Intermolecular Forces

Intermolecular Forces - INTERMOLECULAR FORCES AND CONDENSED...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
1 INTERMOLECULAR FORCES AND CONDENSED STATES I. Intermolecular Forces A. Types of Intermolecular Forces. 1. Van der Waals forces = attractive forces that exist between neutral molecules. a. Are much weaker than the intramolecular force of the covalent bond.( 150 - 1100 kJ/mol) b. Weaker than ion-ion forces (Lattice energies, 200 - 4000 kJ/mol) 2. Dipole-dipole forces . a. Exist between polar molecules. Due to the attraction of the negative end of one dipole and the positive end of another. b. Vary as 1/d 2 ( d = distance ). Fairly strong ( 5-25 kJ/mol), should depend on the dipole moments of the molecules. c. Especially important in understanding the behavior of small polar molecules. 3. Dipole-induced dipole forces. a. The permanent dipole in one molecule can distort, or polarize, the electron density in a neighboring molecule creating, or inducing, a dipole. The resulting force of attraction is called dipole-induced dipole attraction. It can take place between a polar and a nonpolar molecule or between two polar molecules. polar molecule nonpolar molecule Polarization + - + - induced dipole dipole dipole - induced dipole attraction - + b. These forces of attraction will increases as the polarizability of the molecule increases. The polarizability is a measure of the ease of distortion of the electron density of a molecule. c. The polarizability increases as
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 1) the number of electrons increases. 2) the size of the molecule increases d. Forces 1/r 6 and are fairly weak (2-10 kJ/mol). They are important in that they are responsible for the solubility of nonpolar gases, such as O 2 in polar solvents, such as water. 4. Induced dipole-induced dipole forces (London dispersion forces). a. A dipole created by an instantaneous asymmetry in the electron density of one molecule can induce a dipole in a neighboring molecule, giving rise to a force of attraction. b. Example. Consider two He atoms. Each He has two electron in spherically symmetric 1s orbitals. However, it is possible to have an instantaneous asymmetry in the electron density of one of the He atoms. This will induce an asymmetry in neighboring He atoms. If the electrons move in a synchronous manner, there will always be a weak attractive force between the He atoms. +2 Instantaneous Dipole - - - - + + + + + Induced dipole + + + + - - - - Induced Dipole - Induced Dipole Attraction He 1s 2 Symmetric This type of attractive forces are called London Dispersion Forces or Induced Dipole- Induced Dipole forces.
Background image of page 2
3 c. Such forces should increase as the polarizabilities of the molecules increase. It is found in all molecules, polar and nonpolar. It is the only type of attractive forces that exist between nonpolar molecules. Forces 1/d 6 (0.05-40 kJ/mol). 5. Hydrogen Bonding. a. Special force of attraction that exists between unshielded nuclear charge on a H, when it is covalently bonded to an O, N or F and the lone pair of electrons on a neighboring O, N, or F.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 04/07/2008 for the course CHEM 1304 taught by Professor Prof.maguire during the Spring '08 term at SMU.

Page1 / 23

Intermolecular Forces - INTERMOLECULAR FORCES AND CONDENSED...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online