Occurrence and Extraction — Revision Notes

⚡ 30-Second Revision

Occurrence — Never free, always combined (salts, silicates).
Key Minerals

- Li: Spodumene ($LiAl(SiO_3)_2$) - Na: Rock Salt (NaCl), Seawater, Borax ($Na_2B_4O_7 cdot 10H_2O$) - K: Sylvite (KCl), Carnallite ($KCl cdot MgCl_2 cdot 6H_2O$)

Extraction Principle — Electrometallurgy (electrolysis of molten salts).
Downs Process (Na)

- Electrolyte: Molten $NaCl + CaCl_2$ (lowers m.p. to $580^circ C$). - Cathode: $Na^+(l) + e^- \rightarrow Na(l)$ - Anode: $2Cl^-(l) \rightarrow Cl_2(g) + 2e^-$ - Diaphragm: Separates Na and $Cl_2$.

Li Extraction — Electrolysis of molten $LiCl/KCl$.
K Extraction — Chemical reduction of molten KCl with Na vapor ($Na(g) + KCl(l) \rightleftharpoons NaCl(l) + K(g)$) due to K's volatility.
Rb/Cs Extraction — Thermal decomposition of azides ($2MN_3 \rightarrow 2M + 3N_2$) or reduction with Ca/Mg.
Why no aqueous electrolysis — Water is preferentially reduced ($E^circ_{H_2O/H_2} > E^circ_{M^+/M}$).

2-Minute Revision

Alkali metals, being Group 1 elements, are highly reactive due to their single valence electron and low ionization enthalpy. This inherent reactivity means they are never found in their free elemental state in nature; instead, they always occur as stable ionic compounds within various minerals.

For instance, sodium is abundant as rock salt (NaCl) and in seawater, while potassium is found in minerals like sylvite (KCl) and carnallite ($KCl cdot MgCl_2 cdot 6H_2O$). Lithium is extracted from minerals such as spodumene ($LiAl(SiO_3)_2$).

The extraction of these highly electropositive metals requires powerful reducing agents, which chemical methods typically cannot provide. Therefore, the primary industrial method is electrometallurgy, specifically the electrolysis of their molten salts.

For sodium, the 'Downs process' is used, involving the electrolysis of a molten mixture of NaCl and $CaCl_2$. The $CaCl_2$ is crucial as it lowers the melting point of the electrolyte, making the process more energy-efficient.

Sodium metal is produced at the cathode, and chlorine gas at the anode. It's vital to remember that aqueous solutions cannot be used for electrolysis because water would be preferentially reduced over the alkali metal ions.

For potassium, an alternative method involving chemical reduction of molten KCl with sodium vapor is often employed, leveraging potassium's higher volatility for separation. Rubidium and Caesium are typically obtained via thermal decomposition of their azides.

5-Minute Revision

The occurrence and extraction of alkali metals are directly linked to their unique chemical properties. These Group 1 elements (Li, Na, K, Rb, Cs) are characterized by a single valence electron, large atomic radii, and exceptionally low ionization enthalpies.

These factors make them highly electropositive and extremely reactive, causing them to readily lose their valence electron to form stable $M^+$ ions. Consequently, alkali metals are never found in their elemental (free) state in nature; they always exist in combined forms within minerals or dissolved in water.

Key mineral sources include: Spodumene for Lithium; Rock Salt (Halite) and Seawater for Sodium; and Sylvite, Carnallite, and Feldspar for Potassium. Rubidium and Caesium are less abundant, often found as trace elements in other alkali metal minerals.

Due to their high reactivity and strong reducing power, conventional chemical reduction methods (like heating with carbon) are ineffective for extracting alkali metals. The standard method is electrometallurgy, specifically the electrolysis of their molten salts. Using molten salts is critical because in aqueous solutions, water would be preferentially reduced at the cathode over the alkali metal ions (e.g., $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$), preventing metal formation.

The Downs Process is the industrial cornerstone for sodium extraction. It involves the electrolysis of a molten mixture of approximately 40% NaCl and 60% $CaCl_2$. The addition of $CaCl_2$ is vital as it lowers the melting point of the electrolyte from $801^circ C$ (pure NaCl) to around $580^circ C$, significantly reducing energy consumption and minimizing sodium volatilization.

In the Downs cell, $Na^+$ ions are reduced to liquid sodium metal at the cathode ($Na^+(l) + e^- \rightarrow Na(l)$), while $Cl^-$ ions are oxidized to chlorine gas at the anode ($2Cl^-(l) \rightarrow Cl_2(g) + 2e^-$).

A steel gauze diaphragm separates the products to prevent their highly reactive recombination.

