Exam Master Review Sheet - s-Block Elements (JEE & NEET)

CHEMCA

EXAM MASTER REVIEW SHEET

Inorganic Chemistry: s-Block Elements

Group 1 & 2 Strategic Summary for JEE Main, Advanced & NEET

1 Group 1: Alkali Metals ($ns^1$)

Hydration Enthalpy & Mobility

$Li^+ > Na^+ > K^+ > Rb^+ > Cs^+$

The smaller the ion, the higher the hydration enthalpy. Thus, aqueous $Li^+$ is the largest hydrated ion and has the lowest ionic mobility.

Flame Coloration

Li: Crimson Red
Na: Golden Yellow
K: Violet/Lilac
Rb/Cs: Red-Violet / Blue

Reactivity with Oxygen (Air)

Alkali metals tarnish rapidly in air. Depending on the size of the metal, they form different primary products:

$4Li + O_2 \to 2Li_2O$ (Oxide)
$2Na + O_2 \to Na_2O_2$ (Peroxide)
$M + O_2 \to MO_2$ (Superoxide for K, Rb, Cs)

Reason: Large cations stabilize large anions (like peroxide/superoxide) via lattice energy effects.

Solution in Liquid Ammonia

$M + (x+y)NH_3 \longrightarrow [M(NH_3)_x]^+ + [e(NH_3)_y]^-$

• Imparts a Deep Blue Color due to ammoniated electrons.
• Solution is paramagnetic and highly conducting.
• Powerful reducing agent (used in Birch reduction).
• In concentrated solutions (>3M), color changes to Bronze and becomes diamagnetic.

2 Group 2: Alkaline Earth Metals ($ns^2$)

Property	Trend Down the Group ($Be \to Ba$)	Comparison with Group 1
Atomic & Ionic Size	Increases	Smaller than corresponding Gr 1
Ionization Enthalpy ($IE_1$)	Decreases	Higher than Gr 1 (due to smaller size)
Second Ionization ($IE_2$)	Decreases	Much LOWER than Gr 1 (Gr 1 achieves noble gas config after $IE_1$)
Hardness / M.P.	Irregular trend	Harder, denser, higher M.P. than Gr 1

Important Exception (Flame Test): Beryllium ($Be$) and Magnesium ($Mg$) do not impart any color to the Bunsen flame. Their electrons are too tightly bound (high ionization enthalpy) to be excited by the energy of the flame.

3 Solubility Trends & Diagonal Relationships

Solubility & Thermal Stability

Hydroxide Solubility: Increases down the group. (Lattice energy decreases more rapidly than hydration energy).
$Be(OH)_2 < Mg(OH)_2 < Ca(OH)_2 < Ba(OH)_2$
Sulfate/Carbonate Solubility: Decreases down the group. (Hydration energy decreases more rapidly than lattice energy for large anions).
Thermal Stability of Carbonates: Increases down the group. (Polarizing power of cation decreases, less distortion of $CO_3^{2-}$ cloud).

Anomalous Properties (Fajans' Rule)

Lithium and Beryllium exhibit anomalous behavior due to their exceptionally small size and high charge/radius ratio (high polarizing power).

Their compounds have significant covalent character (soluble in organic solvents).
$Li$ is the only alkali metal to form a stable Nitride ($Li_3N$) directly with air.
$Be(OH)_2$ is amphoteric, while other Gr 2 hydroxides are basic.

Diagonal: Lithium & Magnesium

• Both form normal oxides ($Li_2O, MgO$) in air.
• Both form Nitrides directly with nitrogen.
• Their carbonates decompose on heating to form oxides and $CO_2$. (Other alkali carbonates are stable to heat).
• Chlorides ($LiCl, MgCl_2$) are deliquescent and soluble in ethanol.

Diagonal: Beryllium & Aluminium

• Both are rendered passive by conc. $HNO_3$ due to a protective oxide film.
• Both form Amphoteric oxides and hydroxides.
• Their chlorides ($BeCl_2, AlCl_3$) are Lewis acids, soluble in organic solvents, and form bridged polymeric structures in vapor/solid phase.

4 Important Compounds & Biological Role

Solvay Process ($Na_2CO_3$)

$NH_3 + H_2O + CO_2 \to NH_4HCO_3$
$NH_4HCO_3 + NaCl \to NaHCO_3 \downarrow + NH_4Cl$
$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$

Trap: $K_2CO_3$ CANNOT be prepared by this method because $KHCO_3$ is highly soluble in water and does not precipitate out.

Castner-Kellner Cell ($NaOH$)

Electrolysis of Brine ($NaCl$). Uses a Mercury Cathode and Carbon Anode.

Cathode: $Na^+ + e^- \xrightarrow{Hg} Na\text{-amalgam}$
Anode: $Cl^- \to \frac{1}{2}Cl_2 + e^-$
Hydrolysis: $2Na\text{-amalgam} + 2H_2O \to 2NaOH + 2Hg + H_2$

Calcium Compounds

Quick Lime ($CaO$): Produced by roasting $CaCO_3$. Used in cement.
Slaked Lime ($Ca(OH)_2$): $CaO + H_2O$. Used in mortar.
Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$): Obtained by carefully heating Gypsum ($CaSO_4 \cdot 2H_2O$) at 393 K.
Above 473 K: Anhydrous 'dead burnt plaster' forms.

Biological Importance

$Na^+ / K^+$ Pump: Essential for nerve signal transmission and cell membrane potential. $Na^+$ is primarily extracellular; $K^+$ is intracellular.
$Mg^{2+}$: Central atom in Chlorophyll. Crucial for ATP function (all enzymes utilizing ATP require Mg).
$Ca^{2+}$: Present in bones/teeth (as apatite), essential for blood clotting, and muscle contraction.

Search This Blog

Exam Master Review Sheet - s-Block Elements