 3 F (c) CH 3 OH or CH 3 CH 2 OH (d) Hexane (CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 ) or 2,2-dimethylbutane PLAN:Use the formula, structure, and Table 2.2. Bonding forces are stronger than nonbonding (intermolecular) forces. Hydrogen bonding is a strong type of dipole-dipole force. Dispersion forces are always present, but decisive when the difference is molar mass or molecular shape.

SOLUTION: SAMPLE PROBLEM 12.4 Predicting the Types Intermolecular Force continued (a) Mg 2+ and Cl - are held together by ionic bonds while PCl 3 is covalently bonded and the molecules are held together by dipole-dipole interactions. Ionic bonds are stronger than dipole interactions and so MgCl 2 has the higher boiling point. (b) CH 3 NH 2 and CH 3 F are both covalent compounds and have bonds which are polar. CH 3 NH 2 can H bond while that in CH 3 F cannot. Therefore, CH 3 NH 2 has the stronger interactions and the higher boiling point. (c) Both CH 3 OH and CH 3 CH 2 OH can H bond but CH 3 CH 2 OH has more CH for more dispersion force interaction. Therefore, CH 3 CH 2 OH has the higher boiling point. (d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to hold the molecules together. Hexane has the larger surface area and therefore, the greater dispersion forces and higher boiling point.

Figure Summary diagram for analyzing the intermolecular forces in a sample.