Group 7- The Halogens

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  • Group 7 - The Halogens
    • Physical properties
      • gases become darker and denser as you descend group 7
      • Have a familiar pool smell
      • Melting and boiling points increase going down group
        • Atoms become larger as you descend-have lager vdw forces
        • Need more energy to break the larger vdw forced
        • Chlorine more volatile than iodine (solid)
      • Atomic size increases going down group
        • Extra shell is added for each element- has one more outer shell than the one above
        • E.g. fluorine has 3p as its outer shell. Chlorine, which is below flourine, has 4p outer shell
          • Extra shell is added for each element- has one more outer shell than the one above
      • Electronegativity decreases down group
        • The power of an atom to attract the electron density of a covalent bond to itself
        • Valence electrons get further away from nucleus as you go down group and shielding is increased
          • Decreases electrostatic attraction of outer electrons to nucleus valence electrons have less attraction to nucleus
            • It becomes harder to gain electrons or pull them closer due to the lack of attraction  descending the group
          • It becomes harder to gain electrons or pull them closer due to the lack of attraction  descending the group
    • Displacement reactions
      • Halogens reacting in a solution with metal halides- the halide in the compound will be dispaced by more reactive halogen
        • E.g. iodine could not displace chlorine but fluorine could
      • Reactivity of halogens decrease down the group
      • Cl2 + 2NaBr ---> Br2 + NaCl
        • In aqueous solution
    • Oxidising Ability of halogens
      • Decreases down group
      • Halogens are oxidising agets- lose electrons to become -ve ions with a charge of -1
    • Reactions of halide ions
      • Sodium halides and sulfuric acid
        • Sdoum chloride
          • Fumes of HCl and solid sodium hyrdogensulfate are produced
          • NOT A REDOX REACTION as chlorine is too weak a reducing agent. It is an acid-base reaction
        • Sodium Bromide
          • Fumes of hydrogen bromide, brown fumes of bromine and colourless SO2
            • FIRST REACTION IS ACID- BASE SECOND IS REDOX
        • Sodium Iodide
          • Fumes of hydrogen iodide, black solid iodine and bad smell of hydrogen sulfide gas
          • NaI + H2SO4 --> NaHSO4 + HI
            • 8I- + H2SO4 --> H2S + 4H2O + 4I2
              • Redox reaction
            • Acid-base reaction
      • Testing for halide ions
        • Add dilute nitric acid to halide solution to remove impurities e.g. CO32-
        • Then silver nitrate is added-halide ppt forms
          • White ppt=chloride ions     Cream ppt=bromide ions    Yellow ppt=iodide ions
            • Chloride ions soluble in dilute NH3 but bromide ions only dissolve in conc. NH3
          • After pt is formed, ammonia can be added to distinguish between similar colours

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