RW Session ID = MSTCHEM1 Intermolecular Forces

Transcription

1 RW Session ID = MSTCHEM1 Intermolecular Forces Sections 9.4,

2 Intermolecular Forces Attractive forces between molecules due to charges, partial charges, and temporary charges Higher charge, stronger forces Shorter distance, stronger forces Much weaker than bonding forces (intramolecular forces)

3 Intermolecular Forces Dispersion forces (weakest) Dipole-dipole forces Hydrogen bonding Ion-dipole forces Ion-Ion forces (strongest) not always considered an intermolecular force

4 Dispersion Forces Attraction between the temporary dipoles Present in all atoms and molecules Fluctuations in the electron distribution lead to a temporary dipole (instantaneous dipole) The magnitude depends on the polarizability A larger electron cloud is easier to polarize than a small electron cloud

5 Dipole-Dipole Forces Attraction between permanent dipoles Exist in all polar molecules Polar molecules have permanent dipoles Polar liquids are miscible with other polar liquids Polar liquids are not miscible with nonpolar liquids

6 Hydrogen Bonding Very strong dipole-dipole force Occurs in polar molecules containing hydrogen atoms bonded directly to small electronegative atoms (most important when bonded to N, O, or F) Results from a very large dipole HF

7 Ion-Dipole Force Attractive forces between ions and polar molecules Present in a mixture of an ionic compound and polar compound Positive ions are attracted to the partial negative Negative ions are attracted to the partial positive

8 Ion-Ion Forces Attractive forces between ions Present in ionic compounds Ion-ion forces are related to the lattice energy of a compound Lattice energy the energy associated with forming a crystalline lattice of alternating cations and anions from the gaseous ions Based on Coulomb s law, the potential energy decreases when these ions come together to form a lattice = = =

9 Ion charge Trends in Lattice Energy Lattice energy is higher (more negative) for higher charges Larger charges result in a stronger attraction, so more energy is released when the lattice is formed = = = Ion size Lattice energy is higher (more negative) for smaller ions Larger ions cannot get close to each other, so they don t release as much energy when the lattice is formed

10 Intermolecular Forces Dispersion forces (weakest) All molecules Due to temporary dipoles Dipole-dipole forces Polar molecules Due to permanent dipoles Hydrogen bonding Polar molecules containing H-F, H-N, or H-O bonds Due to large permanent dipoles Ion-dipole forces Mixtures of ionic compounds and polar molecules Due to ion interaction with permanent dipoles Ion-ion forces Ionic compounds Due to interaction of ions

11 Physical Properties Affected By Intermolecular Forces Liquids Vapor pressure Boiling point Viscosity Surface tension Solids Melting point Hardness Solubility Capillary action

12 Physical Properties of Liquids Vapor pressure the pressure of a gas in dynamic equilibrium with its liquid Boiling point the temperature at which the vapor pressure of a substance equals the external pressure Viscosity the resistance of a liquid to flow Surface tension the tendency of liquids to minimize their surface area Capillary action the ability of a liquid to flow against gravity up a narrow tube

13 Vapor Pressure Equilibrium Vapor pressure The pressure of a gas in dynamic equilibrium with its liquid The rate of evaporation and condensation are equal An increase in temperature results in an increase in vapor pressure More energy More molecules can escape the liquid phase

14 Vaporization The transition from a liquid to a gas Process requires energy Rate of vaporization increases with: Increasing temperature Increasing surface area Decreasing strength of intermolecular forces Volatile evaporates easily Nonvolatile does not evaporate easily

15 Physical Properties of Liquids Vapor pressure the pressure of a gas in dynamic equilibrium with its liquid Stronger intermolecular forces, lower vapor pressure Weaker intermolecular forces, higher vapor pressure Boiling point the temperature at which the vapor pressure of a substance equals the external pressure Stronger intermolecular forces, higher boiling point Weaker intermolecular forces, lower boiling point

16 Physical Properties of Liquids Viscosity the resistance of a liquid to flow Stronger intermolecular forces, higher viscosity Weaker intermolecular forces, lower viscosity Surface tension the tendency of liquids to minimize their surface area Stronger intermolecular forces, higher surface tension Weaker intermolecular forces, lower surface tension

17 Physical Properties of Liquids Capillary action the ability of a liquid to flow against gravity up a narrow tube Cohesive forces the attraction between molecules in a liquid Adhesive forces the attraction between the liquid molecules and the surface of the tube Adhesive forces > cohesive forces (weaker intermolecular forces) capillary action occurs, the liquid will climb up the tube Cohesive forces > adhesive forces (stronger intermolecular forces) no capillary action, liquid may pull away from the tube