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3.1.2 Group 2

Group 2 properties and reactions

Comparison to Group 1 elements

  • Better electrical conductivity
    • More outer shell electrons \(\rightarrow\) 2 electrons delocalised from each metal atom instead of 1
    • More delocalised electrons to move and carry electric current
  • Higher melting and boiling points
    • More outer shell electrons \(\rightarrow\) 2 electrons delocalised from each metal atom instead of 1
    • Higher charge on metal cation
    • Stronger attraction between the metal ions and the delocalised electrons

Redox reaction of Group 2 elements

  • Two outer shell electrons are both in the outer s sub-shell
  • Lose two electrons to form 2+ ion with an electron configuration of a noble gas
  • Another species gain these two electrons and is reduced
    • Reducing agent as it reduce another species

Redox with oxygen

  • \(M(s) + O_2(g) \rightarrow 2MO(s)\)
  • Each metal atom's oxidation number increases from 0 on LHS to +2 in \(MO\) on RHS
  • Each O decreases from 0 in \(O_2(g)\) on LHS to -2 in \(MO(s)\) on RHS

Redox with water

  • \(M(s) + 2H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)\)
  • Each metal atom increases from 0 in \(M(s)\) on LHS to +2 in \(M(OH)_2(aq)\) on RHS
  • 2 hydrogen atoms in \(H_2O\) decreases from +1 in \(H_2O(l)\) on LHS to 0 in \(H_2(g)\) on RHS
  • The other 2 H atoms do not change their oxidation state and forms \(M(OH)_2(aq)\)
  • Observations
    • More vigorous fizzing / bubbling down the group due to \(H_2\) being produced at a faster rate
    • Metal being more soluble down the group / dissolve faster down the group
    • Solution has a higher pH / more alkaline down the group

Redox with dilute acids

  • Metal + acid \(\rightarrow\) salt + hydrogen
  • Oxidation number change
    • Each metal atom increases from 0 in metal on LHS to +2 in salt on RHS
    • Each H decreases from +1 in acid on LHS to 0 in \(H_2\) on RHS
  • Barium / calcium / strontium sulfate are insoluble in water so reaction stops quickly after the metal solid is coated with insoluble sulfate

Reactivity trend

  • Reactivity increases down the group
  • First and second ionisation energy decreases down the group / easier to remove outer shell electrons
    • Increased atomic radius
    • Increased shielding
    • The effect of increasing nuclear charge is outweighed by the effects of increasing atomic radius and shielding
    • Attraction between the nucleus and outer shell electrons decreases

Reaction of Group 2 oxides with water

  • \(MO(s) + H_2O(l) \rightarrow M^{2+}(aq) + 2OH^-(aq) \rightarrow M(OH)_2(aq)\)
  • \(OH^-\) ions are released and form alkaline solutions of the metal hydroxide
  • Hydroxides only slightly soluble in water
  • When the solution become saturated, addition of further metal oxide causes some ions to come out of the solution and form a solid precipitate: \(M^{2+}(aq) + 2OH^-(aq) \rightarrow M(OH)_2(s)\)
  • Solubility of hydroxides increases down the group so the solution can contain more \(OH^-\) ions and become more alkaline
    • \(Mg(OH)_2(s)\) slightly soluble \(\rightarrow\) low \(OH^-\) concentration, pH \(\approx\) 10
    • \(Ba(OH)_2(s)\) more soluble \(\rightarrow\) higher \(OH^-\) concentration, pH \(\approx\) 13

Uses of Group 2 compounds as bases

  • \(Ca(OH)_2\) in agriculture to neutralise acidic soil
    • Added to fields as lime
    • \(Ca(OH)_2(s) + 2H^+(aq) \rightarrow Ca^{2+}(aq) + 2H_2O(l)\)
    • Sodium or potassium hydroxides are not suitable as they would kill soil organisms and plants (too alkaline)
  • \(Mg(OH)_2\), \(MgCO_3\) and \(CaCO_3\) are used as antacids in treating indigestion
    • They are weak bases that neutralise excess stomach acids (\(HCl\)) that causes the indigestion
    • e.g. milk of magnesia = suspension of \(Mg(OH)_2\) in water
    • \(Mg(OH)_2(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)\)
    • \(CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\)
    • \(KOH\) and \(NaOH\) are not suitable as ingesting them would cause poisoning and potentially death