Chapter 12: Intermolecular Attractions and the Properties of Liquids and solids There are important differences between gases, solids, and liquids: Gases - Liquids - Solids –

Properties can be understood in terms of how tightly the molecules are packed together and the strength of the intermolecular attractions between them.

Intermolecular forces are: Intramolecular forces are:

There are only a few important types of intermolecular forces Strong intramolecular attractions exist between H and Cl within HCl molecules. These attractions control the chemical properties of HCl. Weaker intermolecular attractions exist between neighboring HCl molecules. Intermolecular attractions control the physical properties of this substance.

LIST OF INTERMOLECULAR FORCES

Dipole-dipole attractions Polar molecules tend to align their partial charges The attractive force is about 1% of a covalent bond and drops off as 1/d3 (d=distance between dipoles) The net interaction of the imperfectly aligned molecules is attractive.

Hydrogen bonds Very strong dipole-dipole attraction that occur when H is covalently bonded to to a small, highly electronegative atom (usually F, O, or N) Typically about ten times stronger than other dipole-dipole attractions Are responsible for the expansion of water as it freezes

(a) Polar water molecule (a) Polar water molecule. (b) Hydrogen bonding produces strong attractions in the liquid. (c) Hydrogen bonding (dotted lines) between water molecules in ice form a tetrahedral configuration.

http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/waterdens.html

10 cups of liquid water into the freezer and what do you get… 11 cups of ice!!!

London forces The (very) weak attractions between nonpolar molecules Arise from the interactions of instantaneous dipoles on neighboring molecules An instantaneous dipole on one molecule can produce and induced dipole on another. The net interaction of these over time is attractive.

These instantaneous dipole-induced dipole attractions are called London dispersion forces, London forces, or dispersion forces London forces decrease as 1/d6 (d=distance between molecules) Polarizability is a measure of the ease with which the electron cloud on a particle is distorted It tends to increase as the electron cloud volume increases

The boiling point of the halogens and noble gases demonstrate this: Large electron clouds are more easily deformed than small ones. The magnitude of the resulting partial charge is also larger. The larger molecules experience larger London forces than small molecules. The boiling point of the halogens and noble gases demonstrate this:

London forces depend on the number of atoms in the molecule The boiling point of hydrocarbons demonstrates this trend

Hexane, C6H14, (left) has a BP of 68 Hexane, C6H14, (left) has a BP of 68.7oC while the BP propane, C3H8, (right) is –42.1oC because hexane has more sites (marked with *) along its chain where attraction to other molecules can occur.

Molecular shape affects the strength of London forces More compact molecules tend to have lower London forces than longer chain-like molecules For example the more compact neopentane molecule (CH3)4C has a lower boiling point than n-pentane, CH3CH2CH2CH2CH3 Presumably this is because the hydrogens on neopentane cannot interact as well as those on n-pentane with neighboring molecules

Space filling models of two molecules with formula C5H12 Space filling models of two molecules with formula C5H12. The H atoms in the more compact neopentane cannot interact as well with neighboring molecules as the H atoms in the more chain-like n-pentane.

Ion-dipole and ion-induced dipole attractions are the attractions between an ion and the dipole or induced dipole of neighboring molecules (a) The negative ends of water dipoles surround a cation. (b) The positive ends of water dipoles surround an anion. The attractions can be quite strong because the ions have full charges.

Ion-dipole forces of attraction http://www.chem.purdue.edu/gchelp/liquids/ions.gif

It is sometimes possible to predict physical properties (like BP and MP) by comparing the strengths of intermolecular attractions Ion-dipole attractions hold water molecules in a hydrate. Water molecules are found at the vertices of an octahedron around the aluminum ion in AlCl3·6H2O.

Ion-induced dipole forces of attraction

Dipole-dipole: occur between molecules with permanent dipoles; about 1% - 5% of a covalent bond. Hydrogen bonding: occur when molecules contain N-H and O-H bonds; about 5% to 10% of a covalent bond. London dispersion: present in all substances; are weak, but can lead to large net attractions. Ion-dipole: occur when ions interact with polar molecules; can lead to large net attractions. Ion-induced dipole: occur when an ion induces a dipole on neighboring particle; depend on ion charge and the polarizability of its neighbor