Intramolecular forces

• Intramolecular forces?are forces?within?a molecule and are usually covalent bonds
• Covalent bonds?are formed when the outer electrons of two atoms are?shared
• Single, double, triple and co-ordinate bonds are all types of intramolecular forces

Intermolecular forces

• Molecules also contain weaker?intermolecular forces?which are forces?between?the molecules
• There are three types of intermolecular forces:
  • Induced?dipole – dipole forces?also called?van der Waals?or?London dispersion forces
  • Permanent dipole – dipole forces?are the attractive forces between two neighbouring molecules with a permanent dipole
  • Hydrogen Bonding?are a special type of?permanent dipole - permanent dipole?forces
  • Intramolecular forces?are?stronger?than intermolecular forces
    • For example, a hydrogen bond is about one tenth the strength of a covalent bond

     

  • The strengths of the types of bond or force are as follows:

   

The varying strengths of different types of bonds

Polarity

• When two atoms in a covalent bond have the?same?electronegativity?the covalent bond is?nonpolar

The two chlorine atoms have the same electronegativities so the bonding electrons are shared equally between the two atoms

• When two atoms in a covalent bond have?different electronegativities?the covalent bond is?polar?and the electrons will be drawn towards the?more electronegative?atom
• As a result of this:
  • The negative charge centre and positive charge centre do not?coincide?with each other
  • This means that the?electron distribution?is?asymmetric
  • The?less?electronegative?atom gets a partial charge of δ+ (delta?positive)
  • The?more?electronegative?atom gets a partial charge of δ- (delta?negative)

   

• The greater the difference in?electronegativity?the more polar the bond becomes

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive

Permanent dipole - dipole forces:

• Polar molecules?have?permanent dipoles
• The molecule will always have a?negatively?and?positively charged end

• Forces between two molecules that have permanent dipoles are called?permanent dipole - dipole forces
• The?δ+ end of the dipole?in one molecule and the?δ- end of the dipole?in a neighbouring molecule are?attracted?towards each other

Induced dipole-dipole forces:

• Induced dipole - dipole forces?exist between all atoms or molecules
  • They are also known as?van der Waals forces?or?London dispersion forces

   

• The?electron charge cloud?in non-polar molecules or atoms are constantly moving
• During this movement, the electron charge cloud can be more on one side of the atom or molecule than the other
• This causes a?temporary dipole?to arise
• This?temporary dipole?can?induce?a dipole on neighbouring molecules
• When this happens, the?δ+ end of the dipole?in one molecule and the?δ- end of the dipole?in a neighbouring molecule are?attracted?towards each other
• Because the electron clouds are moving constantly, the dipoles are only?temporary

Relative strength

• For small molecules with?the same number of electrons,?permanent dipoles are?stronger?than induced dipoles
  • Butane and propanone have the same number of electrons
  • Butane is a nonpolar molecule and will have induced dipole forces
  • Propanone is a polar molecule and will have permanent dipole forces
  • Therefore, more energy is required to break the intermolecular forces between propanone molecules than between butane molecules
  • So, propanone has a higher boiling point than butane

   

Pd-pd forces are stronger than id-id forces in smaller molecules with an equal number of electrons

Hydrogen bonding

• Hydrogen bonding?is the?strongest?form of?intermolecular bonding
  • Intermolecular bonds are bonds?between?molecules
  • Hydrogen bonding is a type of?permanent dipole – permanent dipole?bonding

   

• For hydrogen bonding to take place the following is needed:
  • A species which has an?O,?N or F?(very?electronegative) atom bonded to a hydrogen

   

• When hydrogen is covalently bonded to an?O,?N?or?F, the bond becomes highly?polarised
• The H becomes so?δ+?charged that it can form a bond with the?lone pair?of an?O,?N or F?atom in another molecule
• For example, in water
  • Water can form two hydrogen bonds, because the O has two lone pairs

   

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Hydrogen bonding in water