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Intermolecular Forces

Intermolecular Forces. Intermolecular forces. The attraction and repulsion forces between molecules ( intramolecular forces are the forces within the molecule – bonding) Cotton vs plastic raincoat Broken down into three types. Van der Waals Forces. Intermolecular forces

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Intermolecular Forces

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  1. Intermolecular Forces

  2. Intermolecular forces • The attraction and repulsion forces between molecules • (intramolecular forces are the forces within the molecule – bonding) • Cotton vs plastic raincoat • Broken down into three types

  3. Van der Waals Forces • Intermolecular forces • Forces holding molecules in order • But very weak compared to the covalent bonding between elements • Energy to boil water vs the amount to decompose water

  4. Dipole-Dipole Force • Force between dipoles or polar molecules • Strength of the force depends on the polarity of the molecule

  5. London Force • Force between nonpolar molecules • While the overall molecule does not have a dipole the electrons moving within the molecule will create a momentary dipole • The more electrons the molecule has the higher the London Force

  6. Predicting boiling points • Can use the number of electrons as a prediction of the boiling point. • This is because the intermolecular forces are related to how difficult the molecules are to separate

  7. Isoelectronic Molecules • Iso – meaning the same • electronic – electrons • These molecules would have the same number of electrons and the London forces will be similar • However this does not mean that the boiling points would be the same. • Remember the boiling point depends on the London forces AND the dipole-dipole

  8. Summary • More polar -> stronger dipole-dipole -> higher boiling point • More electrons -> larger London Forces -> higher boiling point • Using this knowledge to predict relative boiling points for certain cases

  9. Lab Exercise 3.4

  10. Hydrogen bonding • Hydrogen (proton) is shared with a lone pair. • Hydrogen needs to be bonded with an atom with a high electronegativity • Need to have at least one lone pair on the atom bonded to Hydrogen to give hydrogen bonding to another molecule • Three possible structures: HF (g), anything with –OH and –NH. • Example is sugar C12H22O11(s), or better written as C12H14O3(OH)8, we can see there are 8 –OH groups to hydrogen bond

  11. Examples • DNA and proteins • Water, ice • Reason for different densities

  12. Physical Properties of Liquids • Intermolecular forces (IMF) contribute to various • Surface tension – in the liquid IMF are in all directions but at the surface it is only away from the surface. • Cohesion – attraction between like molecules • Adhesion – attraction between unlike molecules

  13. Properties Cont’d • How easily the liquid evaporates • Weak forces – easy to evaporate – volatility

  14. Summary • Intermolecular forces are the attraction and repulsion of positive and negative charges • All molecules have London forces – with momentary diploes • Polar molecules have dipole-dipole forces • Hydrogen bonding occurs when a hydrogen is attracted to the lone pair from an adjacent molecule • IMF affect many various physical properties

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