halogens

• Halogens


  Group 7 nonmetal elements on the right side of the periodic table
• Halogens


  • Have the electron configuration 1s^2 2s^2 2p^5
  • Fluorine is a pale yellow gas, chlorine is a pale green gas, bromine is a reddish-brown liquid, iodine is a grey solid
• As we go down group 7

  The boiling point of the halogens increases
• As we go down group 7

  The electronegativity of the halogens decreases
• Halogen displacement reactions


  1. More reactive halogen displaces less reactive halide ion (top to bottom)

  2. Electron is gained by the less reactive halide ion
• Halogen displacement reactions


  • Cl2 + Br- → Cl- + Br2

  Cl2 + I- → Cl- + I2

  Br2 + I- → Br- + I2
• Halogens become less oxidizing as we go down group 7
• Bleach production

  Cl2 + 2 NaOH → NaCl + NaClO + H2O

  Disproportionation reaction where chlorine is simultaneously oxidized and reduced
• Bleach


  Sodium hypochlorite (NaClO) solution used for water treatment, disinfection, and whitening
• Water chlorination


  Cl2 + H2O → HCl + HClO

  Disproportionation reaction where chlorine is simultaneously oxidized and reduced
• Sunlight can decompose chlorinated water, reducing the active chlorine (HClO) and producing less effective chloride ions
• Advantages of chlorinating drinking water


  • Destroys microorganisms that cause disease

  Long-lasting disinfection effect

  Reduces algae growth and discoloration
• Disadvantages of chlorinating drinking water


  • Chlorine is toxic and can irritate the respiratory system

  Potential to form carcinogenic byproducts by reacting with organic compounds
• Halide ions

  Ions of the halogen elements that have gained an electron, making them good reducing agents
• As we go down group 7

  The halide ions become better reducing agents
• Testing halide ions with sulfuric acid


  Halide ions can reduce concentrated sulfuric acid, producing different products depending on the halide ion and the oxidation state of sulfur
• Testing halide ions with silver nitrate


  Halide ions can react with silver nitrate to produce a precipitate, with the colour of the precipitate depending on the halide ion
• Reduction products of sulfuric acid

  • Sodium hydrogen sulfate
  • Sulfur dioxide
  • Sulfur
  • Hydrogen sulfide
• Oxidation state of sulfur

  • +6
  • +4
  • 0
  • -2
• Halide ions

  • Chloride
  • Bromide
  • Iodide
• Reaction of chloride ion with concentrated sulfuric acid

  Sodium hydrogen sulfate formed
• Reaction of bromide ion with concentrated sulfuric acid

  1. Bromine gas formed

  2. Sulfur dioxide formed
• Reaction of iodide ion with concentrated sulfuric acid

  1. Iodine formed

  2. Sulfur formed

  3. Hydrogen sulfide formed
• Sodium hydrogen sulfate is not a reduction product, it is a standard chemical reaction between a salt and an acid
• Bromide and iodide can further reduce sulfur in sulfuric acid
• Bromide


  • Less powerful reducing agent than iodide
• Test for halide ions with silver nitrate


  1. White precipitate for chloride

  2. Cream precipitate for bromide

  3. Yellow precipitate for iodide
• Dilute nitric acid is added to remove any interfering carbonate ions
• Further test with ammonia

  1. Chloride precipitate dissolves in dilute ammonia

  2. Bromide precipitate dissolves in concentrated ammonia

  3. Iodide precipitate insoluble in concentrated ammonia
• Flame test for group 2 ions

  • Calcium - dark red
  • Strontium - red
  • Barium - green
• Test for ammonium compounds

  Add sodium hydroxide, heat, ammonia gas produced turns red litmus blue
• Test for hydroxides


  Red litmus turns blue
• Test for carbonates


  1. Add hydrochloric acid, carbon dioxide gas produced

  2. Bubble through limewater, turns cloudy
• Test for sulfates


  Add barium chloride, white precipitate of barium sulfate forms
• Test for ions in a particular order - carbonates, sulfates, halides - to avoid false positives