Salts of Oxoacids — Revision Notes

⚡ 30-Second Revision

Oxoacids: — Acids with acidic H-O bonds (e.g., $H_2SO_4$, $HNO_3$).
Salts: — Formed by replacing acidic H with cations (e.g., $Na_2SO_4$, $CaCO_3$).
Solubility:

- Nitrates ($NO_3^-$): All soluble. - Sulfates ($SO_4^{2-}$): Mostly soluble; exceptions: $BaSO_4$, $PbSO_4$, $SrSO_4$, $CaSO_4$ (sparingly). - Carbonates ($CO_3^{2-}$), Phosphates ($PO_4^{3-}$): Mostly insoluble; exceptions: Group 1 & $NH_4^+$ salts.

Thermal Stability:

- Group 1 & 2 Carbonates/Nitrates: Stability $\uparrow$ down group (due to $downarrow$ polarizing power of cation). - $Li_2CO_3$: Exception, less stable than other Group 1, like Group 2. - Decomposition: $MCO_3 \rightarrow MO + CO_2$; $2MNO_3 \rightarrow 2MNO_2 + O_2$ (Group 1); $2M(NO_3)_2 \rightarrow 2MO + 4NO_2 + O_2$ (Group 2, heavy metals); $NH_4NO_3 \rightarrow N_2O + 2H_2O$.

Redox:

- Oxidizing agents: Central atom in high O.S. (e.g., $NO_3^-$, $ClO_4^-$). - Reducing agents: Central atom in intermediate O.S. (e.g., $SO_3^{2-}$, $NO_2^-$).

Hydrolysis:

- Strong Acid + Strong Base: Neutral. - Strong Acid + Weak Base: Acidic. - Weak Acid + Strong Base: Basic.

2-Minute Revision

Salts of oxoacids are ionic compounds derived from oxoacids, where acidic protons are replaced by cations. Key properties for NEET include solubility, thermal stability, and redox behavior. Solubility rules are crucial: nitrates are almost universally soluble, while most carbonates and phosphates are insoluble, with alkali metal and ammonium salts being the main exceptions.

Sulfates are generally soluble, but $BaSO_4$, $PbSO_4$, and $SrSO_4$ are important insoluble examples. Thermal stability is a high-yield topic; it generally increases down a group for carbonates and nitrates due to decreasing polarizing power of the cation.

Remember the exception of $Li_2CO_3$. Decomposition products vary: Group 1 nitrates yield nitrites and oxygen, while Group 2 and heavy metal nitrates yield metal oxides, nitrogen dioxide (brown gas), and oxygen.

Ammonium nitrate decomposes to $N_2O$ and water. Redox properties depend on the oxidation state of the central atom in the oxoanion: high oxidation states imply oxidizing agents (e.g., $NO_3^-$), while intermediate states imply reducing agents (e.

g., $SO_3^{2-}$). Finally, understand salt hydrolysis to predict if a solution will be acidic, basic, or neutral based on the strengths of the parent acid and base.

5-Minute Revision

Salts of oxoacids are fundamental inorganic compounds, formed from the neutralization of oxoacids by bases. An oxoacid contains oxygen and acidic H-O bonds, yielding an oxoanion (e.g., $SO_4^{2-}$ from $H_2SO_4$).

1. Solubility: This is a critical predictive tool. All nitrates ($NO_3^-$) are soluble. Most sulfates ($SO_4^{2-}$) are soluble, but remember the key insoluble ones: $BaSO_4$, $PbSO_4$, and $SrSO_4$, with $CaSO_4$ being sparingly soluble.

Conversely, most carbonates ($CO_3^{2-}$), phosphates ($PO_4^{3-}$), and sulfites ($SO_3^{2-}$) are insoluble, with the significant exceptions being salts of alkali metals ($Na^+, K^+$ etc.) and ammonium ($NH_4^+$).

*Example:* Is $CuCO_3$ soluble? No, because copper is not an alkali metal or ammonium.

2. Thermal Stability: This property is heavily tested. For carbonates and nitrates of Group 1 and Group 2 metals, thermal stability increases as you go down the group. This is because larger cations have lower polarizing power, leading to less distortion of the large oxoanion and thus greater stability.

For instance, $BaCO_3$ is more stable than $MgCO_3$. A crucial exception is $Li_2CO_3$, which is less stable than other alkali metal carbonates due to the small size and high polarizing power of $Li^+$, making it behave more like Group 2 carbonates.

*Example:* Order of stability: $MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3$.

3. Decomposition Products: Specific reactions are important: * Alkali metal nitrates (except $LiNO_3$): $2MNO_3 \xrightarrow{\Delta} 2MNO_2 + O_2$ * $LiNO_3$, Group 2 nitrates, and heavy metal nitrates (e.g., $Pb(NO_3)_2$): $2M(NO_3)_n \xrightarrow{\Delta} 2MO_x + 2nNO_2 + \frac{n}{2}O_2$. Note the brown $NO_2$ gas. * Ammonium nitrate: $NH_4NO_3 \xrightarrow{\Delta} N_2O + 2H_2O$.

