Chemical Bonding and Interactions The Ties That Bind Chemical Bonding and Interactions

Chemical Bonding and Interactions Stable Electron Configurations Electron-Dot (Lewis) Structures Drawing, Rules for Drawing The Octet Rule Some Exceptions to the Rule Ionic Bonding Naming ionic compounds Drawing Covalent Bonding Naming covalent compounds Electronegativity and Polar Covalent Compounds Molecular Shapes and the VSEPR Theory Intermolecular Forces of Attraction H-bonds, Dipole-Dipole, Ion-Dipole, London Dispersion Forces

INTERMOLECULAR FORCES OF ATTRACTION

A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water 11.1

Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub Generally, intermolecular forces are much weaker than intramolecular forces. 11.2

Intermolecular Forces

Intermolecular Forces Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction 11.2

Intermolecular Forces

11.2

Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11.2

Intermolecular Forces Dipole-Dipole 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. It is possible for two adjacent neutral molecules to affect each other. 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).

Intermolecular Forces London Dispersion Forces

Intermolecular Forces London Dispersion Forces One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom). Instantaneous dipoles are called London Dispersion Forces. 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.

Intermolecular Forces London Dispersion Forces

Intermolecular Forces London Dispersion Forces London dispersion forces increase as molecular weight increases. London dispersion forces exist between all molecules. London dispersion forces depend on the shape of the molecule. 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 Bond The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H … B or A & B are N, O, or F 11.2

Hydrogen Bond 11.2

Why is the hydrogen bond considered a “special” dipole-dipole interaction? Decreasing molar mass Decreasing boiling point 11.2

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.

Ice is less dense than water Water is a Unique Substance Maximum Density 40C Density of Water Ice is less dense than water 11.3

Intermolecular Forces Hydrogen Bonding Hydrogen bonds are responsible for: Protein Structure Protein folding is a consequence of H-bonding. DNA Transport of Genetic Information

Intermolecular Forces Comparing Intermolecular Forces

Strong intermolecular forces Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces High surface tension 11.3

Some Properties of Liquids Surface Tension

Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules Adhesion Cohesion 11.3

Strong intermolecular forces Properties of Liquids Viscosity is a measure of a fluid’s resistance to flow. Strong intermolecular forces High viscosity 11.3

Ultrahydrophobic surfaces

PHASE CHANGES We can use the concepts of intermolecular forces of attraction to explain the physical phase changes

Phase Changes Energy Changes Accompanying Phase Changes Surface molecules are only attracted inwards towards the bulk molecules. Sublimation: solid  gas. Vaporization: liquid  gas. Melting or fusion: solid  liquid. Deposition: gas  solid. Condensation: gas  liquid. Freezing: liquid  solid. Energy Changes Accompanying Phase Changes Energy change of the system for the above processes are:

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. cool gas  condense  cool liquid  freeze  cool solid is exothermic.

Phase Changes Energy Changes Accompanying Phase Changes

Phase Changes Heating Curves

Test yourself Which has a higher boiling point, ethane (C2H6) or dodecane (C12H26)? What kind of IFA will be present in the following combinations/mixtures? Water and ammonia Octane and water CCl4 and CHCl3 Hydrofluoric acid (HF) and water? Acetic acid and cysteine? (see board for structures) Water and NaCl Which has a higher boiling point, neopentane or n-pentane? (See board for structures) Which will have a higher boiling point: Ne or Xe N2 or Kr