The Ties That Bind

The Ties That Bind

Chemical Bonding and Interactions

Chemical Bonding and Interactions1. Stable Electron Configurations2. Electron-Dot (Lewis) Structures

1. Drawing, Rules for Drawing2. The Octet Rule3. Some Exceptions to the Rule

3. Ionic Bonding1. Naming ionic compounds2. Drawing

4. Covalent Bonding1. Naming covalent compounds2. Drawing3. Electronegativity and Polar Covalent Compounds

5. Molecular Shapes and the VSEPR Theory6. Intermolecular Forces of Attraction

1. 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

11.2

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)

Generally, intermolecular forces are much weaker than intramolecular forces.

“Measure” of intermolecular force

boiling point

melting point

Hvap

Hfus

Hsub

Intermolecular ForcesIntermolecular Forces


Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

11.2

Ion-Dipole Interaction


11.2


Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

11.2


Dipole-Dipole ForcesDipole-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.


London Dispersion ForcesLondon 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).


London Dispersion ForcesLondon Dispersion Forces


London Dispersion ForcesLondon 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.


London Dispersion ForcesLondon Dispersion Forces


London Dispersion ForcesLondon 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 ForcesHydrogen Bond

11.2

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 A H…Aor

A & B are N, O, or F

Hydrogen Bond

11.2

Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen 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.

Maximum Density40C

Ice is less dense than water

Density of Water

11.3

Water is a Unique Substance

Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen Bonding Hydrogen bonds are responsible for:

Protein Structure Protein folding is a consequence of H-

bonding. DNA Transport of Genetic Information

Intermolecular ForcesIntermolecular ForcesComparing Intermolecular ForcesComparing 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 LiquidsSome Properties of Liquids

Surface TensionSurface Tension


Cohesion is the intermolecular attraction between like molecules

11.3

Adhesion is an attraction between unlike molecules

Adhesion

Cohesion


Viscosity is a measure of a fluid’s resistance to flow.

11.3

Strong intermolecular

forces

High viscosity

Ultrahydrophobic surfaces

PHASE CHANGES

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

Phase ChangesPhase 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 ChangesEnergy Changes Accompanying Phase Changes Energy change of the system for the above processes

are:

Phase ChangesPhase Changes

Energy Changes Accompanying Phase ChangesEnergy 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.


Energy Changes Accompanying Phase ChangesEnergy Changes Accompanying Phase Changes


Heating CurvesHeating Curves

Test yourself1. Which has a higher boiling point, ethane (C2H6) or dodecane (C12H26)?2. What kind of IFA will be present in the following

combinations/mixtures?1. Water and ammonia2. Octane and water3. CCl4 and CHCl34. Hydrofluoric acid (HF) and water?5. Acetic acid and cysteine? (see board for structures)6. Water and NaCl

3. Which has a higher boiling point, neopentane or n-pentane? (See board for structures)

4. Which will have a higher boiling point: 1. Ne or Xe2. N2 or Kr

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The Ties That Bind