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

Appearance: Silvery grey metals at room temperature.
Melting Points: Higher than alkali metals due to stronger metallic bonding. Decreases down the group.
Hardness and Density: Harder metals with higher densities (stronger metallic bonding).
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

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).
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.
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.
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.
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