group 7 - halogens

• chlorine - green gas

  bromine - reddish liquid

  iodine - grey solid
• what is the trend going down the group ?

  colour - gets darker

  state - solidifies

  melting and boiling point - increase
• what is the test for chlorine ?

  damp litmus paper is put into chlorine gas, it is bleached and turns white
• halogen + metal -> metal halide (salt)

  chlorine + iron -> iron chloride

  bromine + iron -> iron bromide

  iodine + iron -> iron iodide

  reactivity decreases down the group
• hydrogen + halogen -> hydrogen halides

  hydrogen + chloride -> hydrogen chloride

  • hydrogen halides are gases at room temperature
  • they dissolve in water to produce acidic solutions
  • hydrogen chloride dissolves in water to produce hydrochloric acid
• a more reactive halogen can displace a less reactive in an aqueous solution of its salt e.g. chlorine is more reactive than iodine therefore a solution of chlorine can displace iodide from a potassium iodide solution
• displacement reaction : halogen and salt solution

  A) chloride

  B) iodine

  C) no reaction

  
• why are these displacement reactions redox reactions ?


  oxidation - loss of electrons

  reduction - gain of electrons

  more reactive halogen displaces the less reactive halogen which forms a negative ion which is a reduction

  e.g. chlorine with potassium bromide

  chlorine looses 2 electrons = reduction

  bromine gains 2 electrons = oxidation
• why does reactivity decrease down the group ?


  the outer shell is further away from the nucleus as you go down the group, meaning the other shells shield it, and its harder to gain that electron