Some Important Compounds — Revision Notes

⚡ 30-Second Revision

Washing Soda: — $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$. Solvay process. Glass, soap, water softener. Efflorescent.
Baking Soda: — $\text{NaHCO}_3$. From Solvay. Antacid, baking. Decomposes to $\text{CO}_2$ on heating.
Caustic Soda: — $\text{NaOH}$. Castner-Kellner process. Strong base. Soap, paper, bauxite purification.
Quicklime: — $\text{CaO}$. From $\text{CaCO}_3$ calcination. Reacts with $\text{H}_2\text{O}$ (slaking) to $\text{Ca(OH)}_2$.
Slaked Lime: — $\text{Ca(OH)}_2$. From $\text{CaO} + \text{H}_2\text{O}$. 'Milk of lime'. Turns milky with $\text{CO}_2$, clears with excess $\text{CO}_2$.
Limestone: — $\text{CaCO}_3$. Natural form. Decomposes to $\text{CaO} + \text{CO}_2$ on heating.
Plaster of Paris (POP): — $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$. From gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) at $120^circ\text{C}$. Sets with $\text{H}_2\text{O}$.
Dead Burnt Plaster: — Anhydrous $\text{CaSO}_4$. Overheated gypsum, loses setting property.
Thermal Stability: — $\text{Li}_2\text{CO}_3$ least stable alkali carbonate. $\text{BeCO}_3$ least stable alkaline earth carbonate. Stability increases down Group 2.

2-Minute Revision

Important s-block compounds include those of sodium and calcium, vital for various applications. Sodium carbonate, or washing soda ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$), is produced by the Solvay process, which is notable for ammonia regeneration and $\text{CaCl}_2$ as a byproduct.

It's used in glass and water softening. Sodium bicarbonate ($\text{NaHCO}_3$), baking soda, is an intermediate in the Solvay process, used as an antacid and leavening agent due to its thermal decomposition to $\text{CO}_2$.

Sodium hydroxide ($\text{NaOH}$), caustic soda, is a strong base manufactured via the Castner-Kellner electrolytic process, used in soap and paper. Calcium oxide ($\text{CaO}$), quicklime, is made by heating limestone ($\text{CaCO}_3$) and reacts exothermically with water to form calcium hydroxide ($\text{Ca(OH)}_2$), slaked lime.

Slaked lime is crucial for detecting $\text{CO}_2$ (milkiness then clearing). Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$) is formed by heating gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) to $120^circ\text{C}$ and sets into a hard mass with water, used in medical casts.

Remember the thermal stability trend: smaller cations (like $\text{Li}^+, \text{Be}^{2+}$) lead to less stable carbonates.

5-Minute Revision

For NEET, a deep dive into s-block compounds is essential. Start with Sodium Carbonate (Washing Soda), $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$. Its industrial preparation, the Solvay Process, is critical: $\text{NaCl} + \text{NH}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{NaHCO}_3\downarrow + \text{NH}_4\text{Cl}$, followed by $2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$.

Ammonia regeneration ($2\text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \rightarrow 2\text{NH}_3 + \text{CaCl}_2 + 2\text{H}_2\text{O}$) makes it economical. Uses include glass, soap, and water softening. It's efflorescent.

Sodium Bicarbonate (Baking Soda), $\text{NaHCO}_3$, is an intermediate, used as an antacid and leavening agent due to its decomposition. Sodium Hydroxide (Caustic Soda), $\text{NaOH}$, is a strong base from the Castner-Kellner Process (electrolysis of brine with mercury cathode: $\text{Na}^+ + \text{e}^- \xrightarrow{\text{Hg}} \text{Na(amalgam)}$; then $2\text{Na(amalgam)} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2 + 2\text{Hg}$).

Uses: soap, paper, bauxite. Next, Calcium Oxide (Quicklime), $\text{CaO}$, from heating limestone ($\text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2$). It undergoes slaking of lime with water: $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$ (exothermic).

Calcium Hydroxide (Slaked Lime), $\text{Ca(OH)}_2$, forms 'milk of lime' and is used to detect $\text{CO}_2$: $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow$ (milkiness), then $\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3)_2$ (clears).

Calcium Carbonate ($\text{CaCO}_3$) is limestone/marble. Finally, Plaster of Paris (POP), $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$, from heating gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) to $120^circ\text{C}$.

It sets with water: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$. Overheating gives 'dead burnt plaster'. Remember thermal stability: $\text{Li}_2\text{CO}_3$ and $\text{BeCO}_3$ are less stable due to small cation size and high polarizing power; stability increases down Group 2 for carbonates.

