group 2-alkaline earth metals

• trends in reactivity:

  • increase down the group
  • nuclear charge increases, shielding increases and atomic radius increases
  • overall nuclear attraction decreases
  • outer 2 electrons are easy to lose
• trends in ionisation energy:

  • ionisation energy decreases down the group
  • increased shielding, increased atomic radius and increased nuclear charge
  • nuclear attraction on outer electrons decreases
  • less energy is required to remove the outer electrons from the atom
• group 1 metals have lower 1st ionisation energy than group 2 metals, but group 2 metals have the lowest combined 1st and 2nd ionisation energies therefore the charge of a group 2 metal is 2+
• group 2 metals are less reactive than their corresponding group 1 metal in the same period as two electrons need to be lost rather than just one which requires a higher activation energy
• reaction of magnesium with water:

  • reacts very slowly to produce magnesium hydroxide and hydrogen gas
  • Mg (s) + H2O (l) -> Mg(OH)2 (aq) + H2 (g)
• magnesium hydroxide is used as an antacid in indigestion tablets
• reaction of magnesium with steam:

  • forms magnesium oxide and hydrogen gas
  • Mg (s) + H2O (g) -> MgO (s) + H2 (g)
• when magnesium reacts with steam a lot of heat is used to get the reaction going and the initially formed magnesium hydroxide can decompose into magnesium oxide and water
• reaction of calcium with water:

  • reacts vigorously with water to form calcium hydroxide and hydrogen gas
  • Ca(s) + H2O (l) -> Ca(OH)2 (aq) + H2 (g)
• calcium hydroxide is used in agriculture to neutralise acidic soils
• reactions with air:

  • reactivity increases down the group
  • react with air to form a white oxide layer
  • 2Mg(s) + O2(g) -> 2MgO (s)
• reaction with dilute acids:

  • form a salt and hydrogen gas
  • reaction becomes more vigorous down the group
  • Ca (s) + 2HCl (aq) -> CaCl2 (aq) + H2 (g)
• reactions between group 2 oxides and water:

  • react with water to form metal hydroxides
  • MgO (s) + H2O (l) -> Mg(OH)2 (aq)
• group 2 metal hydroxides are varyingly soluble in water and from alkaline solutions with water as they release OH- ions.
• trends in solubility of group 2 metal hydroxides:

  • solubility increases down the group
  • when a hydroxide is more soluble than another it will release more OH- ions and will make a more alkaline solution with a higher pH
• magnesium hydroxide is sparingly soluble in water
• barium hydroxide is more soluble in water than magnesium hydroxide and makes a more alkaline solution than magnesium hydroxide
• as the group descends alkalinity increases with solubility of metal hydroxides
• magnesium hydroxide is less soluble than barium hydroxide because it has a stronger ionic bond and a higher density of charge as magnesium has a smaller atomic radius than barium so the 2+ charge is more concentrated
• magnesium hydroxide solution has a pH of 10.5
• calcium hydroxide solution has a pH of 9.0
• barium hydroxide solution has a pH of 12.3
• magnesium hydroxide neutralises stomach acid

  • Mg(OH)2 +2HCl -> MgCl2 + H20