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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? The answers have to do with …. Intermolecular forces. Intermolecular forces. Overview.

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  1. 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? • The answers have to do with … Intermolecular forces

  2. Intermolecular forces Overview • There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces • Intramolecular bonds are BONDS - ionic, polar covalent or non-polar covalent • Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms 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)

  3. + – H Cl + – + – + – + – Ionic, Dipole - Dipole attractions • We have seen that molecules can have a separation of charge • This happens in both ionic and polar bonds (the greater the EN, the greater the dipoles) • Molecules are non polar if the EN of all bonds is 0 - 0.4 or if it is symmetrical in shape • Molecules are polar if the EN of any bond is 0.5 or greater and if it is non symmetrical in shape

  4. + + + – + – – – + – + – Electronegativity & IMFs • EN essentially defines the type of IMF. • Ionic bonds form if the EN is 1.7 or greater. • Dipole-dipole (polar covalent) is around 0.5-1.9. • Hydrogen bonding is a type of dipole-dipole. • London forces exist in all molecules, but are especially important in non-polar covalent molecules (where EN is less than 0.5). • Recall that ionic are strongest. • Dipole-dipole are not as strong. • Hydrogen bonding is about five times stronger than regular dipole-dipole forces. • London forces are weakest.

  5. H - bonding • H-bonding is a special type of dipole - dipole attraction that is very strong • It occurs when N, O, or F are bonded to H Q- Calculate the EN for HCl and H2O A- HCl = 3.0-2.1 = 0.9, 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

  6. 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

  7. London forces Induced dipole: Instantaneous dipole: Eventually electrons are situated so that tiny dipoles form A dipole forms in one atom or molecule, inducing a dipole in the other

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