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Intermolecular forces

Intermolecular forces. L.O.: Describe intermolecular forces in terms of permanent and instantaneous (van der Waals’ forces) dipoles. In pairs discuss: which of these molecules has a permanent dipole (H 2 O, CO 2 , CCl 4 , NH 3 ). Permanent dipole–dipole forces between HCl molecules.

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Intermolecular forces

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  1. Intermolecular forces L.O.: Describe intermolecular forces in terms of permanent and instantaneous (van der Waals’ forces) dipoles.

  2. In pairs discuss: which of these molecules has a permanent dipole (H2O, CO2, CCl4, NH3)

  3. Permanent dipole–dipole forces between HCl molecules

  4. A permanent dipole-dipole force is a weak attractive force between permanent dipoles in neighbouring polar molecules.

  5. van der Waals’ forces

  6. The size of the van der Walls forces increases with the number of electrons present.

  7. Why do the boiling points of the noble gases increase as the atomic number of the noble gases increase?

  8. Hydrogen bonding • Describe hydrogen bonding between molecules containing –OH and –NH groups. • Describe and explain the anomalous properties of water resulting from hydrogen bonding.

  9. Look at this example of hydrogen bonding between 2 molecules of water. • In pairs discuss: • What is a hydrogen bond? • What is required to make a hydrogen bond?

  10. Hydrogen bonding: a type of intermolecular force in which a hydrogen atom covalently bonded to an electronegative atom interacts with another electronegative atom.

  11. To form a hydrogen bonding we must have: • a hydrogen atom that is bonded to a very electronegative atom (O,N and F). • A very electronegative atom with a lone pair of electrons.

  12. What trend would you expect in the boiling points of: • Ne, Ar, Kr, Xe • CH4, SiH4, GeH4, SnH4, • NH3, PH3, AsH3, SbH3, • HF, HCl, HBr HI • H2O, H2S, H2Se, H2Te,

  13. 100 Mr 0 BOILING POINT / C° 100 140 50 -160 BOILING POINTS OF HYDRIDES GROUP IV The boiling points of the hydrides increase with molecular mass. CH4 has the lowest boiling point as it is the smallest molecule. PbH4 GeH4 SiH4 Larger molecules have greater intermolecular forces and therefore higher boiling points CH4

  14. 100 Mr 0 BOILING POINT / C° 100 140 50 -160 BOILING POINTS OF HYDRIDES GROUP V NH3 has a higher boiling point than expected for its molecular mass. There must be an additional intermolecular force. NH3

  15. 100 Mr 0 BOILING POINT / C° 100 140 50 -160 BOILING POINTS OF HYDRIDES H2O GROUP VI H2O has a very much higher boiling point for its molecular mass. There must be an additional intermolecular force.

  16. 100 Mr 0 BOILING POINT / C° 100 140 50 -160 BOILING POINTS OF HYDRIDES GROUP VII HF has a higher boiling point than expected for its molecular mass. There must be an additional intermolecular force. HF

  17. 100 Mr 0 BOILING POINT / C° 100 140 50 -160 BOILING POINTS OF HYDRIDES H2O The higher than expected boiling points of NH3, H2O and HF are due to intermolecular HYDROGEN BONDING HF NH3 GROUP IV GROUP V GROUP VI GROUP VII

  18. 100 Mr 0 BOILING POINT / C° 100 140 50 -160 BOILING POINTS OF HYDRIDES GROUP IV GROUP V GROUP VI GROUP VII

  19. The boiling points of H2O, HF, NH3, are higher than what we would expect if only van der Waals forces were operating. This is because hydrogen bonding is present between the molecules in each of these compounds.

  20. HYDROGEN BONDING - ICE • each water molecule is hydrogen-bonded to 4 • others in a tetrahedral formation • ice has a “diamond-like” structure • volume is larger than the liquid making it • when ice melts, the structure collapses • slightly and the molecules come closer; they • then move a little further apart as they get • more energy as they warm up • this is why… • water has a maximum density at 4°C • ice floats. hydrogen bonding lone pair

  21. HYDROGEN BONDING - ICE hydrogen bonding

  22. Liquid water: hydrogen bonds break and reform Ice: water molecules are in fixed positions. Molecules are slightly less packed than in liquid water.

  23. In which of the following does hydrogen bonding not occur between molecules: H2O, NH3, HBr, HF • Explain why hydrogen bonds do not form between a) molecules of CH4 b) molecules of CCl4 • Explain why water molecules form on average two hydrogen bonds per molecule, where the ammonia molecule, NH3, forms only one

  24. Hydrogen bonding in water

  25. For H-bonding look out for O-H or N-H bonds

  26. The ice lattice is an open network of H2O molecules

  27. In DNA, three hydrogen bonds link C and G

  28. In DNA, two hydrogen bonds link A and T

  29. Double helix in DNA

  30. Explain why the molecule PH3 has a pyramidal shape and a bond angle of 107o (3 or 4 marks) • 4 electron pairs surround the central atom. 3 bonding pairs and a lone pair • electron pairs repel other electron pairs • lone pair repel more • electron pairs get as far apart as possible

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