LIQUIDS AND SOLIDS Kinetic Molecular Theory of Gases used to account for Ideal Gas Behavior when gases approach high temperatures and low pressures GASES are very different from solids and liquids. We need a different model. Jan 18 8:09 PM 1
MUCH similarity between liquids and solids, therefore we discuss them together. The forces holding liquids and solids are very close and can be seen by examining: 1. Density 2. Compressibilities 3. Heats of Phase Change Jan 18 8:11 PM 2
Intramolecular Forces Forces within a molecule. Atoms form molecules by sharing electrons. Jan 18 8:13 PM 3
Intermolecular Forces Forces between molecules, NOT within the molecules. Jan 18 8:13 PM 4
WATER phase change SOLID LIQUID GAS Molecules remain intact because there is a change in the forces among the molecules not within the molecules. DECOMPOSITION OF WATER O H bonds need to be broken with a molecule. Jan 18 8:14 PM 5
TYPES OF INTERMOLECULAR FORCES 1. Dipole Dipole Forces Partial positive and negative charges of neighboring polar covalent molecules attract. 1% as strong as intramolecular covalent bonds. These become weaker as the distance between the dipoles increase. Jan 18 8:16 PM 6
A polar molecule has a dipole. The value of a dipole depends on a) polarity of bonds b) molecular geometry Jan 18 8:18 PM 7
Formaldehyde Molecules with greater dipole moments have greater attractions to each other. Jan 18 8:20 PM 8
Intermolecular forces (such dipole dipole forces) can occur between different molecules. Acetonitrile Formaldehyde Jan 18 8:23 PM 9
2. Hydrogen Bonds A hydrogen bond is a very strong dipole dipole bond. Molecules with N, O, F covalently bonded to H can form hydrogen bonds. N, O, F atoms have high electronegativity Electrons in sigma bond are greatly polarized. Jan 18 8:32 PM 10
H 2 O Hydrogen bond is much stronger than dipole dipole bond. Hydrogen bonding gives water many important and distinctive properties. Hydrogen bonding holds together DNA helix and proteins. Jan 18 8:34 PM 11
3. London Dispersion Forces (Induced Dipole Induced Dipole) Instantaneous dipoles arise in a molecule as a result of momentary imbalances in electron distribution. These are VERY weak forces that become more important as the size of the atom of interest increases. Jan 18 8:35 PM 12
A molecule can have its electron cloud polarized by neighboring charge. Consider F 2. Fluorine molecule has no dipole. Jan 18 8:36 PM 13
Consider F 2 next to sodium ion, Na +. Negative electron cloud adjusts to move closer to positive ion. Fluorine molecule now has a dipole! Sodium ion induces dipole in fluorine. Thus fluorine may have an induced dipole. Jan 18 8:40 PM 14
Some atoms in molecules have electron clouds that adjust easier than others. Sodium ion induces larger dipole in iodine molecule than in fluorine. Jan 18 8:42 PM 15
The ability of an atom or molecule to adjust its electron cloud in response to an outside charge is called its polarizability. The less tightly bound to the nucleus the electrons are, the more polarizable they are. As a periodic trend, polarizability increases as row increases. Each electron in an atom, molecule or ion contributes to the overall polarizability. Thus, the more electrons a molecule has, the greater the strength of its induced dipole. Jan 18 8:43 PM 16
Charge distribution of molecules fluctuates over time. charge distribution is unequal in nonpolar molecules at times nonpolar molecules become polar for a short time, though average dipole is zero. Jan 18 8:44 PM 17
Spontaneous fluctuation of electron cloud. Consider a single bromine molecule. dipole appears, then disappears, then appears reversed, then disappears, etc Jan 18 8:47 PM 18
Fluctuating dipole induces dipole in another molecule so that they fluctuate together. Consider two bromine molecules Strength of dispersion forces depends on strength of induced dipole. Molecules with greater polarizability have greater dispersion forces. Jan 18 8:50 PM 19
Bond between bromine molecules is stronger than bond between chlorine molecules. Jan 18 8:51 PM 20
IONIC COMPOUNDS Ionic Forces COVALENT COMPOUNDS London Dispersion Forces POLAR COVALENT COMPOUNDS Dipole Forces and/or Hydrogen Bonding REMEMBER Hydrogen bonding must have N H, O H, or F H bond in the molecule. Jan 18 8:53 PM 21