Alkali and Alkaline Earth Metals

Group 1 – The Alkali Metals

• General Properties:

  • s-block elements: Outer electron is in an s-orbital.
  • Reaction: Lose an electron to form M+ ions (oxidation).
  • Properties: Metals and reducing agents.
  • Ionization Energy: Decreases down the group.
    • Reason: Increased shielding and distance between the nucleus and outer electron.
  • Reactivity: Increases down the group.

• Physical Properties:

  1. Soft metals with low densities.
  2. Melting Points: Lower than other metals.
     • Reason: Weaker metallic bonding.
     • Trend: Decreases down the group due to increased ionic radius and weaker metallic bonding.
  3. Good conductors of heat and electricity.
     • Reason: Presence of delocalized electrons.
  4. Appearance: Silvery when freshly prepared but tarnishes quickly in air due to reaction with oxygen.

• Chemical Properties:

  1. Reaction with Water:
     • Reaction: Vigorous reaction with cold water to produce alkaline solution and hydrogen gas.
     • Lithium: Floats, releases hydrogen, forms lithium hydroxide solution.
     • Sodium: Melts into silvery ball, releases hydrogen, forms sodium hydroxide solution.
     • Potassium: Similar reaction, more vigorous, hydrogen released catches fire.
  2. Reaction with Oxygen:
     • Lithium: Burns with red flame, forms lithium oxide.
     • Sodium: Burns with yellow flame, forms sodium oxide and sodium peroxide.
     • Potassium: Burns with lilac flame, forms peroxide and superoxide mixture.
     • Trend: Tendency to form peroxide and superoxide increases down the group.
     • Reason: Decreasing polarizing power of the cation.
     • Lithium Ion: High polarizing power, breaks down peroxide ion to oxide.

Alkali Metal Compounds

• Hydroxides:

  • Preparation: Reacting metal, oxide, or hydride with water.
  • Solubility: Soluble in water, increases down the group.
  • Deliquescence: Solid hydroxides absorb water vapor and form a solution.
    • Implication: Sodium hydroxide solutions must be standardized before titration.
  • Thermal Stability: Generally stable.
    • Exception: Lithium hydroxide decomposes at 650^\circ C to form oxide and water vapor.
    • Trend: Thermal stability increases down the group.
    • Diagonal Relationship: Lithium's behavior similar to magnesium due to similar charge/radius ratio (q/r).

• Carbonates:

  • Preparation: Bubbling carbon dioxide through alkali solution.
  • Sodium Carbonate: Forms sodium carbonate-10-water crystals upon crystallization.
    • Efflorescence: These crystals lose water of crystallization without heating.
  • Thermal Stability: Generally stable.
    • Exception: Lithium carbonate decomposes to oxide and carbon dioxide.
    • Diagonal Relationship: Similar to magnesium.
    • Trend: Thermal stability increases down the group due to decreased q/r of the positive ion.
    • Lithium Ion: High q/r, polarizes carbonate ion, weakens bonds, leading to decomposition upon heating.
    • Other Group 1 Cations: Lower polarizing power, carbonates are stable.

• Hydrogencarbonates:

  • Preparation: Bubbling excess carbon dioxide through carbonate solution.
  • Stability: Stable in solid state at room temperature but decompose when heated to form carbonate, water, and carbon dioxide.
  • Application: Sodium hydrogencarbonate is used in baking soda; release of carbon dioxide has a leavening effect.

• Nitrates(V):

  • Source: Usually mined, can be prepared by neutralization reaction.
  • Decomposition: Break down when heated to form nitrate(III) and oxygen.
    • Exception: Lithium nitrate decomposes to lithium oxide, nitrogen dioxide, and oxygen.
    • Reason: Small size of lithium ion requires further size reduction to stabilize the lattice.

• Sulfates(VI):

  • Preparation: Neutralization.
  • Sodium Sulfate: Forms hydrated crystals when aqueous solution is heated to crystallization.
  • Sodium Hydrogensulfate: Acid salt, prepared with half the alkali volume or double the acid volume.

• Flame Tests

• Uses of Alkali Metal Compounds

Group 2 – The Alkaline Earth Metals

• Beryllium:

  • Appearance: Grey and hard metal, brittle at room temperature.
  • Amphoteric: Reacts with both acids and alkalis to produce salt and hydrogen.
  • Beryllium Oxide: Amphoteric oxide, reacts with acids and alkalis to form salt and water.
  • Be2+ Ion: Very high q/r, high polarizing power.
    • Covalent Character: Beryllium compounds have high covalent character (e.g., beryllium chloride is an electron deficient molecule which polymerizes).
    • Acidity: Solutions containing Be2+ ion are acidic.

