The S-Block Elements (Compounds Of Alkali Metals)

General Characteristics Of The Compounds Of The Alkali Metals

The compounds of alkali metals generally reflect the characteristic properties of these elements – their electropositive nature, low ionization enthalpies, and tendency to form $+1$ ions.

Oxides And Hydroxides

Oxides: Alkali metals react with oxygen to form oxides.

• Lithium: Forms the normal oxide, $Li_2O$.

$4Li + O_2 \rightarrow 2Li_2O$

• Sodium: Forms sodium peroxide, $Na_2O_2$, and a small amount of superoxide, $NaO_2$.

$2Na + O_2 \rightarrow Na_2O_2$

$Na + O_2 \rightarrow NaO_2$ (minor product)

• Potassium, Rubidium, Cesium: Form superoxides ($KO_2$, $RbO_2$, $CsO_2$) and the normal oxide. Superoxides are colored and paramagnetic due to the presence of the $O_2^-$ ion.

$2K + O_2 \rightarrow K_2O_2$ (peroxide, but further reacts) $\rightarrow$ $KO_2$ (superoxide)

• Reactivity: Reactivity increases down the group. Superoxides are strong oxidizing agents.

Hydroxides: Alkali metal oxides react with water to form strong alkali metal hydroxides.

• General Reaction: $M_2O(s) + H_2O(l) \rightarrow 2MOH(aq)$
• Lithium Oxide: $Li_2O(s) + H_2O(l) \rightarrow 2LiOH(aq)$
• Sodium Hydroxide ($NaOH$): Forms a strong, highly soluble alkali.
• Potassium Hydroxide ($KOH$), Rubidium Hydroxide ($RbOH$), Cesium Hydroxide ($CsOH$): Are increasingly soluble and more strongly basic down the group.
• Stability: The basicity and solubility of alkali metal hydroxides increase down the group.

Halides

Formation: Alkali metals react directly with halogens to form ionic halides ($MX$).

General Reaction: $2M(s) + X_2 \rightarrow 2MX(s)$

Properties:

• Ionic Nature: The ionic character of halides decreases slightly down the group (e.g., $LiCl$ has some covalent character due to lithium's small size and high polarizing power, while $CsCl$ is almost completely ionic).
• Solubility: Most alkali metal halides are soluble in water. The solubility trend varies:
• Fluorides: Solubility decreases down the group ($LiF$ is least soluble, $CsF$ is most soluble) due to the very high hydration enthalpy of $F^-$ and high lattice enthalpy of $LiF$.
• Chlorides, Bromides, Iodides: Solubility generally increases down the group ($LiX$ is least soluble, $CsX$ is most soluble).
• Melting and Boiling Points: They have high melting and boiling points due to their ionic nature. These decrease down the group as the ionic size increases, weakening the lattice energy.

Salts Of Oxo-Acids

Formation: Alkali metals react with oxo-acids to form salts. These salts are generally soluble in water and are thermally stable.

Examples:

• Carbonates: $Li_2CO_3$ is sparingly soluble in water and decomposes on heating, unlike other alkali metal carbonates which are soluble and thermally stable.

• Bicarbonates: Only sodium forms a stable bicarbonate ($NaHCO_3$).

$2NaHCO_3(s) \xrightarrow{heat} Na_2CO_3(s) + H_2O(g) + CO_2(g)$

• Nitrates: Alkali metal nitrates ($MNO_3$) are generally soluble and decompose on strong heating to give nitrites and oxygen.

$2MNO_3(s) \xrightarrow{heat} 2MNO_2(s) + O_2(g)$

• Sulfates: Sodium sulfate ($Na_2SO_4$) and potassium sulfate ($K_2SO_4$) are soluble and stable.
• Phosphates: Form various phosphates depending on the ratio of metal to phosphate ion.

Trend in Solubility: For most oxo-acids, the solubility of the alkali metal salts increases from Li to Cs.

Some Important Compounds Of Sodium

Sodium ($Na$) is the second most abundant alkali metal and forms several commercially important compounds.

