Why is sodium chloride purified using $ ext{HCl}$ gas?

Crude sodium chloride obtained from seawater or rock salt contains impurities like $ ext{CaCl}_2$ and $ ext{MgCl}_2$, which are deliquescent and make the salt sticky. To purify it, crude salt is dissolved in minimal water, and then $ ext{HCl}$ gas is passed through the saturated solution. This works on the principle of the common ion effect. The added $ ext{HCl}$ increases the concentration of $ ext{Cl}^-$ ions, shifting the solubility equilibrium of $ ext{NaCl}$ to the left, causing pure $ ext{NaCl}$ to precipitate out. The impurities, being more soluble, remain in the solution.

What is the Chlor-alkali process, and what are its main products?

The Chlor-alkali process is an industrial method for producing sodium hydroxide, chlorine gas, and hydrogen gas by the electrolysis of an aqueous solution of sodium chloride (brine). The name 'Chlor-alkali' refers to chlorine and the alkali metal hydroxide (sodium hydroxide) produced. The main products are $ ext{NaOH}$ (at cathode), $ ext{Cl}_2$ (at anode), and $ ext{H}_2$ (at cathode). Each of these products has significant industrial applications, making this process highly valuable.

Explain the role of ammonia in the Solvay process.

Ammonia plays a crucial role in the Solvay process. First, it reacts with $ ext{CO}_2$ and water to form ammonium bicarbonate ($ ext{NH}_4 ext{HCO}_3$). This ammonium bicarbonate then reacts with $ ext{NaCl}$ to precipitate sodium bicarbonate ($ ext{NaHCO}_3$). Crucially, ammonia is regenerated at the end of the process from ammonium chloride ($ ext{NH}_4 ext{Cl}$) by reacting it with calcium hydroxide ($ ext{Ca(OH)}_2$). This regeneration makes the Solvay process highly economical and environmentally efficient, as ammonia is a relatively expensive raw material.

Why is an aqueous solution of sodium carbonate alkaline?

Sodium carbonate ($ ext{Na}_2 ext{CO}_3$) is a salt formed from a strong base ($ ext{NaOH}$) and a weak acid ($ ext{H}_2 ext{CO}_3$). When dissolved in water, it dissociates into $ ext{Na}^+$ and $ ext{CO}_3^{2-}$ ions. The $ ext{Na}^+$ ions are spectator ions, but the carbonate ions ($ ext{CO}_3^{2-}$) are the conjugate base of a weak acid. They react with water (hydrolyze) to produce bicarbonate ions ($ ext{HCO}_3^-$) and hydroxide ions ($ ext{OH}^-$), as shown: $ ext{CO}_3^{2-} ( ext{aq}) + ext{H}_2 ext{O} ( ext{l}) ightleftharpoons ext{HCO}_3^- ( ext{aq}) + ext{OH}^- ( ext{aq})$. The excess $ ext{OH}^-$ ions in the solution make it alkaline.

What is the difference between soda ash and washing soda?

Soda ash refers to anhydrous sodium carbonate ($ ext{Na}_2 ext{CO}_3$), which is a white powder. It is typically obtained by heating sodium bicarbonate or by dehydrating washing soda. Washing soda, on the other hand, is the decahydrate form of sodium carbonate, $ ext{Na}_2 ext{CO}_3 cdot 10 ext{H}_2 ext{O}$. It is a white crystalline solid and is efflorescent, meaning it loses water of crystallization when exposed to air. Both are used for similar purposes, but washing soda is the common form used in households for cleaning and water softening.

How does sodium carbonate help in softening hard water?

Hard water contains dissolved salts of calcium and magnesium, primarily as bicarbonates, chlorides, or sulfates. Sodium carbonate helps soften water by reacting with these dissolved calcium ($ ext{Ca}^{2+}$) and magnesium ($ ext{Mg}^{2+}$) ions to form insoluble calcium carbonate ($ ext{CaCO}_3$) and magnesium carbonate ($ ext{MgCO}_3$). These insoluble carbonates precipitate out of the water, effectively removing the hardness-causing ions. For example: $ ext{CaSO}_4 ( ext{aq}) + ext{Na}_2 ext{CO}_3 ( ext{aq}) ightarrow ext{CaCO}_3 ( ext{s}) downarrow + ext{Na}_2 ext{SO}_4 ( ext{aq})$. The softened water is then suitable for washing and other uses.

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Sodium carbonate, sodium chloride, and sodium hydroxide are three indispensable inorganic compounds of sodium, playing pivotal roles across various industrial sectors, biological systems, and daily life. Sodium chloride, commonly known as common salt, is a fundamental dietary component and a primary raw material for the synthesis of other sodium compounds. Sodium hydroxide, or caustic soda, is a p…

Quick Summary

Sodium chloride ( $ext{NaCl}$ ), commonly known as common salt, is an essential ionic compound found abundantly in seawater and rock salt deposits. It's crucial for biological functions and serves as a primary raw material for numerous industrial chemicals.

Its purification often involves the common ion effect using $ext{HCl}$ gas. Sodium hydroxide ( $ext{NaOH}$ ), or caustic soda, is a powerful, deliquescent base produced industrially by the Chlor-alkali process, involving the electrolysis of brine.

This process also yields chlorine and hydrogen gas. $ext{NaOH}$ is vital in soap, paper, and textile industries. Sodium carbonate ( $ext{Na}_2\text{CO}_3$ ), known as washing soda (decahydrate form) or soda ash (anhydrous), is a white crystalline solid that forms an alkaline solution due to hydrolysis.

It's manufactured primarily by the Solvay process, which efficiently recycles ammonia and $ext{CO}_2$ . $ext{Na}_2\text{CO}_3$ is extensively used in glass manufacturing, detergents, and water softening.

Understanding the preparation methods, key properties like deliquescence and efflorescence, and diverse applications of these compounds is fundamental for NEET aspirants.

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Key Concepts

Chlor-alkali Process Mechanism

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Solvay Process Key Reactions and Recycling

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Hydrolysis of Sodium Carbonate

Sodium carbonate dissolves in water to produce an alkaline solution. This is due to the hydrolysis of the…

Sodium Chloride ($ ext{NaCl}$): — Common salt. Raw material for

ext{NaOH}

ext{Na}_2\text{CO}_3

. Purified by

ext{HCl}

gas (common ion effect).

Sodium Hydroxide ($ ext{NaOH}$): — Caustic soda. Strong base, deliquescent. Prepared by Chlor-alkali process (electrolysis of brine).

- Anode: $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-$ - Cathode: $2\text{H}_2\text{O} + 2\text{e}^- \rightarrow \text{H}_2 + 2\text{OH}^-$ - Overall: $2\text{NaCl} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{Cl}_2 + \text{H}_2$

Sodium Carbonate ($ ext{Na}_2 ext{CO}_3$): — Washing soda (

ext{Na}_2\text{CO}_3 cdot 10\text{H}_2\text{O}

) / Soda ash (

ext{Na}_2\text{CO}_3

). Efflorescent (decahydrate). Aqueous solution is alkaline (hydrolysis of

ext{CO}_3^{2-}

- Prepared by Solvay process: $ext{NaCl}$ , $ext{CaCO}_3$ , $ext{NH}_3$ are raw materials. - Key step: $ext{NH}_4\text{HCO}_3 + \text{NaCl} \rightarrow \text{NaHCO}_3 downarrow + \text{NH}_4\text{Cl}$ - Heating: $2\text{NaHCO}_3 xrightarrow{\text{Heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$ - Recycling: $ext{NH}_3$ and $ext{CO}_2$ are recycled.