Intermolecular Forces Forces Between Molecules Intermolecular Forces Electrical forces between molecules causing one molecule to influence another Heats of vaporization give a.
Download ReportTranscript Intermolecular Forces Forces Between Molecules Intermolecular Forces Electrical forces between molecules causing one molecule to influence another Heats of vaporization give a.
Intermolecular Forces Forces Between Molecules Intermolecular Forces Electrical forces between molecules causing one molecule to influence another Heats of vaporization give a measure of the strength of attractions present between molecules – the energy required to separate molecules when changing from liquid to gas state Ionic Compounds The forces of attractions in ionic compounds are the electrostatic force between ions A relatively strong force Hvap /100 kJ/mol Molecular Compounds Polar Molecules Polar Molecules Force of attraction between molecules is a dipole-dipole attraction + - + - Molecules - - are+ electrically - neutral overall but organize + themselves by attractions of head to tail dipole orientation Dipole-dipole forces are smaller than ion- ion forces Hvap .20 kJ/mol Hydrogen Bonding A Special Dipole-Dipole Interaction Hydrogen Bonding - + - + :X-H ....... :X-H The energy of the Hbond depends on the electronegativity of the X-atom F > O > N . Cl Heats of Vaporization olecule Hvap (kJ/mol) C non-polar molecule Molecular Compounds Non-Polar Molecules Non-Polar Molecules Non-polar molecules do not possess permanent dipoles Force of attraction between molecules is a London Force Hvap increases with increasing numbers of electrons Molecule Hvap (kJ/mol) F2 6.5 Cl2 20.4 I2 41.9 Principles of Solubility Solubility is dependent on intermolecular forces Liquid-Liquid “like dissolves like” liquids with similar structures (similar type & magnitude intermolecular forces) will be soluble in each other in all proportions. Example Both are held together by London Forces When a pentane molecule passes into a volume of hexane molecules, there is no significant environment change hexane pentane Oil Slicks Non-polar substances have little water solubility – Water molecules are held together by H-bonds – Non-polar are held together by London Forces H-bonds must be broken to dissolve appreciable quantities of non-polar substances in water Oil Slicks For substances to be soluble, there must be compensation for any forces broken in the dissolution process. Since there is no compensating force between a non-polar molecule and a water molecule, enough energy is not available to break the H-bonds Water Solubility of Polar Molecules Water will dissolve some polar molecules CH3OH and CH3CH2OH are capable of forming H-bonds Intermolecular forces between these alcohols and water are similar to those forces in pure alcohol and pure water. Water Solubility of Alcohols Solubility decreases as length of carbon chain increases As the chain gets longer, more H-bonds in the water must be broken to make room for the alcohol. Not enough H-bonds can be reformed to compensate Non-Polar & Slightly Polar Substances Most soluble in solvents of low polarity Least soluble in H-bonding solvents The DDT Story Soluble in non-polar or slightly polar solvents Concentrates in fatty tissue of fish, birds & game Quite water insoluble – isn’t washed out of contaminated soil Solid-Liquid Solids always have limited solubility in liquids – due to differences in the magnitudes of intermolecular forces in solid vs. liquid state – at 25oC a solid has much stronger intermolecular forces than a liquid Solid-Liquid The closer a solid is to its mp, the better its intermolecular forces will match up with a liquid Typically, solubility increases as the temperature increases Low mp solids tend to exhibit greater solubility than high mp solids