Halogens

• The halogens are the most reactive non-metallic group
• On earth, the halogens occur as stable, negative halide ions dissolved in sea water or combined with sodium or potassium as solid deposits like in salt mines containing NaCl.
• At room temp and pressure, all halogens exist as diatomic molecules
• Going down the halogens, there are more electrons in each atom, so stronger London forces, so more energy required to break the intermolecular forces, so the boiling point increases.
• Each halogen has seven outer shell electrons, just one electron short of the electron configuration of a noble gas.

  Two of these electrons are in the outer s sub-shell and 5 of them are in the outer p sub-shell.

  When they become halide ions, it increases to 6 electrons in the outer p sub-shell.
• • Fluorine is a pale yellow gas
  • chlorine is a pale green gas
  • bromine is a red/brown liquid
  • iodine is a shiny grey/black solid
  • Astatine has never been seen
• Solution in water:

  • chlorine is pale green
  • bromine is orange/brown
  • iodine is brown


  Solution in cyclohexane (top layer):

  • chlorine is pale green
  • bromine is orange
  • iodine is deep violet
• The non-polar halogens dissolve more readily in an organic non-polar solvent than in water
• Astatine is extremely rare because its radioactive and decays rapidly. The element has never really been seen but it is predicted to be the least reactive halogen
• In redox reactions, halogens react by gaining electrons. Down the group, the tendency to gain an electron decreases and so the reactivity decreases. This is because:

  • atomic radius increases
  • more inner shells so shielding increases
  • less nuclear attraction to capture an electron form another species
  • reactivity decreases
• Redox reactions are the most common type of reaction of the halogens.

  They are oxidising agents- they oxidise the other species in the reaction by taking an electron from them.
• Disproportionation is a redox reaction in which the same element is both oxidised and reduced.

  E.g. chlorine with water- for each chlorine molecule, one chlorine atom is oxidised and the other one is reduced:

  $Cl_2(aq) +$$H_2O(l) -> HClO(aq) +$$HCl(aq)$
• In their solid states the halogens form lattices with simple molecular structures
• Displacement reactions of halogens with halide ions can be carried out on a test tube scale. The results of the displacement reactions show that the reactivity of the halogens decreases down the group.

  • A solution of each halogen is added to aqueous solutions of the other halides
  • if the halogen added is more reactive than the halide present, a reaction takes place, the halogen displacing the halide from solution. The halogen added is reduced to become an ion and the halide is oxidised back into its diatomic element.
  • The solution changes colour
• • When chlorine is formed it is pale green, bromine is orange, and iodine is brown
  • Iodine and bromine are similar so to tell apart, organic non-polar solvent like cyclohexane is added. Iodine turns violet
• Fluorine is a pale yellow gas. It reacts with almost any substance that it comes into contact with
• In redox reactions, halogens react by gaining electrons. Down the group, the tendency to gain an electron decreases and the halogens become less reactive because:

  • atomic radius increases
  • more inner shells so shielding increases
  • less nuclear attraction to capture an electron from another species
  • reactivity decreases
• In the halogens, fluorine is the strongest oxidising agent, gaining electrons from other species more readily than the other halogens. The halogens become weaker oxidising agents down the group
• Disproportionation is a redox reaction in which the same element is both oxidised and reduced. The reaction of chlorine with water and with cold, dilute sodium hydroxide are two examples of disproportionation reactions
• When small amounts of chlorine are added to water, a disproportionation reaction takes place. For each chlorine molecule, one chlorine atom is oxidised and the other chlorine atom is reduced.
• The two products of chlorine reacting with water are both acids:

  • chloric(I) acid, HClO
  • hydrochloric acid, HCl
  • Bacteria are killed by chloric(I) acid and chlorate (I) ions, ClO-, rather than by chlorine
  • Chloric (I) acid also acts as a weak bleach: add indicator solution to a solution of chlorine in water; it will first turn red then the colour will disappear from the chloric acid bleaching
• The reaction of chlorine with water is limited by the low solubility of chlorine in water. If the water contains dissolved sodium hydroxide, much more chlorine dissolves and another disproportionation reaction takes place. The resulting solution contains a large concentration of chlorate(I) ClO- ions from the sodium chlorate(I), NaClO, that is formed. This solution finds a use as household bleach, which is made by reacting chlorine with cold dilute aqueous sodium hydroxide
• Although chlorine is beneficial in ensuring that our water is fit to drink and that bacteria are killed, chlorine is also a toxic gas. It is a respiratory irritant in small concentrations and fatal in large concentrations
• Chlorine in drinking water can react with organic hydrocarbons such as methane formed from decaying vegetation. This forms chlorinated hydrocarbons which are suspected of causing cancer. However, the overall risk to health of not adding chlorine to the water supply is far greater than the risk of chlorinated hydrocarbons
• Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides. This reaction forms the basis for a test for the presence of halides. $Ag^+$$(aq) +$$X^-(aq) -> AgX(s)$