Intermolecular Forces Ch 11 Liquids & Intermolecular Forces

Remember the Kinetic Theory of Molecular Motion Remember the Kinetic Theory of Molecular Motion. Solids have fixed volume and shape. Liquids have a fixed volume with changeable shape. Gases spread to fill any shape and volume. Lets look at the properties of solid and liquids as well as the intermolecular forces.

Intermolecular forces Van der Waals forces are any forces between molecules. The more forces between molecules, the higher the melting point and boiling point. Dipole – dipole attraction Hydrogen bonding London dispersion forces

Dipole – Dipole forces Molecules with dipole moments can attract other molecules with dipole moments. (Polar molecules) Positive end and negative sides line up. These are permanent forces.

Ionic solids exhibit electrostatic forces of attraction because of the positive and negative ions.

N O O O No dipole DEN = 0.0 bp = -195.6oC Polar DEN = 0.5 bp = -151.6oC No dipole DEN = 0.0 bp = -182.0oC Three molecules that have similar arrangement. N2 and O2 which exhibit no dipole moment, will evaporate at lower temperatures than NO which has a small dipole moment. If the molecule has an electrical dipole moment, the molecules will be attracted to each other.

Boiling points are simply a function of the molecular mass when the molecules are non-polar.

An increase in the dipole moment, causes an increase in the boiling point. The molecules set up a “network” of attractions between positive and negative ends. Note that all the molecules listed an similar molecular mass, but the boiling point is a function of the dipole moment.

Hydrogen bonding: A bond formed when a H atom bonded to F, O, or N is attracted to the F, O, or N in a neighboring molecule. Hydrogen bonding is not as strong as a covalent bond, but is very important.

Some of the molecules exhibiting hydrogen bonding which are very important. Water DNA Proteins One hydrogen bond is very weak. But 100’s to 1000’s of hydrogen bonds become very strong.

Because of hydrogen bonding, water exhibits : Adhesion : attraction to other objects Cohesion: attraction to itself Adhesion: water adhering to the xylem in a plant stem, results in capillary action Cohesion: water sticking to water, force that allows water to stick together in capillary action High surface tension: water resisting an increase in the surface area High Surface Tension: hydrogen bonding at the surface, High Specific Heat: energy required to raise the temperature of 1 gram of water by 1oC Viscosity: resistance to flow of a liquid

Comparison of boiling points of covalent hydrides Comparison of boiling points of covalent hydrides. Note that the boiling points exhibited within a family increase as molecular mass increases. However, H2O, because of the hydrogen bonding between molecules, boils at 100oC.

Comparison of the boiling points Ethane CH3CH3 bp = -88 Comparison of the boiling points Ethane CH3CH3 bp = -88.63oC ethane without hydrogen bonding Ethanol CH3CH2OH bp = 78oC ethanol with hydrogen bonding Bond energy Covalent bond = 150 KJ/mole Hydrogen bond = 25 KJ/mole

Hydrogen bonding in DNA The complementary nitrogen bases on the two strands hydrogen bond together.

The secondary structures of proteins include beta (b) pleated sheets and alpha (a) helix. Both structures are held in their shapes by hydrogen bonding.

Induced Dipole – London Dispersion forces An atom at one instant of time may have a concentration of e- in one region of the atom. An instantaneous dipole moment is formed.

The ability to induce a dipole is directly dependent on: 1 The ability to induce a dipole is directly dependent on: 1. an increased number of e- 2. an increasing molar mass 3. long linear molecules Ethane CH3CH3 30.06 m b.p. -88.63oC Propane CH3CH2CH3 44.09 m b.p. -42.5oC Hexane C6H14 86.16 m b.p. 68oC Decane C10H22 142.25 m b.p. 174oC An induced dipole causes an increase in the boiling point of a molecule.

Classification of solids Crystalline solids have a highly regular repeating arrangement of components Amorphous solids have a great amount of disorder in their structure.

Metallic solids exhibit a sea of e- which can flow over the entire solid. This explains the ability of metals to conduct electrical current.

Phase Changes Gas Liquid Solid Sublimation Condensation Deposition Melting or Liquefaction Freeze or Solidification Condensation Evaporation or Vaporization

All substances go through phase changes All substances go through phase changes. A Phase Change Chart can be constructed for any substance.

When changing the temperature of a substance, the same formula is used. H = m c DT The specific heat (c) is different for each substance, and phase of the substance.

H = Hf m H = Hv m Melting Evaporating (Vaporizing) The plateau’s represent phase changes. Melting H = Hf m Evaporating (Vaporizing) H = Hv m Hf and Hv are unique values for each substance. For water, Hf = 344.9j/goC Hv =2260.4j/goC

Triple Point: all three phase can exit Critical Point: point where vapor and liquid are indistinguishable.

760 torr (1 atm) is shows the normal evaporation temperature.. The vapor pressure (point when the liquid turns into a gas) is a function of the temperature. The line at 760 torr (1 atm) is shows the normal evaporation temperature..

Liquid Crystals Viscous – milky state between a liquid & solid See page 468 Viscous – milky state between a liquid & solid Used as a pressure – temperature sensor in liquid crystal displays (LCD) Liquid – molecules are random Nematic crystals align in liquid with the long axis pointing the same way Smectic A – crystal in liquid axis align with layers Smectic C – crystals in liquid axis align in layers with an incline Cholesteric – crystals align in layers with the axis rotating each layer