3.2.3 Group 7(17), the halogens

• M.p/b.p increases down group 7 because:

  • molecules become larger,
  • they have more electrons
  • therefore have stronger Van der Waals forces between the molecules. 
• Electronegativity decreases down group 7. 
• Reactivity decreases down group 7 because the atomic radius increases and there is a greater number of shells
• Chlorine will displace both Br- and I-2.
• What are the colours of solution for each type of free halogen?

  • Chlorine = Pale Green / Colourless
  • Bromine = Yellow/Orange
  • Iodine = Brown
• Testing for halide ions:

  • Test solution is made acidic with nitric acid
  • Silver nitrate solution is added dropwise.
• What is the role of nitric acid in the silver nitrate test for halide ions?

  • Reacts with any carbonates to prevent formation of the precipitate Ag2 CO3 
  • This would mask the desired observations.
• Reducing power of halides increases down group 7 This is because:

  • They have a greater tendency to lose electrons. 
  • ions increase in size,
  • shielding increases
  • easier for electrons to be removed.
• What is the role of H2SO4 in acid-base reactions? is

  a proton donor
• Disproportionation reaction: A reaction in which a species is both oxidised and reduced
• How does Chlorine react with cold water? Cl2 + H2O --> ClO- + Cl- +2H+
• Bleach: Chlorine can be mixed with cold, aqueous sodium hydroxide to produce sodium hypochlorite. 

  
  

  Cl2 + 2NaOH --> NaCl + NaClO + H20
• Iodide ions are stronger reducing agents than chloride ions.
• Adding ammonia solution can confirm that a precipitate is silver bromide because it dissolve the precipitate
• Bromine will displace I- .
• Iodide ions are larger than chlorine ions - with more electron shells and a larger atomic radius.
• Electronegativity decreases down group 7. Because:

  
  

  • more occupied electron shells
  • → greater atomic radius and outer electrons are further from the positive charge of the nucleus
  • → lower force of attraction between the nucleus and electron pair in the covalent bond
• Why do you add HNO3 ? Why not HCl?

  • To remove CO32-
  • Adding HCl would add Cl- ions
  • giving a false positive result
• Result and equation for Cl- test:

  • Result: white precipitate
  • Equation: Ag+ + Cl- → AgCl (s)
• Test for I-?

  • Result: Yellow precipitate
  • Equation: Ag+ + I- → AgI (s)
• What is the trend in oxidising ability down group 7?

  • Decreases down group
  • Because:
  • Cl has fewest occupied electron shells,
  • greatest force of attraction between outer electrons and nucleus,
  • easiest to gain electrons and be reduced
  • → best oxidising agent
• Write the equation for Cl2 oxidising 2l-:

  2ICl2 + 2I- → 2Cl- + I2
• Why is chlorine added to drinking water? Forms ClO- ions which oxidise and therefore kill all microorganisms in water
• NaClO is bleach
• Desalination: Converts saltwater into clean, potable water. Either by reverse osmosis (using a smart membrane) or by vacuum distillation at low pressure and low temperature
• What is the result and equation for the test for Br-?

  • Cream precipitate
  • Ag+ + Br- → AgBr (s)