Salts of Oxoacids
Thermal Stability, Solubility Trends, and Decomposition Traps.
Oxoacids are acids containing oxygen (e.g., $H_2CO_3, H_2SO_4, HNO_3$). Since s-block elements are highly electropositive metals, they readily form stable ionic salts with the anions of these oxoacids (Carbonates, Bicarbonates, Sulfates, Nitrates). The trends in their thermal stability and solubility are massive cornerstones of competitive exams.
1. The Golden Rule of Thermal Stability
For salts containing large polyatomic anions (like $CO_3^{2-}, SO_4^{2-}, NO_3^-$), thermal stability is governed by Fajans' Rules (Polarization).
- Small cations (like $Li^+, Be^{2+}$) have a very high charge density and strongly polarize the electron cloud of the oxygen atom in the polyatomic anion.
- This intense polarization weakens the bond within the anion (e.g., the $C-O$ bond in carbonate), making the salt unstable to heat. It decomposes easily to yield the metal oxide.
- As cation size increases down the group, polarizing power decreases. The anion is less distorted, and the salt becomes more thermally stable.
2. Carbonates and Bicarbonates
Group 1 (Alkali Metals)
Alkali metal carbonates ($M_2CO_3$) and bicarbonates ($MHCO_3$) are generally highly stable to heat and highly soluble in water.
- The Lithium Anomaly: Because $Li^+$ is so small, $Li_2CO_3$ is the ONLY alkali metal carbonate that is thermally unstable. It decomposes on heating:
$Li_2CO_3 \xrightarrow{\Delta} Li_2O + CO_2$ - Bicarbonate Solid State Trap: All alkali metals form solid bicarbonates except Lithium. Lithium bicarbonate ($LiHCO_3$) only exists in aqueous solution because the small $Li^+$ ion cannot stabilize the large $HCO_3^-$ ion in a solid crystal lattice.
Group 2 (Alkaline Earth Metals)
Group 2 carbonates ($MCO_3$) are insoluble in water and decompose on heating.
- Thermal stability increases down the group: $BeCO_3 \lt MgCO_3 \lt CaCO_3 \lt SrCO_3 \lt BaCO_3$.
- Beryllium Trap: $BeCO_3$ is so unstable that it is usually kept in an atmosphere of $CO_2$ to prevent it from spontaneously decomposing into $BeO$.
- Bicarbonate Trap: NO Group 2 element forms a solid bicarbonate. Group 2 bicarbonates exist only in aqueous solution.
3. Sulfates and Solubility Trends
Group 1 sulfates ($M_2SO_4$) are readily soluble in water. However, the solubility trend for Group 2 sulfates ($MSO_4$) is highly specific and frequently tested.
Solubility Trend in Group 2:
Why does solubility DECREASE down Group 2?
Solubility depends on the balance between Hydration Enthalpy and Lattice Energy. The sulfate ion ($SO_4^{2-}$) is so massive that the Lattice Energy remains relatively constant down the group. However, as the metal cation gets larger (from Be to Ba), its Hydration Enthalpy plummets drastically.
4. Nitrates and Thermal Decomposition
The thermal decomposition of nitrates provides one of the most reliable ways to distinguish between elements based on their polarizing power.
Group 1 (Except Lithium):
Alkali metal nitrates (Na, K, Rb, Cs) decompose to form metal nitrites and Oxygen gas. They DO NOT yield nitrogen dioxide ($NO_2$).
Lithium and Group 2 Metals:
Because $Li^+$ and Group 2 ions ($Mg^{2+}, Ca^{2+},$ etc.) are highly polarizing, they severely distort the nitrate ion. Their nitrates decompose completely to the Metal Oxide, Nitrogen Dioxide (brown gas), and Oxygen.
Group 2: $2Ca(NO_3)_2 \xrightarrow{\Delta} 2CaO + 4NO_2 \uparrow + O_2 \uparrow$
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