Lithium is similarly extracted by the electrolysis of a molten $LiCl/KCl$ mixture. For potassium, while molten KCl electrolysis is possible, an alternative and often preferred method for high purity involves the chemical reduction of molten KCl with sodium vapor at $850^circ C$.

This equilibrium reaction ($Na(g) + KCl(l) \rightleftharpoons NaCl(l) + K(g)$) is driven to the right by continuously distilling off the more volatile potassium vapor. Rubidium and Caesium, being even more volatile, are typically extracted by the thermal decomposition of their azides (e.

g., $2CsN_3(s) xrightarrow{Delta} 2Cs(l) + 3N_2(g)$) or by reduction of their halides with active metals like calcium or magnesium. Understanding these specific methods and the underlying reasons (reactivity, volatility, reduction potentials) is key for NEET preparation.

Prelims Revision Notes

Occurrence and Extraction of Alkali Metals (NEET Revision)

1. Occurrence:

Reactivity: — Alkali metals (Li, Na, K, Rb, Cs) are highly reactive due to:

* Single valence electron ($ns^1$). * Large atomic size. * Very low ionization enthalpy. * High electropositivity.

State in Nature: — Never found in free (native) state. Always occur in combined forms as ionic compounds (salts, silicates, aluminosilicates).
Key Minerals:

* Lithium (Li): Spodumene ($LiAl(SiO_3)_2$), Lepidolite. * Sodium (Na): Rock Salt (Halite, NaCl), Seawater (2.7% NaCl), Chile Saltpetre ($NaNO_3$), Borax ($Na_2B_4O_7 cdot 10H_2O$). * Potassium (K): Sylvite (KCl), Carnallite ($KCl cdot MgCl_2 cdot 6H_2O$), Feldspar ($KAlSi_3O_8$). * Rubidium (Rb) & Caesium (Cs): Less abundant, found as minor constituents in Li and K minerals (e.g., Pollucite for Cs).

2. Extraction Methods:

General Principle: — Electrometallurgy (electrolysis of molten salts) is the primary method due to high reactivity and very negative standard reduction potentials.
Why not aqueous electrolysis? — Water is preferentially reduced at the cathode over alkali metal ions ($E^circ_{H_2O/H_2} = -0.83,\text{V}$ vs. $E^circ_{Na^+/Na} = -2.71,\text{V}$). Products would be $H_2$ and $OH^-$, not the metal.
Why not carbon reduction? — Alkali metals are stronger reducing agents than carbon; reduction with carbon is thermodynamically unfavorable.

A. Extraction of Sodium (Na) - Downs Process:

* Raw Material: Fused NaCl. * Electrolyte: Molten mixture of $NaCl$ (40%) and $CaCl_2$ (60%). * **Role of $CaCl_2$:** Lowers melting point from $801^circ C$ (pure NaCl) to $580^circ C$, reducing energy cost and Na volatilization.

* Cell: Downs cell with central graphite anode and annular iron cathode. * Reactions: * Cathode (Reduction): $Na^+(l) + e^- \rightarrow Na(l)$ (Liquid sodium collected). * Anode (Oxidation): $2Cl^-(l) \rightarrow Cl_2(g) + 2e^-$ (Chlorine gas collected).

* Diaphragm: Steel gauze diaphragm separates Na and $Cl_2$ to prevent recombination.

B. Extraction of Lithium (Li):

* Electrolysis of molten $LiCl/KCl$ mixture (KCl lowers m.p. of LiCl). * Reactions similar to Downs process: $Li^+(l) + e^- \rightarrow Li(l)$ at cathode, $Cl^-(l) \rightarrow Cl_2(g)$ at anode.

C. Extraction of Potassium (K):

* Electrolysis of molten KCl is possible but less efficient due to K's high volatility. * Preferred Method: Chemical reduction of molten KCl with sodium vapor at $850^circ C$. * $Na(g) + KCl(l) \rightleftharpoons NaCl(l) + K(g)$ * Potassium is more volatile than sodium; continuous distillation of K vapor shifts equilibrium to the right.

D. Extraction of Rubidium (Rb) & Caesium (Cs):

* Thermal Decomposition of Azides: E.g., $2CsN_3(s) xrightarrow{Delta} 2Cs(l) + 3N_2(g)$. * Reduction with Active Metals: E.g., $2RbCl(l) + Ca(s) xrightarrow{Delta} 2Rb(g) + CaCl_2(l)$ (volatile Rb distilled off).

Key Takeaways: High reactivity dictates combined occurrence. Electrometallurgy is key. Downs process specifics are crucial. Understand 'why' certain methods are used.

Vyyuha Quick Recall

Little Nice Kids Really Care: Li from Spodumene, Na from Rock Salt (Downs), K from Carnallite (Na vapor reduction), Rb & Cs from Azides (thermal decomp).