Topic 3- REDOX

• Measuring carbon dioxide production

  1. Place hand over sample

  2. Measure time for lime water to turn cloudy

  3. Use gas syringe to quantify gas production
• Flame test

  Test for positive ions (cations) in a compound
• Flame test


  • Electrons jump between shells, absorbing and releasing energy as different wavelengths of light
  • Nichrome wire used to hold sample and place in blue Bunsen flame
• Flame test colours


  • Lithium - crimson
  • Sodium - yellow-orange
  • Rubidium - red
  • Cesium - blue
  • Calcium - dark red
  • Strontium - crimson
  • Barium - green
• Halogens


  • Fluorine - pale yellow gas
  • Chlorine - pale green gas
  • Bromine - brown-orange liquid
  • Iodine - grey solid
• As we go down Group 7

  Boiling point increases, physical state changes from gas to solid
• Electronegativity


  Ability of an atom to attract electrons in a covalent bond, decreases down Group 7
• Displacement reactions with halogens

  1. Add organic solvent like hexane

  2. Observe colour of organic layer

  3. More reactive halogen displaces less reactive halide ion
• Halogen displacement reactions


  • Chlorine displaces bromide, forming bromine
  • Chlorine displaces iodide, forming iodine
  • Bromine displaces iodide, forming iodine
• Disproportionation reactions


  Halogens react with cold alkalis to be simultaneously oxidised and reduced
• Disproportionation with hot alkalis

  1. Halogen reacts to form halate (V) ion and halide ion

  2. e.g. 3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O
• Bleach


  Sodium hypochlorite solution formed by reaction of chlorine and sodium hydroxide, used to kill bacteria
• Halide ions as reducing agents


  Halide ions lose electrons, so are good reducing agents
• As we go down Group 7

  Halide ion reducing power increases
• HCl ionizes


  1. Produces chlorate 1 ion

  2. Reacts with water

  3. Produces chloride ions

  4. Produces hydroxide ions
• Chloric 1 acid reacting with water produces chloride ions and hydroxide ions which makes it acidic
• As we go down the group, the ionic radius of halide ions increases, making them better reducing agents
• Reaction of halide ions with sulfuric acid


  1. Produces sodium hydrogen sulfate

  2. Produces sulfur dioxide (for bromide and iodide)

  3. Produces sulfur (for iodide)
• Chloride ions are not very powerful reducing agents and only produce sodium hydrogen sulfate
• Bromide and iodide ions can further reduce sulfuric acid to produce sulfur dioxide and sulfur
• Reaction of iodide ions with sulfuric acid


  1. Produces sodium hydrogen sulfate

  2. Produces sulfur dioxide

  3. Produces hydrogen sulfide
• Hydrogen halides


  React with ammonia gas to form ammonium halide salts
• Testing for halide ions using silver nitrate


  1. Add dilute nitric acid to remove carbonates

  2. Chloride forms white precipitate of silver chloride

  3. Bromide forms cream precipitate of silver bromide

  4. Iodide forms yellow precipitate of silver iodide
• Further testing with ammonia

  1. Silver chloride dissolves in dilute ammonia

  2. Silver bromide dissolves in concentrated ammonia

  3. Silver iodide does not dissolve in ammonia
• Fluoride ions do not form a precipitate with silver nitrate as silver fluoride is soluble
• Testing for carbonates

  1. Add hydrochloric acid

  2. Produces carbon dioxide gas
• Testing for sulfates


  1. Add hydrochloric acid to remove carbonates

  2. Add barium chloride

  3. Forms white precipitate of barium sulfate
• Testing for ammonium compounds

  1. Add sodium hydroxide and heat

  2. Produces ammonia gas

  3. Turns red litmus paper blue
• Testing for hydroxides


  Turns red litmus paper blue