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Inter molecular Forces Topic 4.3

Inter molecular Forces Topic 4.3. between. Intra vs. Inter molecular forces. Intra- strong forces (ionic or covalent) that hold the atoms in a molecule together takes 464 kJ/ mol to break the H-O bonds within a water molecule responsible for chemical properties. Inter-

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Inter molecular Forces Topic 4.3

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  1. Intermolecular ForcesTopic 4.3 between

  2. Intra vs. Inter molecular forces • Intra- • strong forces (ionic or covalent) that hold the atoms in a molecule together • takes 464 kJ/mol to break the H-O bonds within a water molecule • responsible for chemical properties

  3. Inter- • weak forces that holds molecules to one another • takes only 19 kJ/mol to break the bonds between water molecules • the strength of the intermolecular forces determines the physical properties of the substance • melting, boiling, reacting, solubility, conductivity, volatility

  4. 3 main “types” of intermolecular forces • temporary/induced/instantaneous dipole-dipole forces called Van der Waals’ • permanent dipole-dipole forces (polar molecules) • a stronger type of dipole-dipole bonding called hydrogen bonding strength increases

  5. 1. van der Waals’ forces

  6. van der Waals’ YouTube (:20) • also known as London Dispersion Forces • even nonpolar molecules have forces that hold them together • the distribution of electrons around an individual atom, at a given instant in time, may not be perfectly symmetrical • this can produce temporary/instantaneous dipoles (polar molecule) • this can then induce a nearby molecule to be polar and therefore a very weak attraction between the two molecules • the more electrons in an atom, the greater the Van der Waals’ forces

  7. Sticky secret. Tiny hairs on geckos' feet help maximize contact with surfaces, allowing van der Waals forces to go to work.

  8. Dipole-Dipole Forces • attractive forces between the positive end of one polar molecule and the negative end of another polar molecule • must be in close proximity for the dipole-dipole forces to be significant • the more polar the molecule, the greater the dipole-dipole force • stronger than van der Waals’ forces

  9. Hydrogen Bonding

  10. YouTube Hydrogen Bonding (1:40) • YouTube Hydrogen Bonding Video (:58) • a specific type of dipole-dipole type interactions • stronger than other dipole-dipole and Van der Waals’ • the hydrogen (H) in a molecule is bonded to a small, highly electronegative element (usually an N, O or F atom) on another molecule H-NOF

  11. Intermolecular forces affect on boiling point (4.3.2) • the greater polarity of a molecule, the higher the boiling point • In HF, His 2.1 and F is 4.0, difference of 1.9 • In HCl, H is 2.1 and Cl is 3.0, difference of 0.9 BP is 20°C BP is -85°C

  12. H-NOF ? • H2O vs. H2S? • In H2O, H is 2.1 and O is 3.5, difference of 1.4 • water molecules can hydrogen bond to each other • BP is 100°C • In H2S, H is 2.1 and Cl is 2.5, only a difference of 0.4 • H2S can only dipole-dipole bonding to each other • BP is -60°C H-NOF ? no H-NOF ? yes

  13. H-NOF ? • CH3OCH3 vs. CH3CH2OH? • -24°C 78°C only dipole – dipole has hydrogen bonding • NH3 vs. PH3? • -33°C -87°C • has hydrogen bondingonly dipole – dipole

  14. CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ? only van der Waals’ low BP dipole-dipole medium BP hydrogen bonding highest BP

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