Jeff Venables Northwestern High School Unit 4 States of Matter CHEMISTRY 2 HONORS Jeff Venables Northwestern High School

Multiple Bonds -Bonds: electron density lies on the axis between the nuclei. All single bonds are -bonds. -Bonds: electron density lies above and below the plane of the nuclei. A double bond consists of one -bond and one -bond. A triple bond has one -bond and two -bonds. Often, the p-orbitals involved in -bonding come from unhybridized orbitals.

Ethylene, C2H4, has: Each C-H bond is a σ bond one - and one -bond between the C atoms; both C atoms sp2 hybridized; both C atoms with trigonal planar electron pair and molecular geometries.

Delocalized p Bonding So far all the bonds we have encountered are localized between two nuclei. In the case of benzene there are 6 C-C  bonds, 6 C-H  bonds, each C atom is sp2 hybridized, and there are 6 unhybridized p orbitals, one on each C atom.

Delocalized p Bonding

In benzene there are two options for the 3  bonds localized between C atoms or delocalized over the entire ring (i.e. the  electrons are shared by all 6 C atoms). Experimentally, all C-C bonds are the same length in benzene. Therefore, all C-C bonds are of the same type (recall single bonds are longer than double bonds).

General Conclusions Every two atoms share at least 2 electrons. Two electrons between atoms on the same axis as the nuclei are  bonds. -Bonds are always localized. If two atoms share more than one pair of electrons, the second and third pair form -bonds. When resonance structures are possible, delocalization is also possible.

Examples – Draw Lewis Structures (including resonance) Examples – Draw Lewis Structures (including resonance). Determine the total number of σ and π bonds in the molecule, and the bond order of each bond: O3 SO3 CO2 2σ and 1π Each bond 1.5 3σ and 1π each bond 1.33 2σ and 2π each bond 2

A Molecular Comparison of Liquids and Solids Physical properties of substances understood in terms of kinetic molecular theory: Gases are highly compressible, assume shape and volume of container: Gas molecules are far apart and do not interact much with each other. Liquids are almost incompressible, assume the shape but not the volume of container: Liquids molecules are held closer together than gas molecules, but not so rigidly that the molecules cannot slide past each other.

Solids are incompressible and have a definite shape and volume: Solid molecules are packed closely together. The molecules are so rigidly packed that they cannot easily slide past each other.

London dispersion forces Converting a gas into a liquid or solid requires the molecules to get closer to each other: cool or compress. Converting a solid into a liquid or gas requires the molecules to move further apart: heat or reduce pressure. The forces holding solids and liquids together are called intermolecular forces. Hydrogen Bonds Dipole-dipole forces London dispersion forces

Summary of Intermolecular Forces Ionic Compound – contains positive ions (usually metals) and negative ions. Held together by ionic bonds. Covalent Compound – contains 2 or more nonmetals (no ions) Nonpolar – contains only London dispersion forces (LDF) Polar – contains LDF and dipole-dipole forces Polar with H bonded to N, O, or F (with unshared pair) – contains LDF, dipole-dipole forces, and hydrogen bonds. Larger molecule, stronger LDF (all other factors equal) Mixture – ions mixed with polar molecules – contains ion-dipole forces (very strong) – like Na+ and Cl- ions in water.

Relative Strengths of Forces Bonds (covalent, ionic, metallic) Ion-dipole forces Hydrogen bonds Dipole-dipole forces London dispersion forces

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 Tension Bulk molecules (those in the liquid) are equally attracted to their neighbors.

Viscosity

Surface Tension

Cohesive forces bind molecules to each other. Surface Tension Surface molecules are only attracted inwards towards the bulk molecules. Therefore, surface molecules are packed more closely than bulk molecules. 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.

Meniscus is the shape of the liquid surface. If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass). If cohesive forces are greater than adhesive forces, the meniscus is curved downwards. Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube. Surface molecules are only attracted inwards towards the bulk molecules.

Phase Changes Examples – Name each of the following phase changes: solid  gas: liquid  gas: solid  liquid: gas  solid: gas  liquid: liquid  solid:

Phase Changes solid  gas: sublimation liquid  gas: vaporization (boiling, evaporation) solid  liquid: melting gas  solid: deposition gas  liquid: condensation liquid  solid: freezing