Prelims Revision Notes

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**Sodium Carbonate (Washing Soda, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$):**

* Preparation: Solvay process. Raw materials: $\text{NaCl}$, $\text{NH}_3$, $\text{CaCO}_3$ (for $\text{CO}_2$ and $\text{Ca(OH)}_2$). * Key Steps: $\text{NH}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{NH}_4\text{HCO}_3$; $\text{NaCl} + \text{NH}_4\text{HCO}_3 \rightarrow \text{NaHCO}_3\downarrow + \text{NH}_4\text{Cl}$; $2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$.

* Ammonia Regeneration: $2\text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \rightarrow 2\text{NH}_3 + \text{CaCl}_2 + 2\text{H}_2\text{O}$. $\text{CaCl}_2$ is a byproduct. * Properties: White crystalline, efflorescent, alkaline solution (hydrolysis of $\text{CO}_3^{2-}$).

* Uses: Glass, soap, borax, paper, water softener, laundry.

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**Sodium Bicarbonate (Baking Soda, $\text{NaHCO}_3$):**

* Preparation: Intermediate in Solvay process. Also $\text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow 2\text{NaHCO}_3$. * Properties: White solid, sparingly soluble. Decomposes on heating ($>100^circ\text{C}$) to $\text{Na}_2\text{CO}_3$, $\text{H}_2\text{O}$, $\text{CO}_2$. * Uses: Baking powder (releases $\text{CO}_2$), antacid, fire extinguishers.

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**Sodium Hydroxide (Caustic Soda, $\text{NaOH}$):**

* Preparation: Castner-Kellner process (electrolysis of brine). Anode: $\text{Cl}_2$. Cathode: $\text{Na(amalgam)}$. Amalgam reacts with $\text{H}_2\text{O}$ to give $\text{NaOH}$, $\text{H}_2$, $\text{Hg}$. * Properties: White, deliquescent, strong corrosive base, highly soluble in $\text{H}_2\text{O}$ (exothermic). * Uses: Soap, paper, artificial silk, dyes, petroleum refining, bauxite purification.

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**Calcium Oxide (Quicklime, $\text{CaO}$):**

* Preparation: Calcination of limestone: $\text{CaCO}_3 \xrightarrow{1000-1200^circ\text{C}} \text{CaO} + \text{CO}_2$. * Properties: White amorphous solid, high melting point. Basic oxide. Reacts vigorously with $\text{H}_2\text{O}$ (slaking of lime) to $\text{Ca(OH)}_2$. * Uses: Cement, glass, steel, flux, sugar purification, neutralizing acidic soils.

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**Calcium Hydroxide (Slaked Lime, $\text{Ca(OH)}_2$):**

* Preparation: Slaking of lime: $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$. * Properties: White powder, sparingly soluble. Aqueous solution is 'lime water', suspension is 'milk of lime'.

* **Reactions with $\text{CO}_2$:** $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$ (milkiness). $\text{CaCO}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Ca(HCO}_3)_2$ (clears).

* Uses: Whitewash, mortar, neutralizing acids, bleaching powder.

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**Calcium Carbonate (Limestone, Marble, Chalk, $\text{CaCO}_3$):**

* Occurrence: Natural forms. Main component of shells. * Properties: White, insoluble. Decomposes on heating to $\text{CaO} + \text{CO}_2$. Reacts with acids to release $\text{CO}_2$. * Uses: Building material, cement, quicklime, antacid.

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**Plaster of Paris (POP, $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$):**

* Preparation: Heating gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) to $120^circ\text{C}$ ($393,\text{K}$). Overheating leads to 'dead burnt plaster' (anhydrous $\text{CaSO}_4$, loses setting property).

* Properties: White powder. Sets with $\text{H}_2\text{O}$ to form gypsum: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$.

* Uses: Surgical bandages, casts, dental impressions, fire-proofing, smooth surfaces.

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Thermal Stability of Carbonates:

* Alkali Metals: Generally very stable, except $\text{Li}_2\text{CO}_3$ (less stable due to small $\text{Li}^+$). * Alkaline Earth Metals: Stability increases down the group ($\text{BeCO}_3 < \text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3$). $\text{BeCO}_3$ is least stable due to small $\text{Be}^{2+}$ with high polarizing power.

Vyyuha Quick Recall

For Calcium compounds: Quicklime Slakes Cool People.

Quicklime ($\text{CaO}$) reacts with water to Slake, forming Calcium hydroxide ($\text{Ca(OH)}_2$).
Plaster of Paris ($\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$) is made from gypsum.