24.2    Group IV Chlorides

Introduction to Group IV chlorides

• All Group IV chlorides are simple covalent molecules which exist as liquid at room temperature with the general formula of XCl4, where X = A Group IV element.

• All of them have a tetrahedral shape with a bond angle of 109.5°. A typical example is carbon tetrachloride:

• The boiling point of Group IV tetrachlorides increases down the This is because as the number of electrons increases, more temporary dipoles can be set up between the molecules. Hence the van der Waals’ forces of attraction are stronger.Note: Carbon tetrachloride, CCl4 is an exception.

Reaction with water, H2O(Hydrolysis)

• 1. Carbon tetrachloride does not react with water. If carbon tetrachloride is added to water, two immiscible layers are formed.
  2. There are two reasons for this:
     • Carbon is a small atom cluttered with big chlorine atoms. This causes the water molecule to have difftculty approaching the bulky molecule. Even if the water molecule successfully gets in, the transition state is very cluttered and unstable, the activation energy is very high
     • Also, there isn’t any convenient orbital on the carbon that the lone pair of the water molecule can attaches itself to.
• 1. Silicon, germanium and tin tetrachlorides(also called silicon(IV) chloride, germanium(IV) chloride and tin(IV) chloride) hydrolyses in water to form an oxide and steamy fumes of hydrogen chloride.
     XCl4 + 2H2O → XO2 + 4HCl       ; where X = Si/Ge/Sn/Pb
  2. Unlike carbon tetrachloride, this is possible because the Group IV atoms are bigger, the molecules are less bulky.

• Lead(IV) chloride behaves similarly as silicon, germanium and tin tetrachlorides. However, the heat released during the reaction would tend to decompose lead(IV) chloride, PbCl4 to lead(II) chloride, PbCl2.
  PbCl4 → PbCl2 + Cl2

• The hydrolysis reactions become more vigorous going down the This is because going down the Group, the X-Cl bond becomes longer. A longer bond has a lower bond energy and it is more easy to be broken.

Relative stability of the oxidation states of Group IV chlorides

• A fact about Group IV elements is as such:
  1. Going down the Group, compounds with a +2 oxidation state Group IV element become more stable.
  2. Going up the Group, compounds with a +4 oxidation state Group IV element become more stable
• This means that the +4 oxidation is less favourable down the Group while the +2 oxidation state is more favourable down the Group.
• From carbon to tin, the +4 oxidation state is more stable than +2 oxidation state. However, for lead, the +2 oxidation is more This is why lead(IV) chloride decomposes to lead(II) chloride on heating.