Intermolecular Forces  In the particles of a liquid the particles are much closer than in a gas. This is because they have greater intermolecular forces.  Intermolecular forces exist between particles and there are different types.  They have a huge impact on molecular properties.

Ion-Dipole Forces  These are among the strongest IMFs. Ion-Ion forces are stronger.  These forces occur between an ion and a polar covalent compound.  The strength of the force depends on the distance, the charge of the ion, and the magnitude of the dipole.

Enthalpy of Hydration  This enthalpy measures the change of energy involved with the hydration of ions.  The enthalpy depends on 1/d. The d is distance.  As the atomic radius increases, the enthalpy decreases.  Why is the enthalpy larger for sodium than cesium?  Why is magnesium even bigger than sodium or cesium?  Would fluorine or chlorine have a higher enthalpy? Why?

Dipole-Dipole Attractions  These happen when two polar molecules interact. The positive end of one molecule is attracted to the negative end of another.  These influence evaporation and condensation.  Which process is endothermic? Which is exothermic?  Greater IMFs cause the energy required to separate molecules increase. This is why polar compounds typically have higher boiling points than nonpolar compounds.

Like Dissolves Like  This is a common statement in chemistry. It is saying that a polar molecules can be dissolved in a polar solvent, but not a nonpolar one.  This is a result of the IMFs.  Water and oil don’t mix because water is polar while oil is nonpolar.

Hydrogen Bonding  The hydrogen involved must be bonded to a nitrogen, oxygen or fluorine. It will be attracted to a nitrogen, oxygen or fluorine with a lone pair of electrons.  Which molecule will have a higher boiling point? Ethanol of dimethyl ether?  Draw the Lewis structure for methanol (CH 3 OH). Describe how the hydrogen bonding occurs in methanol.

Hydrogen Bonding in Water  Hydrogen bonding in water explains one of the unique characteristics of water.  Water is one of the few substances that has a higher density as a liquid than a solid.  Water’s density is greatest at 4°C. Water colder than that will be on top and eventually freezes. It keeps aquatic species alive as a result.  Hydrogen bonding is also responsible for the high heat capacity of water. This is why large bodies of water have a large effect on the weather.

Dipole/Induced Dipole Forces  Some nonpolar substances such as oxygen and nitrogen can have a dipole induced by polar molecules. The dipole of a polar molecule can affect the electrons in the nonpolar molecule and induce a dipole.  The process of inducing a dipole is called polarization.  Larger atoms are easier to polarize.

London Dispersion Forces  These are also called induced dipole/induced dipole forces.  In nonpolar substances, a dipole can be induced when molecules are close and their electrons repel. This warps the electron cloud and creates dipoles.  This also occurs in polar molecules, but it is the only IMF for nonpolar molecules.

Types of IMFs A term for all attractive IMFs is van der Waals forces. Type of InteractionApproximate Energy (kJ/mol) Ion-dipole Dipole-dipole20-30 Hydrogen bonding5-30 Dipole/induced dipole2-10 London Dispersion Forces

Practice  If you have a mixture of solid iodine, water and liquid carbon tetrachloride, what IMFs exist between each set of molecules?  What IMFs will exist in a mixture of water, carbon tetrachloride, and hexane?

More Practice  Which IMF is most important in each situation? Rank in order of increasing strength.  Liquid methane, CH 4  Mix of water and methanol  Solution of bromine in water  Which IMF is involved in each situation? Rank in order of increasing strength.  Liquid oxygen  Liquid methanol  Nitrogen dissolved in water

Assignment  Page 581 2, 4, 8, 10

Vaporization  Vaporization is when a liquid changes into a gas.  Molecules at a particular temperature have a range of energies. The molecules with the highest energies can evaporate.  There is energy exchanged when a liquid is vaporized. The molar enthalpy of vaporization, Δ vap H, gives us how many kilojoules per mole are needed to vaporize a liquid.  They are given in a table on page 572.  Will the values for the enthalpy of vaporization be positive or negative?

Condensation  Condensation is when a gas turns into a liquid.  How can we determine the molar enthalpy of condensation?

Example  You put 925 mL of water in a pan at 100°C and the water evaporates. How much energy was transferred as heat? Density of water at 100°C = g/mL

Practice  The molar enthalpy of methanol, CH 3 OH, is 35.2 kJ/mol. How much energy is required to evaporate 1.00 kg of methanol?

Vapor Pressure  If a liquid is in a sealed container, it will evaporate until it reaches an equilibrium. Molecules evaporate and condense at the same rate.  When the equilibrium occurs, the pressure of the gas is the vapor pressure.  A liquid with a large vapor pressure at equilibrium is called volatile.  As temperature increases, vapor pressure increases.  A vapor pressure curve gives you pressure and temperature and any point on the curve tells a pressure and temperature where an equilibrium occurs.

Example  If you place 2.00 L of water in a room with a volume of 4.25X10 4 L. Will the water evaporate at 25°C? Vapor pressure=23.8 mm Hg

Practice  What is the vapor pressure of ethanol at 40°C?  Is there an equilibrium if the temperature is 60°C and the pressure is 600 mm Hg? If not, do you have more liquid or vapor?

Assignment  Page 575 Exercise 12.7  Page , 14, 16, 20

Clausius-Clapeyron Equation

Boiling Point and Pressure  The boiling point varies with pressure. In Salt Lake City, the pressure is about 650 mm Hg and the boiling point of water is about 95 °C.  The line between liquid and solid does not continue forever. Once the critical point is reached. At pressures and temperatures above the critical point, the substance is a supercritical fluid.  A supercritical fluid is like a gas with high pressure and the density is closer to a liquid’s density and the viscosity is close to a gas. Supercritical carbon dioxide is used to decaffeinate coffee.

Characteristic of Liquids  Surface Tension- the energy required to break through the surface or disrupt a liquid drop.  Capillary Action- water molecules are attracted to polar substances, like glass or paper, and rise up with paper due to IMFs.  Viscosity- the resistance of liquids to flow. Water has a low viscosity, while molasses has a very high viscosity.  Longer carbon chains are more viscous.  Which is more viscous? or

Assignment  Page , 24, 26, 28, 30, 32, 36, 40, 42, 44, 58