Reactivity trends

• Group 2 elements

  • Reactive metals
  • Do not occur in elemental form naturally
  • Found in stable compounds like calcium carbonate
• Reducing agents

  Group 2 elements have two outer shell electrons and lose them to form 2+ ions
• Redox reactions of Group 2 elements

  1. Metal atom is oxidised, losing two electrons

  2. Another species gains the two electrons and is reduced
• Reactivity of Group 2 elements

  Increases down the group
• Ionisation energies of Group 2 elements decrease down the group
• Decreasing ionisation energies

  Cause Group 2 elements to become more reactive and stronger reducing agents down the group
• Group 2 oxides

  • React with water to form alkaline hydroxide solutions
  • Hydroxides have increasing solubility down the group
• Uses of Group 2 compounds

  • Calcium hydroxide added to fields as lime to increase soil pH
  • Magnesium and calcium carbonates used as antacids
• Halogens


  • Most reactive non-metallic group
  • Do not occur in elemental form in nature, found as halide ions
• Halogens exist as diatomic molecules at room temperature and pressure
• Trend in boiling points of halogens

  Increases down the group due to increasing number of electrons and stronger London forces
• Redox reactions of halogens

  Halogens are oxidising agents, gaining one electron to form halide ions
• Halogen-halide displacement reactions


  1. More reactive halogen displaces less reactive halide from solution

  2. Reaction shows decreasing reactivity down the group
• Fluorine is the strongest oxidising agent, astatine is the weakest
• Disproportionation


  Redox reaction where the same element is both oxidised and reduced
• Reaction of chlorine with water

  One chlorine atom is oxidised, the other is reduced
• Chlorine is used in water purification to kill harmful bacteria
• A₂


  Reactivity decreases
• In the halogens, fluorine is the strongest oxidising agent, gaining electrons from other species more readily than the other halogens. The halogens become weaker oxidising agents down the group.
• Disproportionation


  A redox reaction in which the same element is both oxidised and reduced
• Disproportionation reactions


  • The reaction of chlorine with water
  • The reaction of chlorine with cold, dilute sodium hydroxide
• The reaction of chlorine with water

  1. Chlorine atom is oxidised

  2. The other chlorine atom is reduced

  3. Products are chloric acid and hydrochloric acid
• When small amounts of chlorine are added to water

  A disproportionation reaction takes place
• Chloric acid and chlorate ions, rather than chlorine, kill the bacteria
• Chloric acid also acts as a weak bleach
• The reaction of chlorine with cold, dilute aqueous sodium hydroxide

  1. Chlorine is reduced

  2. Chlorine is oxidised

  3. Products are sodium chlorate and sodium chloride
• The resulting solution contains a large concentration of chlorate ions from the sodium chlorate that is formed
• This solution finds a use as household bleach, which is made by reacting chlorine with cold dilute aqueous sodium hydroxide
• Chlorine


  • It is beneficial in ensuring that our water is fit to drink and that bacteria are killed
  • It is also an extremely toxic gas
• Chlorine in drinking water can react with organic hydrocarbons


  Chlorinated hydrocarbons are formed, which are suspected of causing cancer
• The overall risk to health of not adding chlorine to the water supply is far greater than the risk posed by the chlorinated hydrocarbons
• After a natural disaster one of the very first, life-saving tasks is to ensure that the survivors have a safe water supply
• Precipitation reactions with aqueous silver ions

  Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides
• Tests for anions


  • Tests based on gases
  • Tests based on precipitates
• Carbonate test


  1. Carbonates react with acids to form carbon dioxide gas

  2. Bubble the gas through lime water to form a white precipitate of calcium carbonate
• Sulfate test


  Add aqueous barium ions to form a white precipitate of barium sulfate
• Halide test


  Add aqueous silver ions to form precipitates of silver halides of different colours
• The reactions with Ag+ ions and the solubilities of the silver halide precipitates in aqueous ammonia are summarised in Table 1
• Barium meals


  • Used to enable doctors to see the outline of the gut, stomach, and upper small intestine to identify abnormalities
• Barium ions in solution are extremely toxic, but patients are not poisoned by this treatment