1. Sodium Chloride ($NaCl$) (Common Salt):

• Preparation: Obtained from sea-salt (evaporation of seawater) and from rock deposits (rock salt).
• Properties: White crystalline solid, soluble in water, melts at 1074 K. It is electrically non-conductive in solid state but conducts when molten or dissolved in water.
• Uses:
  • Source of sodium and chlorine.
  • Electrolysis of molten $NaCl$ yields sodium metal and chlorine gas.
  • Electrolysis of aqueous $NaCl$ yields $H_2$, $Cl_2$, and $NaOH$.
  • Used in cooking, de-icing roads, and as a raw material in chemical industries.

2. Sodium Hydroxide ($NaOH$) (Caustic Soda):

• Preparation: Primarily manufactured by the electrolysis of brine (concentrated aqueous $NaCl$) in the Castner-Kellner process or diaphragm cell process.

Overall reaction: $2NaCl(aq) + 2H_2O(l) \xrightarrow{electrolysis} 2NaOH(aq) + Cl_2(g) + H_2(g)$

• Properties: White, crystalline solid, highly soluble in water, deliquescent, absorbs moisture and carbon dioxide from the air. It is a strong base.
• Uses:
  • Manufacture of soaps and detergents.
  • Paper and rayon industries.
  • Used in refining petroleum.
  • Used in cleaning agents (drain cleaners).
  • Used in laboratories.

3. Sodium Carbonate ($Na_2CO_3$) (Washing Soda):

• Preparation: Primarily produced by the Solvay process.
• Properties: White crystalline solid, soluble in water, forms Sodium Carbonate decahydrate ($Na_2CO_3 \cdot 10H_2O$) which effloresces in dry air.
• Uses:
  • Manufacture of glass, soap, and paper.
  • Used in laundry detergents as a water softener.
  • Used in the manufacture of other sodium compounds.

4. Sodium Hydrogencarbonate ($NaHCO_3$) (Baking Soda):

• Preparation: Made by reacting sodium carbonate with carbon dioxide and water (a by-product of the Solvay process).

$Na_2CO_3(aq) + H_2O(l) + CO_2(g) \rightarrow 2NaHCO_3(s)$

• Properties: White crystalline solid, sparingly soluble in water. It is a mild, non-corrosive base.
• Uses:
  • Used in baking (as a leavening agent, it produces $CO_2$ upon heating or reaction with acid).
  • Used in fire extinguishers (releases $CO_2$ upon heating).
  • Used as an antacid to neutralize excess acid in the stomach.

5. Sodium Nitrate ($NaNO_3$) (Chile Saltpeter):

• Occurrence: Found naturally in Chile.
• Properties: White crystalline solid, soluble in water.
• Uses:
  • Used in the manufacture of fertilizers.
  • Used in the manufacture of explosives (gunpowder).
  • Used as a food preservative.

6. Sodium Carbonate ($Na_2CO_3$) (Washing Soda):

• Preparation: Primarily produced by the Solvay process.
• Properties: White crystalline solid, soluble in water, forms Sodium Carbonate decahydrate ($Na_2CO_3 \cdot 10H_2O$) which effloresces in dry air.
• Uses:
  • Manufacture of glass, soap, and paper.
  • Used in laundry detergents as a water softener.
  • Used in the manufacture of other sodium compounds.

7. Sodium Hydrogencarbonate ($NaHCO_3$) (Baking Soda):

• Preparation: Made by reacting sodium carbonate with carbon dioxide and water (a by-product of the Solvay process).

$Na_2CO_3(aq) + H_2O(l) + CO_2(g) \rightarrow 2NaHCO_3(s)$

• Properties: White crystalline solid, sparingly soluble in water. It is a mild, non-corrosive base.
• Uses:
  • Used in baking (as a leavening agent, it produces $CO_2$ upon heating or reaction with acid).
  • Used in fire extinguishers (releases $CO_2$ upon heating).
  • Used as an antacid to neutralize excess acid in the stomach.