Intermolecular Interactions The forces between particles

Intermolecular Interactions Weakest Dispersion – Simply ‘chemical gravity’ Dipole-Dipole – The interaction of two opposing dipoles as an attractive force StrongestHydrogen Bonding – The interaction of two opposing dipoles, which contain H, make them very strong dipoles

Disperson Forces (weakest) “Dispersion force is a weak intermolecular force arising from quantum-induced instantaneous polarization multipoles in molecules. They can therefore act between molecules without permanent multipole moments.” That is – in real speak – randomly all the electrons are on one side of the molecule (just by statistical chance) creating an instantaneous dipole. This leads to a slight interaction.

Dipole-Dipole “Dipole-dipole interactions are electrostatic interactions between permanent dipoles in molecules. These interactions tend to align the molecules to increase attraction. An example of a dipole-dipole interaction can be seen in hydrogen chloride (HCl): the positive end of a polar molecule will attract the negative end of the other molecule and influence its position. Polar molecules have a net attraction between them.”

Hydrogen Bonding “A hydrogen bond is the attraction between the lone pair of an electronegative atom and a hydrogen atom that is bonded to either nitrogen, oxygen, or fluorine. The hydrogen bond is often described as a strong electrostatic dipole-dipole interaction”

Melting points and Boiling Points The energy it takes to break these intermolecular interactions are measured as the melting and boiling points. The greater the interactions (i.e. stronger intermolecular interactions) the harder they are to break and therefore have higher melting and boiling points.

Solids There are four major families of solids: – Covalent Network – Ionic – Molecular – Metallic Each of these sub-families has very different properties as a result of their strusctures.

Covalent Network Examples: Diamond, quartz, glass The atoms are all held to each other by covalent bonds (strong). This means they are very hard, strong and have very high melting points. It also means that they are not water soluble.

Ionic Examples: NaCl, PbF 2, CaO The ions are held in a lattice by strong electronegative interactions. This means that these too ave high melting points but they are brittle. They are water soluble and then can conduct electricity.

Molecular Examples: Caffiene pills, sugar cubes, ice. The individual molecules are held by whatever intermolecular forces they have depending upon their dipoles etc… This means that they generally have low melting points an varying water solubility.

Metallic Examples: Iron rod, Copper wire The electrons inside a metals structure are delocalised, they are free to move around a lattice or nuclei. This means that they conduct electricity and heat, they are ductile and malleable, hard, durable and have high melting points. This electronic structure is commonly referred to as “a sea of electrons”.

Summary of Solids TypeExamplesConstituentsBondingProperties Networkdiamond, quartzatomscovalent bondshigh melting points poor conductors insoluble IonicNaCl, CaF 2 cations and anionsionic bondshigh melting points good conductors brittle hard often water soluble MolecularH 2 O, S 8, sugarmoleculesintermolecular forces low melting points poor conductors solubility dependent on polarity MetallicCopper, zinccations, electronsmetallic bondshigh melting points good conductors

Summary FOUR type of solids & bonding they have: Covalent network – covalent Metallic – sea of electrons Ionic – electrostatic Molecular – dispersion / dipole-dipole / hydrogen bonding