4. Redox Properties: The ability to act as an oxidizing or reducing agent depends on the oxidation state of the central atom in the oxoanion. If the central atom is in its highest oxidation state (e.

g., N in $NO_3^-$ (+5), S in $SO_4^{2-}$ (+6), Cl in $ClO_4^-$ (+7)), the salt is an oxidizing agent. If it's in an intermediate oxidation state (e.g., N in $NO_2^-$ (+3), S in $SO_3^{2-}$ (+4)), it can act as a reducing agent (and sometimes an oxidizing agent via disproportionation).

*Example:* $Na_2SO_3$ is a reducing agent because sulfur can be oxidized from +4 to +6.

5. Hydrolysis: The pH of an aqueous solution of an oxoacid salt depends on the strengths of its parent acid and base. Salts of strong acid/strong base (e.g., $K_2SO_4$) are neutral. Salts of strong acid/weak base (e.g., $NH_4NO_3$) are acidic. Salts of weak acid/strong base (e.g., $Na_2CO_3$) are basic.

Prelims Revision Notes

Salts of Oxoacids: NEET Revision Notes

1. Definition & Formation:

Oxoacid: — Acid with acidic H-O bonds (e.g., $H_2SO_4$, $HNO_3$, $H_2CO_3$).
Salt Formation: — Replacement of acidic H by metal/ammonium cation (neutralization reaction).

* Example: $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$

2. Solubility Rules (Key for NEET):

Nitrates ($NO_3^-$): — ALL are soluble. (e.g., $NaNO_3$, $Pb(NO_3)_2$)
Sulfates ($SO_4^{2-}$): — Mostly soluble.

* Insoluble/Sparingly Soluble: $BaSO_4$, $PbSO_4$, $SrSO_4$, $CaSO_4$.

Carbonates ($CO_3^{2-}$), Phosphates ($PO_4^{3-}$), Sulfites ($SO_3^{2-}$): — Mostly insoluble.

* Soluble Exceptions: Alkali metal salts ($Li^+, Na^+, K^+, Rb^+, Cs^+$) and Ammonium salts ($NH_4^+$).

3. Thermal Stability (High Yield Topic):

General Trend: — Stability $\uparrow$ down a group for carbonates and nitrates.

* Reason: Cation size $\uparrow$, Polarizing power $\downarrow$, less distortion of anion, stronger ionic bond, higher stability.

Group 1 Carbonates: — Very stable, except $Li_2CO_3$.

* $Li_2CO_3 \xrightarrow{\Delta} Li_2O + CO_2$ (like Group 2). * Other $M_2CO_3$ are stable to very high temperatures.

Group 2 Carbonates: — Less stable than Group 1. Stability: $BeCO_3 < MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3$.

* Decomposition: $MCO_3(s) \xrightarrow{\Delta} MO(s) + CO_2(g)$

Nitrates Decomposition:

* **Alkali Metal Nitrates (except $LiNO_3$):** $2MNO_3(s) \xrightarrow{\Delta} 2MNO_2(s) + O_2(g)$ * **$LiNO_3$, Group 2 Nitrates, Heavy Metal Nitrates (e.g., $Pb(NO_3)_2$, $Cu(NO_3)_2$):** $2M(NO_3)_n(s) \xrightarrow{\Delta} 2MO_x(s) + 2nNO_2(g) + \frac{n}{2}O_2(g)$ * Key: $NO_2$ is a reddish-brown gas.

* Ammonium Nitrate: $NH_4NO_3(s) \xrightarrow{\Delta} N_2O(g) + 2H_2O(g)$ ($N_2O$ is dinitrogen monoxide, colorless).

4. Redox Properties:

Oxidizing Agents: — Central atom in highest oxidation state (can only be reduced).

* Examples: $NO_3^-$ (N=+5), $ClO_4^-$ (Cl=+7), $MnO_4^-$ (Mn=+7), $Cr_2O_7^{2-}$ (Cr=+6).

Reducing Agents: — Central atom in intermediate oxidation state (can be further oxidized).

* Examples: $SO_3^{2-}$ (S=+4), $NO_2^-$ (N=+3), $S_2O_3^{2-}$ (S=+2).

5. Hydrolysis of Salts:

Strong Acid + Strong Base: — Neutral solution (e.g., $NaCl$, $K_2SO_4$).
Strong Acid + Weak Base: — Acidic solution (cation hydrolysis, e.g., $NH_4Cl$, $CuSO_4$).
Weak Acid + Strong Base: — Basic solution (anion hydrolysis, e.g., $Na_2CO_3$, $CH_3COONa$).
Weak Acid + Weak Base: — pH depends on relative $K_a$ and $K_b$ (e.g., $CH_3COONH_4$).

6. Important Examples:

$CaCO_3$: Limestone, marble, chalk.
$NaHCO_3$: Baking soda.
$Na_2CO_3$: Washing soda.
$CaSO_4 cdot 2H_2O$: Gypsum.
$NH_4NO_3$, $(NH_4)_2SO_4$, $KNO_3$: Fertilizers.

Vyyuha Quick Recall

Never Stop Calculating Problems: Nitrates Soluble, Sulfates Soluble (except Big People Sit Carefully - $Ba, Pb, Sr, Ca$), Carbonates & Phosphates Insoluble (except Always Always - Alkali Metals & Ammonium).