Effect of Heat on Nitrates, Carbonates, and Hydrogen Carbonates

I. The Facts

Alkali metal compounds in Group 1 are generally more stable to heat than the corresponding compounds in Group 2. However, lithium compounds often exhibit behavior similar to Group 2 compounds, while the rest of Group 1 compounds display different thermal stability characteristics.

Nature of Carbonates, Bicarbonates, and Nitrates

• Carbonates (M₂CO₃) and bicarbonates (MHCO₃) of alkali metals are highly stable to heat. As we move down the group from lithium (Li) to cesium (Cs), the stability of these salts increases due to the increasing electropositive character of the metals.
• Nitrates (MNO₃) decompose upon strong heating to form the corresponding nitrites (MNO₂) and oxygen (O₂). The exception to this rule is lithium nitrate (LiNO₃), which decomposes differently.

Decomposition Reactions

Nitrates

For most alkali metals:

$2{NaNO}_3 → 2{NaNO}_2 + {O}_2$

For lithium nitrate:

$4{LiNO}_3 → 2{Li}_2{O} + 4{NO}_2 + {O}_2$

Carbonates

Carbonates of alkali metals are thermally stable and do not decompose upon heating. However, lithium carbonate (Li₂CO₃) decomposes to lithium oxide (Li₂O) and carbon dioxide (CO₂):

${Li}_2{CO}_3 → {Li}_2{O} + {CO}_2$

Bicarbonates

Bicarbonates decompose upon heating to form the carbonate, water, and carbon dioxide:

$2{MHCO}_3 → {M}_2{CO}_3 + {H}_2{O} + {CO}_2$

II. Explaining the Trend in Terms of the Polarizing Ability of the Positive Ion

The stability of alkali metal salts can be explained by considering the polarizing ability of the alkali metal cations.

Polarization and Covalent Character

• When an alkali metal cation (M⁺) approaches an anion, it attracts the outermost electrons of the anion and repels the anion’s nucleus. This interaction causes distortion or polarization of the anion.
• The distortion results in a partial sharing of electrons between the cation and the anion, imparting some covalent character to the ionic bond.

Size and Polarizing Power

• Smaller cations (like Li⁺) are more effective at polarizing anions compared to larger cations. This is because smaller cations have a higher charge density, which leads to stronger distortion of the anion.
• As a result, lithium salts tend to have more covalent character, making them less stable to heat compared to salts of larger alkali metals, which are predominantly ionic.

Summary

• Thermal Stability: Carbonates and bicarbonates of alkali metals are generally thermally stable, with stability increasing down the group. Lithium carbonate, however, decomposes upon heating.
• Nitrates: Most alkali metal nitrates decompose to nitrites and oxygen, except for lithium nitrate, which decomposes differently.
• Polarization: The trend in stability is due to the polarizing power of the alkali metal cations, with smaller cations like lithium causing more polarization and resulting in more covalent character in the bonds.