I 11 1 A Molecular Comparison of Gases Liquids and Solids Chapter 11 Liquids and Intermolecular Forces Intermolecular forces Forces that exist between molecules A Intermolecular forces in liquids and solids are much stronger than gasses Recap in kinetic molecular theory we pretty much ignore intermolecular forces Just sayin In liquids the intermolecular forces are strong enough to hold particles together making liquids far less compressible than gasses Unlike gases liquids have a definite volume Like gases liquids conform to the shape of its container This is because the attractive forces in liquids are not strong enough to keep particles from moving past each other In solids intermolecular forces are strong enough to virtually lock particles into place We often refer to solids and liquids as condensed phases The state of a substance depends largely on the balance between kinetic energies of the particles Apparently in solids particles vibrate and increasingly vibrate with rising temperature atoms molecules or ions and the interparticle energies of attraction Interparticle attractions tend to draw particles together As we move down the periodic table intermolecular forces become stronger As kinetic energy decreases attraction between particles increase Increased pressure makes intermolecular forces more effective 11 2 Intermolecular Forces B Intermolecular forces are generally much weaker than intramolecular forces ionic metallic or covalent bonds Less energy is required to vaporize a liquid or melt a solid than to break covalent bonds Boiling point of a substance reflects the strength of its intermolecular forces Molecules of a liquid must overcome their intermolecular forces in order to separate and form a vapor The stronger the intermolecular forces the higher the temperature at which a liquid boils Similarly melting points of solids increase as the strength of intermolecular forces increase Chemical bonds are much stronger than intermolecular forces Three types of intermolecular attractions exist between electrically neutral molecules o Dispersion forces o Dipole dipole attractions o Hydrogen bonding Dispersion forces and dipole dipole attractions are collectively called vand der waals forces Another type of attractive force is the ion dipole force important in solutions All intermolecular attractions are electrostatic involving the attractions between positive and negative species kind of like ionic bonds Recap electrostatic interactions get stronger as the magnitude of the charges increases and weaker as the distance between charges increases Charges responsible for intermolecular forces are generally much smaller than the charges of ionic compounds Also the distances between molecular are often larger than the distances between atoms held together by chemical bonds 1 Dispersion Forces The motion of electrons in an atom or molecule can create an instantaneous or momentary dipole moment The instantaneous distribution of electrons can be different from the average distribution The motion of electrons in one atom influence the motions of electrons in its neighbors An instantaneous dipole on one atom can induce an instantaneous dipole on another atom causing the atoms to be attracted to each other This attractive interaction is called dispersion force Dispersion force is only significant when molecules are very close together The strength of the dispersion force depends on the ease with which the charge distribution in a molecule can be distorted to induce an instantaneous dipole The ease with which the charge distribution is distorted is called the molecule s polarizability Polarizability can be thought of as the squishiness of its electron cloud More polarizable molecules have larger dispersion forces In general polarizability increases as the number of electrons in an atom or molecule increases The strength of the dispersion forces tends to increase with increasing atomic or molecular size Because molecular size and mass generally parallel each other dispersion forces tend to increase in strength with increasing molecular weight Higher molecular weights translate into stronger dispersion forces which in turn lead to higher boiling points Intermolecular forces are greater for molecular shapes in which molecules can come in contact over their entire length longer and flatter compared to compact spherical shapes 2 Dipole Dipole Forces The presence of a permanent dipole moment in molecules gives rise to dipole dipole forces Dipole dipole forces originate from electrostatic attractions between the partially positive end of one molecule and the partially negative end of a neighbor molecule Dipole dipole forces are only effective when molecules are very close In polar solids there are mostly only dipole dipole attractions but in liquids there are both dipole dipole attractions and repulsions However there are more attractive interactions than repulsive ones The overall net attraction in polar liquids is strong enough to keep molecules in liquid rather than moving apart to form a gas For molecules of approximately equal mass and size the strength of intermolecular attractions increases with increasing polarity 3 Hydrogen Bonding The boiling points of hydrogen combined with elements from group 4A increase systematically moving down the group This is an expected trend because polarizability and hence dispersion forces generally increase as molecular weight increases The boiling points of hydrogen combined with the three heavier member of groups 5A 6A and 7A generally increase as molecular weight increases NH3 H20 and HF have boiling points that are much higher than expected because of abnormally strong intermolecular forces Hydrogen bonding is a special type of intermolecular attraction between the hydrogen atom in a polar bond particularly H F H O and H N and a nonbonding electron pair on a nearby small electronegative ion or atom usually F O or N in another molecule Hydrogen bonds can be considered a type of dipole dipole attraction Because N O and F are so electronegative a hydrogen bond to any of these elements is quite polar Hydrogen has no inner electrons so when it bonds the positive side of the dipole has a concentrated charge of a nearly bare hydrogen nucleus Because the electron poor hydrogen is so small it can approach an electronegative atom very closely and thus interact strongly with it The energies of hydrogen bonds vary from about 5 25kJ mol Hydrogen bonds are