PowerPoint - Intermolecular Forces - Ionic, Dipole, London

PowerPoint - Intermolecular Forces - Ionic, Dipole, London

INTERMOLECULAR FORCES QUESTIONS Why do some solids dissolve in water but others do not? Why are some substances gases at room temperature, but others are liquid or solid? Why is water such a good solvent? The answers have to do with Intermolecular forces

Intermolecular Forces There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces We have already looked at intramolecular bonds (ionic, polar, non-polar) Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule) IONIC, DIPOLE - DIPOLE ATTRACTIONS Molecules are attracted to each other by the +ve and ve forces created by the partial charges of a dipole. This is called dipole-dipole

attraction. A dipole (and its partial charges) can also be attracted to the oppositely charged ion in an ionic compound. This is called ion-dipole attraction. + +

+ + + H Cl

H - BONDING H-bonding is a special type of dipole - dipole attraction that is very strong It occurs when F, O, or N are bonded to H Q- Calculate the EN for HCl and H2O A- HCl = 2.9-2.1 = 0.8, H2O = 3.5-2.1 = 1.4 The high EN of NH, OH, and HF bonds cause these to be strong forces (about 5x stronger than normal dipole-dipole forces) They are given a special name (H-bonding) because compounds containing these bonds are important in biological systems

LONDON FORCES Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids? London forces are named after Fritz London (also called van der Waal forces) London forces are due to small dipoles that exist in non-polar molecules Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical The resulting tiny dipoles cause attractions between atoms/molecules LONDON FORCES

Instantaneous dipole: Induced dipole: Eventually electrons A dipole forms in one atom are situated so that or molecule, inducing a tiny dipoles form dipole in the other DO INTERMOLECULAR FORCES MATTER? All molecules have intermolecular forces. The strength of the attraction depends on the EN

value of the dipoles for polar molecules. For non-polar molecules the strength of the attraction depends on how many London forces there are and the size of the atom. Intermolecular forces are responsible for the unique properties of some substances, boiling point, melting point, and are particularly important in biological function.

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