The S-Block Elements (Compounds Of Alkaline Earth Metals)

General Characteristics Of Compounds Of The Alkaline Earth Metals

The compounds of alkaline earth metals reflect their $+2$ oxidation state, relatively higher ionization enthalpies compared to alkali metals, and the significant ionic character of their bonds, though beryllium and magnesium compounds can show some covalent character.

Oxides and Hydroxides

Oxides ($MO$):

• Formation: Formed by direct reaction with oxygen, or by thermal decomposition of carbonates or nitrates.
• Reactivity: Reactivity increases down the group. $BeO$ is amphoteric, $MgO$ is basic, and $CaO$, $SrO$, $BaO$ are increasingly basic.
• Stability: They are very stable and have high melting points.
• Reactions with Water: React with water to form hydroxides. Reactivity with water increases down the group.
• $BeO$ is insoluble.
• $MgO$ is sparingly soluble.
• $CaO$, $SrO$, $BaO$ are soluble.

Hydroxides ($M(OH)_2$):

• Basicity and Solubility: Basicity and solubility increase down the group from Be to Ba.
• $Be(OH)_2$ is amphoteric (reacts with both acids and bases).
• $Mg(OH)_2$ is a weak base and sparingly soluble.
• $Ca(OH)_2$, $Sr(OH)_2$, $Ba(OH)_2$ are strong bases and increasingly soluble.

Halides ($MX_2$)

Formation: Prepared by direct combination of elements or by reaction with acids.

Properties:

• Ionic Character: The ionic character of halides increases down the group. Beryllium halides ($BeCl_2$, $BeBr_2$, $BeI_2$) are covalent and soluble in organic solvents. Magnesium halides are ionic but also soluble in organic solvents to some extent. Calcium, Strontium, and Barium halides are ionic and generally soluble in water.

• Solubility: Solubility trend:
• Fluorides: $BeF_2$ and $MgF_2$ are sparingly soluble. $CaF_2$, $SrF_2$, $BaF_2$ are insoluble (decrease down the group).
• Chlorides, Bromides, Iodides: Solubility generally increases down the group ($BeCl_2$ and $MgCl_2$ are soluble and hygroscopic, $CaCl_2$, $SrCl_2$, $BaCl_2$ are increasingly soluble and deliquescent).
• Melting Points: Decrease down the group due to decreasing lattice energy.

Salts Of Oxo-Acids

Properties: The salts of oxo-acids of alkaline earth metals are generally insoluble or sparingly soluble in water (except for those of $Ca, Sr, Ba$ with some acids).

• Carbonates ($MCO_3$): All carbonates are insoluble in water and are thermally decomposed to form oxides and $CO_2$. Thermal stability increases down the group. ($BeCO_3$ and $MgCO_3$ decompose easily).
• Sulfates ($MSO_4$): Sulfates of $Ca, Sr, Ba$ are less soluble than the corresponding sulfates of alkali metals. Solubility decreases down the group ($BeSO_4$ and $MgSO_4$ are soluble, $CaSO_4$ is sparingly soluble, $SrSO_4$ and $BaSO_4$ are insoluble).
• Nitrates ($M(NO_3)_2$): All nitrates are soluble in water and decompose on heating to give oxides.
• Bicarbonates ($MHCO_3$): Only $Ca, Sr, Ba$ form soluble bicarbonates. $Mg(HCO_3)_2$ and $Be(HCO_3)_2$ are not very stable.

Thermal Stability: The thermal stability of carbonates and nitrates increases down the group.

Flame Coloration: Compounds of Ca, Sr, Ba impart characteristic colors to the flame.

Some Important Compounds Of Calcium

Calcium ($Ca$) is a highly reactive alkaline earth metal, and its compounds are abundant and have significant industrial and biological importance.

1. Calcium Oxide ($CaO$) (Quicklime):

• Preparation: Prepared by heating calcium carbonate ($CaCO_3$) in a kiln at about 1200 K.