• General Properties of Other Alkaline Earth Metals:

  • Ionic Compounds: Form ionic compounds with M2+ ions.
  • Lattice Enthalpies: Higher than corresponding alkali metal compounds due to higher q/r of cation.
  • Thermal Stability: Compounds more thermally unstable than Group 1 compounds (greater polarizing power of cation).
  • Ionization Energy: Decreases down the group (increased shielding, increased distance).
  • Reactivity: Increases down the group.

Physical Properties of Alkaline Earth Metals

1. Appearance: Silvery grey metals at room temperature.
2. Melting Points: Higher than alkali metals due to stronger metallic bonding. Decreases down the group.
3. Hardness and Density: Harder metals with higher densities (stronger metallic bonding).
4. Conductivity: Good conductors of heat and electricity (delocalized electrons).

• Chemical properties
  1. Reaction with oxygen
     • The alkaline earth metals burn vigorously in air to produce a simple oxide. However, barium tends to form the peroxide when heated in air due to a lower q/r of the Ba^{2+} when compared to the other Group 2 cations.
     • When heated in air, the Group 2 metals (except beryllium) form nitrides. This reaction is also given by lithium. These nitrides react with water to release ammonia.
     • M3N2 + 6H2O \rightarrow 3M(OH)2 + 2NH_3
  2. Reaction with water
     • Beryllium does not react with cold water or steam but reacts immediately with acids.
     • Magnesium reacts very slowly with cold water but reacts immediately with steam to produce magnesium oxide and hydrogen.
     • Mg + H2O \rightarrow MgO + H2
     • Calcium is more reactive than magnesium and therefore undergoes a steady reaction with cold water to produce a solution or suspension of calcium hydroxide and hydrogen.
     • Ca + 2H2O \rightarrow Ca(OH)2 + H_2
     • Strontium and barium react more vigorously than calcium and their reactivity is similar to that of lithium.
     • Sr + 2H2O \rightarrow Sr(OH)2 + H_2
     • Ba + 2H2O \rightarrow Ba(OH)2 + H_2

Alkaline Earth Metal Compounds

1. Hydroxides

   • Mg(OH)_2: Very sparingly soluble, prepared by precipitation.
   • Solubility: Increases down the group (decrease in lattice dissociation enthalpy).
   • Ca(OH)_2: Sparingly soluble, prepared by reaction of metal or oxide with water.
   • Sr(OH)_2: Soluble
   • Ba(OH)_2: Soluble
   • Thermal Stability: Decompose when heated to give oxide and water vapor. Thermal stability increases down the group (decrease in q/r of the cation).
     • Note: Barium hydroxide is soluble, so adding sodium hydroxide to barium ions should not give a precipitate unless carbonate ions are present (forming barium carbonate precipitate).

2. Carbonates

   • Insoluble: Prepared by precipitation.
   • Solubility: Decreases down the group (decrease in enthalpy of hydration).
   • Thermal Decomposition: Break down when heated to give oxide and carbon dioxide.

3. Hydrogencarbonates

   • Less stable than Group 1: Higher q/r of the cation.
   • Existence: Only found in solution.
   • Decomposition: Heating the solution yields solid carbonate, water, and carbon dioxide. Reaction relevant to temporary hard water.

4. Sulfates(VI)

   • MgSO4: Mined as Epsom Salts (MgSO4 "." 7H2O), contributes to permanent hardness, soluble. Prepared by reacting dilute sulfuric acid with Mg/MgO/Mg(OH)2/MgCO_3
   • Solubility: Decreases down the group (decrease in enthalpy of hydration).
   • CaSO4: Mined as anhydrite (CaSO4) or gypsum (CaSO4 "." 2H2O), gives Plaster of Paris (CaSO4 "." 1/2 H2O) on heating, contributes to permanent hardness, sparingly soluble, prepared by precipitation.
   • SrSO_4: Insoluble, prepared by precipitation.
   • BaSO_4: Insoluble, prepared by precipitation.
   • Distinguishing Ca^{2+}(aq) and Mg^{2+}(aq): Dilute sulfuric acid or sulfate solution can be used. Calcium forms insoluble calcium sulfate, magnesium forms soluble magnesium sulfate.

5. Nitrates(V)

   • Manufacture: Prepared by reacting dilute nitric acid with a metal oxide, hydroxide, or carbonate.
   • Thermal Decomposition: Break down when heated to give an oxide, nitrogen dioxide, and oxygen.

• Uses of Alkaline Earth Metal Compounds