Intermolecular Forces and Bonding: London Dispersion, Dipole-Dipole, and Hydrogen Bonding, Slides of Molecular Chemistry

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Intermolecular Bonding

Intermolecular Forces

nThe forces that hold two or more molecules to each other are called intermolecular forces. nSuch forces create intermolecular bonds.

Comparing intermolecular bonds

nAt room temperature, intermolecular forces are stronger in. (solid, liquid or gas) nEX: F 2 (g) boils at -188°C. Br 2 (l) boils at 58°C. nBr 2 (l) has stronger intermolecular forces between its molecules. solids

Ex: water

van der Waals forces

nJohannes van der Waals first suggested that weak forces of attraction exist between molecules in 1873. nIntermolecular forces are sometimes called van der Waals forces.

Inter vs. intra

nVan der Waals suggested intramolecular forces are much stronger than intermolecular forces.

Types of intermolecular forces

nLondon dispersion (LD) nDipole - dipole (d-d) nHydrogen bonding (HB)

London dispersion force (LD)

n…exist between molecules of any type. n…are caused by the attraction between the e¯ of one molecule and the p+^ in the nucleus of neighbouring molecules.

Factors that effect LD

nNumber of e¯: nThe more e¯ in a substance’s molecules, the stronger the LD force between the molecules nMolecular shape: nShape affects the strength of LD forces in ways that are beyond the scope of Chem 20

Ex:

nWhich has stronger LD forces between their molecules, nitrogen or neon? ncount e¯ in each molecule nthe more e¯ the stronger the LD forces

Evidence

nThat N 2 has stronger LD forces (intermolecular bonds) than Ne, is reflected in their boiling points: nN 2 b.p. = °C nNe b.p. = °C

Dipole-dipole forces (d-d)

nDipole-dipole forces create intermolecular bonds between the molecules, if the molecules are polar.

Ex.

nCompare the intermolecular forces between molecules of F 2 and between molecules of fluoromethane, CH 3 F.

LD forces

n (^) Both have LD (all substances do) n (^) Count e¯ of each to compare strength of LD forces. n F 2 : 9e¯ x 2 = 18e¯ n (^) CH 3 F: n (^) (1 x 6e¯) + (3 x 1e¯) + (1 x 9e¯) n (^) = 18e¯