Salts of Oxoacids — Core Principles

Core Principles

Salts of oxoacids are ionic compounds formed when the acidic hydrogen atoms of an oxoacid (an acid containing oxygen, like $H_2SO_4$) are replaced by metal or ammonium cations. The resulting anion, called an oxoanion (e.

g., $SO_4^{2-}$), contains a central atom bonded to oxygen. Key properties include solubility, thermal stability, and redox behavior. Most nitrates are soluble, while most carbonates and phosphates are insoluble (except for alkali metals and ammonium).

Thermal stability trends are crucial: for Group 1 and 2 carbonates and nitrates, stability generally increases down the group due to decreasing polarizing power of the cation. Salts with central atoms in high oxidation states (e.

g., nitrates, perchlorates) act as oxidizing agents, while those in lower oxidation states (e.g., sulfites, nitrites) act as reducing agents. Hydrolysis of these salts can lead to acidic, basic, or neutral solutions depending on the parent acid and base strengths.

These salts are vital in agriculture, construction, and various industries.

Important Differences

vs Thermal Stability of Group 1 vs. Group 2 Carbonates

Aspect	This Topic	Thermal Stability of Group 1 vs. Group 2 Carbonates
General Trend within Group	Group 1 Carbonates ($M_2CO_3$)	Group 2 Carbonates ($MCO_3$)
Thermal Stability Down the Group	Increases (e.g., $Na_2CO_3 < K_2CO_3$)	Increases (e.g., $MgCO_3 < CaCO_3$)
Absolute Stability	Generally more stable (higher decomposition temperatures)	Generally less stable (lower decomposition temperatures)
Exception	$Li_2CO_3$ is an exception, behaving more like Group 2 carbonates due to small $Li^+$ size and high polarizing power.	No significant exceptions to the general trend within the group.
Decomposition Products	Typically stable, except $Li_2CO_3 \rightarrow Li_2O + CO_2$. Other alkali metal carbonates are very stable.	Decompose to metal oxide and carbon dioxide ($MCO_3 \rightarrow MO + CO_2$).
Reason for Difference	Larger cation size and lower charge ($+1$) lead to lower polarizing power and greater stability.	Smaller cation size and higher charge ($+2$) lead to higher polarizing power and lower stability compared to Group 1.

The thermal stability of carbonates generally increases down both Group 1 and Group 2. However, Group 1 carbonates are, on the whole, significantly more thermally stable than Group 2 carbonates. This difference arises from the higher polarizing power of the smaller, more highly charged Group 2 cations ($M^{2+}$) compared to the larger, singly charged Group 1 cations ($M^+$). The greater polarizing power of Group 2 cations distorts the carbonate anion more effectively, weakening its bonds and facilitating decomposition at lower temperatures. Lithium carbonate is a notable exception in Group 1, exhibiting lower stability akin to Group 2 carbonates due to the exceptionally small size of the $Li^+$ ion.