CHM1046 liquids solids and intermolecular forces Chapter 11 Study guide 1 Intermolecular forces attractive forces that between molecules and atoms Intermolecular forces State of matter depends on the magnitude of the intermolecular forces The stronger the intermolecular force the closer molecules are to each other If molecules can get away easily not as strongly bonded Intermolecular forces originate from the interactions between charges partial charges and temporary charges on molecules Generally much weaker than bonding forces o Bonding forces are the result of large charges interacting at very close o Intermolecular forces are the result of smaller charges interacting at distances greater distances The more heat the higher the kinetic energy the higher the more intermolecular forces Polarity is the separation of charges difference in electronegativity The s and s result from difference in electronegativity between two atoms which allows them to bond to each other Chemical bonds occur between individual atoms within a molecule whereas intermolecular forces occur between molecules Boiling points increase with increasing molar mass Properties of solids liquids and gases Molecules in liquid water and ice are in close contact with one another touching while those in gaseous water are separated by larger distances Intermolecular forces in gases o Are weakest o Molecules are weakly attracted to one another normally find lots of space between molecules o If forced into a container by applying external pressure volume decreases o As pressure INCREASES the Volume DECREASES for a gas o Gases are compressible Intermolecular forces in liquids o Are stronger than in gases but they aren t to strong o Molecules are in close contact and liquids are incompressible cant compress not easily compressed Intermolecular forces in solids o Are the strongest o Molecules are strongly attracted to each other and are packed closely together o The form a certain arrangement of space o Molecules don t have the freedom to move around o Solids may be either Crystalline molecules or atoms are arranged in a well ordered 3D array Amorphous randomly arranged The more pressure the stronger intermolecular force the more compact the molecules are The stronger the intermolecular forces in the molecule the higher the boiling point Intermolecular forces the forces that hold condensed phases together Dispersion force Vander walls London dispersion o Electrons are in constant motion within orbitals o All atoms and molecules have electrons they all exhibit dispersion forces polarization electron cloud o Movement of electrons in an atom or molecule cause its o The magnitude of dispersion forces depends on the size of the The closer the electrons are to the nucleus the less electrons can move Larger electron clouds have electrons which can move easily creating STRONGER dispersion forces Electrons further away fro the nucleus less attracted to nucleus stronger dispersion forces o An instantaneous dipole temporary dipole is created and progates to other atoms o What makes dispersion forces different The size of the electron cloud Strength of intermolecular forces o Dispersion forces are GREATER for molecules or atoms with a LARGER molecular weight within a family of similar elements or compounds Example He MW of 4g mol noble gas Boiling point 4 2k Dipole Dipole forces Ne MW of 20 18g mol noble gas Boiling point 27 K Dispersion forces are greater in Ne because it has a higher molecular weight has a larger electron cloud larger dispersion force o Why do they have a different boiling point Different dispersion forces The strength of dispersion forces are different in each species Surface area could be a factor o The magnitude of dispersion forces can also depend on surface area of a molecule The larger surface area the more interaction between molecules higher boiling point The smaller area of interaction the lower the boiling point o Exists in all molecules that are polar only o Are stronger than dispersion forces o Polar molecules have permanent dipoles that interact with permanent dipoles of neighboring molecules The positive end of one permanent dipole is attracted to the negative end of another this attraction is the dipole dipole force o Because dipole dipole has stronger intermolecular forces they have higher boiling points o All molecules have dispersion forces polar molecules also have dipole dipole boiling points This additional attractive force raises melting point and o Miscibility the ability to mix without separating into two phases Is determined by polarity A polar liquid mixed with another polar liquid A non polar liquid mixes with another nonpolar liquid Polar molecule Nonpolar molecule These two molecules are not miscible Hydrogen bonding o Exist between polar molecules that have a hydrogen bonded to a strongly electronegative element N O F and molecules that have an atom with a lone pair of electrons o Strongest of the intermolecular forces Ion Dipole force o Occurs when an ionic compound is mixed with a polar compound o Example sodium chloride Strength of dispersion force Dispersion Dipole dipole Hydrogen bonding ion dipole Weakest Strongest Summary Dispersion forces are the weakest present in all molecules increase with molar mass Dipole dipole forces are present in polar molecules Hydrogen bonds the strongest of intermolecular forces that can occur in pure substances most be directly bonded to F O N Ion dipole forces are present in mixtures of ionic compounds and polar compounds very strong and important in aqueous solutions of ionic compounds Intermolecular forces in action surface tension viscosity and capillary action Surface Tension the tendency of liquids to minimize their surface area o The stronger the intermolecular force the more surface tension o Surface tension of a liquid is the energy needed to increase the surface area by a unit amount interior of the liquid o Molecules on the surface have less interactions than molecules in the o Since molecules on the surface have a higher potential energy than those in the interior liquids tend to minimize their surface area this tendency creates a kind of skin at the surface that resists penetration For a fisherman s hook to sink in water the water surface area must increase slightly an increase which is resisted by the surface tension o Surface area decreases with decreasing intermolecular forces Viscosity is the resistance of a liquid to flow o
View Full Document
Unlocking...