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Intermolecular Forces When studying the gas laws, we learned the ideal gas law describes ideal gases as having no intermolecular forces of attraction between them. This is why real gases deviate from the ideal gas law. In gases, intermolecular forces of attraction have very little effect on the individual molecules because of the high amount of kinetic energy. However, if you decrease the amount of available kinetic energy, the intermolecular forces have a greater effect on the molecules present, resulting in the liquid and solid phases The strength of the intermolecular forces between species affect: • Physical states • Melting point (m.p.) • Boiling point (b.p.) • Volume • Order of arrangement (structure) Intramolecular forces Forces of attraction within molecules (i.e. covalent bonds, dipole moments) Intermolecular forces Forces of attraction between molecules and ions The intermolecular forces: 1. Ion-ion force 2. Ion-dipole force 3. Hydrogen bonding 4. Dipole-dipole force 5. Induced Dipoles (a.k.a. Dispersion forces) Strongest Strong Weak 1 Ion-ion Force Dipole-dipole interactions The electrostatic force of attraction between ions of opposite charge (ionic bond) Dissociation Energy of Alkali Halides D Hdissoc (kJ/mol) These are the strongest forces. Lead to solids with high melting temperatures. • Force of attraction between the partial charges of polar molecules 1100 δ+H – Cl δ- δ+ H – Cl δ- F 900 Cl 700 Br I 500 Li Na K Rb Cs Ion-Dipole Force Force of attraction between an ion and the partial charge on the end of a polar molecule. Attraction Between Ions and Permanent Dipoles •• - •• water dipole H H + O Water is highly polar and can interact with positive ions to give hydrated ions in water. The Dissolution of Ionic Solids Into Solution (Hydration) • Many metal ions are hydrated. This is the reason metal salts dissolve in water. Induced Dipoles London Dispersion Forces Dispersion forces are the forces of attraction created by the formation of instanteous dipoles created from electron movement As the molecular weight increases, the dispersion forces increase between the molecules 2 FORCES INVOLVING INDUCED DIPOLES 28 How can nonnon-polar molecules such as O2 and I2 dissolve in water? FORCES INVOLVING INDUCED DIPOLES 29 Solubility increases with mass the gas Dipole-induced Dipole Dipole-induced dipole dipole The water dipole INDUCES a dipole in the O2 electric cloud. FORCES INVOLVING INDUCED DIPOLES Formation of a dipole in two nonpolar I2 molecules. 31 Hydrogen Bonding The force of attraction between the hydrogen of one molecule and a highly electronegative atom (F, O, N) in a different molecule Induced dipole Induced dipoledipole-induced induced dipole dipole H-bonding is a special form of dipole-dipole attraction, which enhances dipole-dipole attractions. H-bonding is strongest when X and Y are N, O, or Hydrogen Bonding in H22O 15 H-bonding is especially strong in water because • the O— O—H bond is very polar • there are 2 lone pairs on the O atom Accounts for many of water’ water’s unique properties. F 3 Hydrogen Hydrogen Bonding Bonding in in H H22O O • H bonds ---> ---> abnormally high specific heat capacity of water (4.184 J/g J/g•K). – This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy. 18 The intermolecular forces between species are directly related to the available kinetic energy of a substance. For example, all three phases of water have the same acting intermolecular forces, but different amounts of kinetic energy • Ice has open latticelattice-like structure. • Ice density is < liquid and so solid floats on water. What is the relationship between kinetic energy, intermolecular forces and the phases? Effect of IMF’s on Phase Gases- Effect of IMF’s on Phase Liquids- • Kinetic energy completely overcomes the intermolecular forces • Kinetic energy partially overcomes intermolecular forces • Assume volume and shape of container • Takes the shape of their container (no inherent structure) • Virtually incompressible • Compressible • Slow diffusion • Rapid diffusion/flows easily Effect of IMF’s on Phase Physical Properties of Alkanes Solids• Kinetic energy overcome by intermolecular forces gases • Does not flow liquids • Retains own shape • Virtually incompressible • No to little diffusion Heptadecane (17) • • • • London forces are the only I.M.F’s of attraction. Boiling point increases with length of chain. Insoluble in water due to being non-polar (hydrophobic). All are less dense than water 4 Physical properties of halocarbons: Higher boiling points than hydrocarbons of similar molecular weight but still relatively low. •Due to polarity of carbon/halogen bond Relatively soluble Physical Properties of Alcohols The –OH group is polar and capable of hydrogen bonding. This makes low molecular weight alcohols highly soluble in water. Hydrogen bonding in a water-methanol solution: •Slightly soluble due to being slightly polar. Some common halocarbons include: • (CFC’S) i.e. Freon CClF2CClF2 • (HFC’S) CF3CHF2 • PVC -[-CH2-CHCl-]- Physical Properties of Alcohols, cont. •Solubilities in water of selected primary (1o) alcohols. Physical Properties of Alcohols, cont. Larger alcohols have greater hydrophobic regions and are less soluble or insoluble in water. Water interacts only with the hydrophilic –OH group of 1-heptanol: •Notice as chain length increases, solubility decreases Physical Properties of Alcohols, cont. The –OH group can hydrogen bond between alcohol molecules leading to relatively high boiling points. Hydrogen bonding in pure ethanol: Physical Properties of Alcohols, cont. •Boiling points of selected primary (1o) alcohols compared to alkanes and ethers of similar molecular weights •Notice B.P.’s increase with increasing M.W. 5 Physical Properties of Carbonyls The carbonyl group is moderately polar, but it doesn’t have any hydrogens attached, so it cannot hydrogen bond between molecules. Physical Properties of Carbonyls, cont. The C=O group can hydrogen bond with water molecules. This makes low molecular weight aldehydes and ketones water soluble (they have small hydrophobic sections). Notice, The boiling points of carboxylic acids are higher that that of alcohols Physical Properties of Carbonyls, cont. Because of the polarity of the C=O group, these groups can interact, but the attraction is not as strong as hydrogen bonding. This makes the boiling point of aldehydes and ketones higher than alkanes, but lower than alcohols. Physical Properties of Carboxylic Acids The carboxylate group is very polar. Hydrogen bonding between carboxylates creates dimers (two identical molecules bonded together). This gives carboxylic acids high boiling points (greater than alcohols). Physical Properties of Carboxylic Acids, cont. Carboxylate groups can hydrogen bond with water. If the hydrophobic –R group is not too large, carboxylic acids are very water soluble. Long chained carboxylic acids were first extracted from fats and are called “fatty acids” 6 Physical Properties of Esters Esters are slightly polar, however have lower b.p.’s than alcohols or aldehydes. Low solubility Many have very pleasant odors Physical Properties of Amines Effects of hydrogen bonding in primary alcohols vs. primary amines Physical Properties of Esters, cont. Primary and secondary amines can form hydrogen bonds between molecules. The N-H bond is not quite as polar as the O-H bond. The hydrogen bonds are not as strong as those of alcohols, so amine boiling points are somewhat lower than those of alcohols. Physical Properties of Amines, cont. Amines can hydrogen bond with water, making smaller amine molecules usually water soluble. 7