Edexcel A Level Chemistry:复习笔记1.5.4 Intermolecular Forces & Physical Properties

Intermolecular Forces & Physical Properties

 

Branching

  • The larger the surface area of a molecule, the more contact it will have with adjacent molecules
  • The surface area of a molecule is reduced by branching
  • The greater its ability to induce a dipole in an adjacent molecule, the greater the London (dispersion) forces and the higher the melting and boiling points
  • This point can be illustrated by comparing different isomers containing the same number of electrons:

1.3-Chemical-Bonding-Contact-Points

Boiling points of molecules with the same numbers of electrons but different surface areas

Number of electrons

  • The greater the number of electrons (or the greater the molecular mass) in a molecule, the greater the likelihood of a distortion and thus the greater the frequency and magnitude of the temporary dipoles
  • The dispersion forces between the molecules are stronger and the enthalpy of vaporisation, melting and boiling points are larger
  • The greater boiling points of the noble gases illustrate this factor:

1.3-Chemical-Bonding-Enthalpy-and-Boiling-Point

As the number of electrons increases more energy is needed to overcome the forces of attraction between the noble gases atoms

10.1.1-Alkanes-Boiling-Point-Graph

Graph showing the increase in boiling point as the number of electrons increases

Alcohols

  • Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen
  • In an alcohol, there are O-H bonds present in the structure
  • Therefore hydrogen bonds set up between the slightly positive hydrogen atoms (δ+ H) and lone pairs on oxygens in other molecules
  • The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms
  • In alkanes, the only intermolecular forces are temporary induced dipole-dipole forces
  • Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules
  • Therefore, the boiling point of alkanes is lower than the boiling point of the respective alcohols
  • For example, the boiling point of propane is -42 oC and the boiling point of propanol is 97 oC

 

Hydrogen Halides 

  • The boiling points of the hydrogen halides are as follows

2-2-3-the-boiling-points-of-the-hydrogen-halides

 

  • The boiling points of the rest of the hydrogen halides increase as the molecules become larger
  • The extra electrons allow greater temporary dipoles and so increase the amount of London dispersion forces between the molecules
  • Hydrogen fluoride also has hydrogen bonding between the HF molecules
  • The bond is very polar so that the hydrogen has a significant amount of positive charge and the fluorine a significant amount of negative charge. In addition, the fluorine has small intense lone pairs
  • Hydrogen bonds can form between the hydrogen on one molecule and a lone pair on the fluorine in its neighbour

 

Choosing Solvents

Solubility

  • The general principle is that 'like dissolves like' so non-polar substances mostly dissolve in non-polar solvents, like hydrocarbons and they form dispersion forces between the solvent and the solute
  • Polar covalent substances generally dissolve in polar solvents as a result of dipole-dipole interactions or the formation of hydrogen bonds between the solute and the solvent
  • A good example of this is seen in organic molecules such as alcohols and water:

 

4.1.14-Hydrogen-bonds-between-ethanol-and-water-1 

Hydrogen bonds form between ethanol and water

 

 

  • As covalent molecules become larger their solubility can decrease as the polar part of the molecule is only a smaller part of the overall structure
    • This effect is seen in alcohols for example where ethanol, C2H5OH, is readily soluble but hexanol, C6H13OH, is not

     

  • Polar covalent substances are unable to dissolve well in non-polar solvents as their dipole-dipole attractions are unable to interact well with the solvent
  • Giant covalent substances generally don't dissolve in any solvents as the energy needed to overcome the strong covalent bonds in the lattice structures is too great

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