$CaCO_3(s) \xrightarrow{heat} CaO(s) + CO_2(g)$

• Properties: White amorphous solid, melts at 2870 K. Reacts with water in a highly exothermic reaction (slaking of lime) to form calcium hydroxide ($Ca(OH)_2$).

$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) \quad (\Delta H \text{ is highly negative})$

• Amphoteric Nature: Reacts with acidic oxides like $SiO_2$ to form salt-like compounds used in cement.

$CaO(s) + SiO_2(s) \rightarrow CaSiO_3(s)$

• Uses:
• One of the important constituents of cement.
• Used in the manufacture of steel as a basic flux.
• Used for drying gases (basic drying agent).
• Used in the manufacture of washing soda from sodium carbonate (Solvay process).

2. Calcium Hydroxide ($Ca(OH)_2$) (Slaked Lime):

• Preparation: Prepared by the reaction of calcium oxide with water (slaking).
• Properties: White powder, sparingly soluble in water. The aqueous solution is known as lime water, and the suspension is known as milk of lime.
• Uses:
• In the Solvay process for the manufacture of sodium carbonate.
• In whitewashing of walls. It reacts with $CO_2$ in the air to form a thin layer of calcium carbonate, giving a shiny finish.

$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$

• Used in glass, cement, and bleaching powder manufacturing.
• Used for neutralization of acidity in soils.

3. Calcium Carbonate ($CaCO_3$) (Limestone, Chalk, Marble):

• Occurrence: Found abundantly in nature as limestone, marble, chalk, seashells, etc.
• Properties: White solid, insoluble in pure water but soluble in water containing dissolved $CO_2$ due to the formation of soluble calcium hydrogencarbonate.

$CaCO_3(s) + H_2O(l) + CO_2(aq) \rightarrow Ca(HCO_3)_2(aq)$

(This is responsible for temporary hardness of water and formation of stalactites and stalagmites in caves).

• Thermal Decomposition: Decomposes on heating above 1200 K to form calcium oxide and carbon dioxide.
• Reaction with Acids: Reacts with acids to produce salt, water, and $CO_2$.
• Uses:
• Essential component of cement and construction industry (limestone, marble).
• Used in the Solvay process.
• Used as a dietary calcium supplement.
• Used in toothpaste as a mild abrasive.

4. Calcium Sulfate ($CaSO_4$):

• Occurrence: Found in nature as gypsum ($CaSO_4 \cdot 2H_2O$) and anhydrite ($CaSO_4$).
• Properties: Gypsum is dihydrate. When heated to about 393 K, it loses water molecules to form plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$).

$CaSO_4 \cdot 2H_2O(s) \xrightarrow{393 K} CaSO_4 \cdot \frac{1}{2}H_2O(s) + \frac{3}{2}H_2O(g)$

• Plaster of Paris: When mixed with water, it sets into a hard mass ($CaSO_4 \cdot 2H_2O$), known as plaster of Paris.

$CaSO_4 \cdot \frac{1}{2}H_2O(s) + 1\frac{1}{2}H_2O(l) \rightarrow CaSO_4 \cdot 2H_2O(s)$

• Uses:
• Plaster of Paris is used in making toys, decorative materials, and for plastering walls.
• Gypsum is used in the manufacture of cement.

5. Calcium Chloride ($CaCl_2$):

• Preparation: Prepared by the reaction of calcium carbonate with hydrochloric acid.

$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$

• Properties: White crystalline solid, highly soluble in water, deliquescent (absorbs moisture from the atmosphere).
• Uses:
• Used for drying gases and oils.
• Used as a de-icing agent on roads.
• Used in refrigerators as brine.

6. Calcium Carbonate (Biological Significance): Calcium carbonate is the major component of seashells, coral reefs, and eggshells. It is vital for bone formation in animals and the structural integrity of many plants.