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Intermolecular forces

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Intermolecular Forces

Intermolecular Forces

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  • 1. Intermolecular FORCES
    PRESENTED BY
    PRATIBHA KOHLI
    DEPARTMENT OF CHEMISTRY, DELHI PUBLIC SCHOOL, RK PURAM
  • 2.
    • Why do some solids dissolve in water but others do not?
    • 3. What happens when noble gases liquefy?
    • 4. Why are some substances gases at room temperature, but others are liquid or solid?
    The answers lie in………
    Intermolecular Forces
  • 5. Intermolecular forces may be attractive or repulsive.
    Johannes D van der Waals, Dutch, was the first to postulate intermolecular forces in developing a theory to account for properties of real gases.
  • 6. van der Waals forces include
    • London forces
    • 7. Dipole - dipole forces
    • 8. Dipole - induced dipole forces
    Other intermolecular forces are
    • Ion - dipole interactions
    • 9. Ion - induced dipole interactions
    • 10. Hydrogen Bonding
  • London Forces (Dispersion Forces)
    These arise from temporary variations in electron density in atoms and molecules. At any instant , the electron distribution may be unsymmetrical and hence produce an instantaneous dipole. This can cause an induced transient dipole in the neighbouring molecule and cause the molecules to be attracted.
  • 11. London forces
  • 12. Dispersion forces are present between all molecules , whether polar or non-polar.
    Dispersion forces are stronger in molecules that are easily polarizable.
    Larger and heavier molecules exhibit stronger dispersion forces than smaller and lighter ones.
  • 13. Shapes of molecules affect magnitude of dispersion forces
    The cylindrical shape of n-pentane has greater surface area .Hence, n-pentane has greater London forces .
    The spherical shape of neo-pentane has lesser surface area. So, it has lesser London attractive forces.
  • 14. Dipole-dipole forces(Keesom Forces)
    These forces arise due to interaction between oppositely charged ends of polar molecules. Greater the dipole moment of molecules , greater are the forces of attraction.
  • 15. DIPOLE-DIPOLE FORCES
  • 16. Dipole-induced dipole forces
    These operate between polar molecules having permanent dipole and the molecules having no permanent dipole.The polar molecule induces a dipole in the neighbouring non-polar molecule.The interaction energy depends on
    • Dipole moment of polar molecule
    • 17. Polarisability of nonpolar molecule
    • Oxygen gas can dissolve in water because the permanent dipole in water can induce a dipole in oxygen molecules.
    • 18. Non-polar iodine dissolves in polar ethanol due to dipole-induced dipole forces.
  • Ion –dipole Interactions
    These interactions depend on
    • Charge and size of ion
    • 19. Magnitude of dipole moment of polar molecule
    • 20. Size of polar molecule
  • Hydration
    Of
    Sodium ions
  • 21. Hydration
    of
    Chloride Ions
  • 22. Hydrogen BONDING
    • It is an electrostatic force of attraction that exists between covalently bonded hydrogen atom of one molecule and the electronegative atom of another molecule.
    • 23. It is represented by a dotted line - - - .
  • Conditions for Hydrogen Bond formation
    A hydrogen atom attached to a relatively electronegative atom is a hydrogen bond donor. This electronegative atom is usually fluorine, oxygen, or nitrogen.
    An electronegative atom such as fluorine, oxygen, or nitrogen is a hydrogen bond acceptor, regardless of whether it is bonded to a hydrogen atom or not.
  • 24. INTERMOLECULAR HYDROGEN BOND
    • It is formed between two different molecules of the same or different substances, eg.
    • 25. Hydrogen bond between molecules of ammonia
    • 26. Hydrogen bond between molecules of water and alcohol
    INTRAMOLECULARHYDROGEN BOND
    • It is formed between the hydrogen atom and a highly electronegative atom present in different bonds within the same molecule, eg.
    • 27. o-salicylaldehyde
    • 28. o- nitrobenzoic acid
  • Association of water molecules
  • 29. HYDROGEN FLUORIDE In the solid state , hydrogen fluoride consists of long zig-zag chains of molecules associated together by hydrogen bonding. Hence, hydrogen fluoride molecule can also be represented as (HF)n. However, in gaseous or liquid state , the chain becomes linear.
  • 30. Alcohols have high miscibility in water due to formation of intermolecular hydrogen bonds between water and alcohol molecules.
  • 31. Higher melting and boiling points
    Dramatically higher boiling points of NH3, H2O, and HF compared to the heavier analogues PH3, H2S, and HCl.
  • 32. Hydrogen Bonding causes
    • Higher boiling point in ethanol as compared to diethyl ether
    • 33. Higher viscosity of sulphuric acid and glycerol
    • 34. Water is a liquid, whereas H2S is a gas
    • 35. Dimerisation of carboxylic acid molecules
  • Hydrogen bonding in CHCl3?
    A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl3. The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, nevertheless represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen-bond acceptor.
  • 36. Intramolecular Hydrogen Bonding
    When hydrogen bonding exists within the molecule it is called intramolecular hydrogen bonding. In such type of hydrogen bonding two groups of the same molecule link through hydrogen bond, forming a stable five or six membered ring structure e.g.,salicylaldehyde, o-chlorophenol, acetylacetone, ethylacetoacetate etc.
  • 37. This intramolecular hydrogen bonding was first called chelation (after the Greek word "Chela" meaning, claw) because in the same molecule the formation of a ring hydrogen bonding is a pincer like action resembling the closing of a Crab's claw. Some more examples of intramolecular hydrogen bonding are :
  • 38. Hydrogen Bonding in DNA
  • 39. Secondary structure of Proteins-Hydrogen Bonding
  • 40. CAGE LIKE STRUCTURE OF
    WATER MOLECULES
    IN ICE
    CAUSES LOWER DENSITY OF ICE THAN WATER
  • 41. Unique nature of water
    • When ice melts, this cage like structure collapses and the molecules move closer.
    • 42. Hence, for the same mass of water, volume decreases and density increases. Hence ice has lower density than water at 273K.
  • Comparison of intermolecular forces