Group 2 /7

• Reducing agent

  Electron donor
• Reducing strength of halides

  • Increases down the group
  • Ionic radius increases
  • Weaker attraction between outer electron and nucleus
• Reducing ability of halides demonstrated through reactions with H2SO4

  1. Oxidation half-equations: I- → I2, Br- → Br2

  2. Reduction half-equations: H2SO4 → SO2, H2SO4 → S, H2SO4 → H2S
• Fluoride and chloride are too weak as reducing agents to reduce H2SO4, instead they undergo acid-base reactions with H2SO4
• Halide tests


  1. Dissolve sample in water

  2. Add HNO3 acid

  3. Add AgNO3(aq) solution

  4. Add dilute NH3

  5. Add concentrated NH3
• Precipitates


  • AgF(aq)
  • AgCl(s)
  • AgBr(s)
  • AgI(s)
• What NH3 does


  • AgCl + 2NH3 → Ag(NH3)2+ + Cl-
  • AgBr + 2NH3 → Ag(NH3)2+ + Br-
• Halogens


  • Diatomic molecules
  • Behave as oxidising agents in reactions
• Oxidising agent


  Electron acceptor
• Trend in oxidising ability of halogens

  • Decreases down group
  • Larger radius
  • Weaker attraction between nucleus and incoming electron
• Halogen displacement reactions

  More-reactive (stronger oxidising agent) halogen will displace a less-reactive (weaker oxidising agent)
• Ionic equations

  • Cl2 + 2NaBr → Br2 + 2NaCl
  • Cl2 + 2NaI → I2 + 2NaCl
  • Br2 + 2NaI → I2 + 2NaBr
• F2 can't be used in displacement reactions, because it's so reactive that it will react with the water dissolving the halide compound
• Disproportionation reaction


  • Cl2 reduced to HCl (oxidation-state change: 0 to -1)
  • Cl2 oxidised to HClO (oxidation-state change: 0 to +1)
• Reactivity of group 2 metals

  • Decreases down the group
• Mg reacts with liquid water

  Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g)
• Mg reacts with steam

  Mg(s) + H2O(g) → MgO(s) + H2(g)
• Observations: White solid (MgO) produced, Bright light
• Use of Mg in extraction of Ti from TiCl4


  TiCl4 + 2Mg → 2MgCl2 + Ti
• Role of Mg in TiCl4 extraction

  • Mg oxidation state changes from 0 to +2
  • Mg is oxidised
  • Mg is a reducing agent
• Solubility of group 2 hydroxides

  • Increases down the group
• Mg(OH)2


  • White solid that is almost insoluble in water
  • Reaction can be used to test for presence of Mg2+ or OH- ions
• Simplest ionic equation: Mg2+(aq) + OH-(aq) → Mg(OH)2(s)
• Observation: White precipitate forms
• What to use to test for Mg2+ or OH- ions

  • NaOH(aq) to test for Mg2+
  • Mg(NO3)2(aq) to test for OH-
• Uses of Mg(OH)2


  • Indigestion relief
  • Often called 'milk of magnesia'
• Hydroxides


  • Mg(OH)2: Sparingly soluble
  • Ca(OH)2: Sparingly soluble
  • Sr(OH)2: More soluble
  • Ba(OH)2: Completely dissolves
• Solubility of group 2 sulphates

  • Decreases down the group
• BaSO4


  • White solid that is insoluble in water
  • Reaction can be used to test for presence of Ba2+ or SO42- ions
• Simplest ionic equation: Ba2+(aq) + SO42-(aq) → BaSO4(s)
• What to use to test for Ba2+ or SO42- ions

  • H2SO4(aq) to test for Ba2+
  • BaCl2(aq) to test for SO42-
• Uses of BaSO4


  Taking X-rays of the stomach (Barium meal)