Chapter 11 Liquids, solids, and intermolecular forces

Intermolecular forces Intermolecular forces – attractive forces between particles composed of matter (atoms, ions, and molecules) These forces dictate many properties of an atom Melting point, boiling point, pattern in a lattice, miscibility, solubility, etc. Driving factor for intermolecular forces is Coulomb’s law!!!! Positive and negative charges attract!!! Depending on the magnitude of the negative or positve charge, the intermolecular force that attracts the particles will be stronger or weaker

Dispersion forces Dispersion/London forces – the result of fluctuations in the electron distribution within molecules or atoms At the point in time when the electrons are localized on one side of the atom, the region has a partial negative charge, while the opposite side of the atom would have a partial positive charge

Dispersion force: Atom size Larger atoms will have greater dispersion forces More electrons, more space, more volume in which electrons can polarize the atom Greater dispersion force results in higher melting points and boiling points since it take more energy (higher temperature) to separate the attracted particles

Dispersion forces: Molecular shape In general, higher molar mass results in greater dispersion forces Molecular shape must also be considered, since it will determine the surface area of interaction C5H14

Dipole-dipole force Dipole-dipole force – attractive force between polar molecules If a molecule has a net dipole it is polar and the positive region of one molecule can attract to the negative region in another molecule

Dipole-dipole interactions are a stronger force than dispersion forces since the dipoles are permanent instead of temporary Every molecule has dispersion forces, but only polar molecules have dipole-dipole forces

Dipole-dipole force: Dipole moment The greater the magnitude of the positive and negative charge in the dipole, the greater the force Increasing dipole moment = stronger dipole-dipole force

Dipole-dipole force: Miscibility Miscibility – the ability to mix without separating into two states Dipole-dipole forces are present between one polar molecule and another, so they will be miscible Dipole-dipole forces are not present between polar molecules and nonpolar molecules Water is a polar molecule Oil is a non polar molecule Water and oil are not miscible, they don’t mix

Hydrogen bonding Hydrogen bonding – attractive force that occurs between the H that is bonded to a very electronegative atom (O, N, F) on one molecule and a very electronegative atom on another molecule Since there is a large electronegativity difference, the magnitude of the dipole moment is larger, resulting in a very strong intermolecular force, larger than regular dipole-dipole forces

Ion-dipole force Ion-dipole force – attractive force between ions and a polar compound Ionic compound composed of cation and anion Cation is attracted to negative end of dipole Anion is attracted to positive end of dipole

Summary of intermolecular forces

Practice Identify the major interaction expected for the following molecules Cl2 NH3 C10H22 KI HBr Identify the major interaction expected between Water and ammonia (NH3) Formaldehyde (CH2O) and methane (CH4)