8.2 The Halogens

8.2 The Halogens

1. Trends in Boiling Points

   • At RTP, all the halogens exist as diatomic molecules, X2. The state of the elements changes as you go down the group, with fluorine and chlorine being gas, bromine being liquid and Iodine being solid.
     • Halogen Molecule
       • No. of Electrons
         • Boiling Point/ degrees C
           • Appearance and State at RTP
             • This is because as you go down the group, the number of electrons increases so therefore there are stronger London Forces and in turn, more energy is required to break these forces so the boiling point increases
       • F2
         • Cl2
           • Br2
             • I2
               • At2
                 • 170
                   • 230
                     • Never been seen
               • 106
                 • 184
                   • Shiny grey-black solid
             • 70
               • 59
                 • Red-Brown liquid
           • 34
             • -34
               • Pale green gas
         • 18
           • -188
             • Pale yellow gas

2. Redox Reactions

   • Each halogen has 7 outer-shell electrons. Two electrons are in the outer s sub-shell, and 5 are in the outer p sub-shell = s2p5
     • Each halogen atom is reduced, gaining one electron to form a 1- halide ion with the electron configuration of the nearest noble gas
       • E.g/ Cl2 + 2e- --> 2Cl- Chlorine is Reduced
         • Another species will lose its electrons to halogen atoms - therefore halogens are called Oxidising agents, because it has oxidised another species
   • Most common type of reaction in the halogens

3. Halogen - Halide displacement Reactions

   • The result of these 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. For example, a solution of chlorine (Cl2) is added to two aqueous solutions containing bromine (Br-) and Iodine (I-) ions. If the halogen added is more reaction than the halide present then...
       • 1) a reaction take place, the halogen displacing the halide from solution
       • 2) the solution changes colour
       • In water, chlorine appears pale green, bromine appears orange and Iodine appears very dark brown.
         • Halogen
           • Cl2 (aq)
             • Br2 (aq)
               • I2 (aq)
           • Cl- (aq)
             • Br- (aq)
               • I- (aq)
                 • Violet colour from I2 formation Cl2 (aq) + 2I- (aq) --> 2Cl- (aq) + I2 (aq)
                   • Violet colour from I2 formation Br2 (aq) + 2I- (aq) --> 2Br- (aq) + I2 (aq)
                     • N/A
               • Orange colour from Br2 formation Cl2 (aq) + 2Br- (aq) --> 2Cl- (aq) + Br2 (aq)
                 • N/A
                   • No Reaction
             • N/A
               • Chlorine cannot displace itself
               • No reaction
                 • Reactivity DECREASES down the group remember! So bromine won't be able to displace chlorine.
                 • No Reaction
                   • Reactivity DECREASES down the group remember! So Iodine won't be able to displace chlorine.
         • Results
           • Chlorine has clear reacted with both Br- and I- so it must be the most reactive out of the three. However Bromine has only reacted with I- showing it isn't as reactive as chlorine but is still more reactive than Iodine. On the other hand, Iodine hasn't reacted at all, showing it is the least reactive out of the three
         • Reaction of Chlorine with Bromide Ions
           • Full Equation: Cl2 (aq) + 2NaBr (aq) --> 2NaCl (aq) + Br2 (aq)
             • Ionic Equation: Cl2 (aq) + 2Br- (aq) --> 2Cl- (aq) + Br2 (aq)
               • Each Br- increases by one and is oxidised
               • Each Cl decreases by 1 and is reduced

4. Trend In reactivity

   • Reactivity DECREASES down the halogen group. This is because going down the group, the atomic radius increases so the outer-electrons are further away from the nucleus, plus, there are more inner shells so shielding has a significant effect. These factors means there is less nuclear attraction to capture an electron from another species and so reactivity decreases.

5. Disproportionation

   • Disproportionation is a redox reaction in which the same element is both oxidised and reduced
     • Learn this definition for your exams!
     • The reaction of chlorine with water
       • Used in water purification
         • Cl2 (aq) + H2O (l) --> HClO (aq) + HCl (aq)
           • 1 Chlorine goes from 0 (element) to -1 (HCl) so it is reduced. The other chlorine goes from 0 (element) to +1 (HClO) so it is oxidised.
             • It is the HClO (Chloric Acid) and the ClO- ions that kill the bacteria rather than the chlorine. To test this if added any indicator would turn red.
     • The reaction of chlorine with cold, dilute aqueous sodium hydroxide
       • The reaction of chlorine with water is limited by the solubility of chlorine in water. If the water contains dissolved sodium hydroxide, much more chlorine dissolves and another disproportionation reaction takes places
         • Cl2 (aq) + 2NaOH (aq) --> NaClO (aq) + NaCl (aq) + H2O (l)
           • One chlorine goes from 0 (element) to +1 in NaClO and is oxidised. The other chlorine goes from 0 (element) to -1 in NaCl and is reduced.

6. Benefits and Risks of chlorine use

   • One benefit to chlorine in water treatment is that it purifies the water and kills the bacteria in it
     • However the risks of chlorine are that it could produce chlorine gas which is highly toxic and it could also from chlorinated hydrocarbons which are suspected of causing cancer
       • But the overall risk of health by not adding chlorine to the water supply is far greater than the risk posed by the chlorinated hydrocarbons.

7. Test for Halide Ions

   • Precipitation reactions with aqueous Silver Ions
     • Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides, as shown by the general equation: Ag+ (aq) + X- --> AgX (s)
       • Chloride ions form a white precipitate
       • Bromide ions form a cream precipitate
       • Iodide ions form a yellow precipitate