Presentation on theme: "Intermolecular Forces. The attractive forces holding solids and liquids together are called intermolecular forces. The covalent bond holding a molecule."— Presentation transcript:
The attractive forces holding solids and liquids together are called intermolecular forces. The covalent bond holding a molecule together is an intramolecular force Intermolecular forces are much weaker than intramolecular forces (chemical bonds) (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl). When a substance melts or boils the intermolecular forces are broken (not the covalent bonds). When a substance condenses intermolecular forces are formed.
Dipole-Dipole Forces Molecules orient themselves to maximaize the + --- - interactions and minimize the + --- + and - --- - forces. There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble. If two molecules have about the same mass and size, then dipole- dipole forces increase with increasing polarity.
Intermolecular Forces London Dispersion Forces Weakest of all intermolecular forces. Exist primarily between noble gases and nonpolar molecules The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). For an instant, the electron clouds become distorted. In that instant a dipole is formed (called an instantaneous dipole). Relatively weak and short-lived
Intermolecular Forces London Dispersion Forces One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom). Polarizability is the ease with which an electron cloud can be deformed. The larger the molecule (the greater the number of electrons) the more polarizable. London dispersion forces increase as molecular weight increases. London dispersion forces exist between all molecules. The greater the surface area available for contact, the greater the dispersion forces. London dispersion forces between spherical molecules are lower than between sausage-like molecules.
Intermolecular Forces Hydrogen Bonding Special case of dipole-dipole forces. By experiments: boiling points of compounds with H- F, H-O, and H-N bonds are abnormally high. Intermolecular forces are abnormally strong. H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N). –Electrons in the H-X (X = electronegative element) lie much closer to X than H. –H has only one electron, so in the H-X bond, the + H presents an almost bare proton to the - X. –Therefore, H-bonds are strong.
Intermolecular Forces Hydrogen Bonding Hydrogen bonds are responsible for: –Ice Floating Solids are usually more closely packed than liquids; therefore, solids are more dense than liquids. Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water. In water the H-O bond length is 1.0 Å. The O…H hydrogen bond length is 1.8 Å. Ice has waters arranged in an open, regular hexagon. Each + H points towards a lone pair on O. Ice floats, so it forms an insulating layer on top of lakes, rivers, etc. Therefore, aquatic life can survive in winter.
Intermolecular Forces Hydrogen Bonding Hydrogen bonds are responsible for: –Protein Structure Protein folding is a consequence of H-bonding. DNA Transport of Genetic Information
Some Properties of Liquids Viscosity Viscosity is the resistance of a liquid to flow. A liquid flows by sliding molecules over each other. The stronger the intermolecular forces, the higher the viscosity. Surface TensionSurface Tension Bulk molecules (those in the liquid) are equally attracted to their neighbors. Surface tension is the amount of energy required to increase the surface area of a liquid. Cohesive forces bind molecules to each other. Adhesive forces bind molecules to a surface.
Phase Changes Surface molecules are only attracted inwards towards the bulk molecules. Sublimation: solid gas. Vaporization: liquid gas. Melting or fusion: solid liq. Deposition: gas solid. Condensation: gas liquid. Freezing: liquid solid. Energy Changes Accompanying Phase Changes Sublimation: H sub > 0 (endo) Vaporization: H vap > 0 (endo) Melting or Fusion: H fus > 0 (endo) Deposition: H dep < 0 (exo) Condensation: H con < 0 (exo) Freezing: H fre < 0 (exo)
Phase Changes Energy Changes Accompanying Phase Changes All phase changes are possible under the right conditions (e.g. water sublimes when snow disappears without forming puddles). The sequence heat solid melt heat liquid boil heat gas is endothermic. The sequence cool gas condense cool liquid freeze cool solid is exothermic.
Phase Changes Energy Changes Accompanying Phase Changes
Phase Changes Heating Curves Plot of temperature change versus heat added is a heating curve. During a phase change, adding heat causes no temperature change. –These points are used to calculate H fus and H vap.
Vapor Pressure Explaining Vapor Pressure on the Molecular Level Some of the molecules on the surface of a liquid have enough energy to escape the attraction of the bulk liquid. These molecules move into the gas phase. As the number of molecules in the gas phase increases, some of the gas phase molecules strike the surface and return to the liquid. After some time the pressure of the gas will be constant at the vapor pressure.
Vapor Pressure Explaining Vapor Pressure on the Molecular Level Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface. Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium.
Vapor Pressure Volatility, Vapor Pressure, and Temperature If equilibrium is never established then the liquid evaporates. Volatile substances evaporate rapidly. The higher the temperature, the higher the average kinetic energy, the faster the liquid evaporates.
Vapor Pressure Vapor Pressure and Boiling Point Liquids boil when the external pressure equals the vapor pressure. Temperature of boiling point increases as pressure increases. Two ways to get a liquid to boil: increase temperature or decrease pressure. –Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked. Normal boiling point is the boiling point at 760 mmHg (1 atm).
Phase Diagrams Phase diagram: plot of pressure vs. Temperature summarizing all equilibria between phases. Given a temperature and pressure, phase diagrams tell us which phase will exist. Features of a phase diagram: –Triple point: temperature and pressure at which all three phases are in equilibrium. –Vapor-pressure curve: generally as pressure increases, temperature increases. –Critical point: critical temperature and pressure for the gas. –Normal melting point: melting point at 1 atm.
Phase Diagrams Any temperature and pressure combination not on a curve represents a single phase.
Phase Diagrams The Phase Diagrams of H 2 O and CO 2 Triple point at 0.0098 C and 4.58 mmHg. Normal melting (freezing) point is 0 C. Normal boiling point is 100 C. Critical point is 374 C and 218 atm Triple point at -56.4 C and 5.11 atm. Normal sublimation point is -78.5 C. (At 1 atm CO 2 sublimes it does not melt.) Critical point occurs at 31.1 C and 73 atm.