Occurrence and Extraction — Revision Notes

⚡ 30-Second Revision

Occurrence: — Never free. Always as compounds (carbonates, sulfates, silicates, halides).

- Be: Beryl ($Be_3Al_2Si_6O_{18}$) - Mg: Magnesite ($MgCO_3$), Dolomite ($CaCO_3 cdot MgCO_3$), Carnallite ($KCl cdot MgCl_2 cdot 6H_2O$), Seawater. - Ca: Limestone/Marble/Chalk ($CaCO_3$), Gypsum ($CaSO_4 cdot 2H_2O$). - Sr: Strontianite ($SrCO_3$), Celestite ($SrSO_4$). - Ba: Barytes ($BaSO_4$), Witherite ($BaCO_3$). - Ra: Pitchblende (Uranium ore).

Extraction Principle: — Primarily electrolysis of molten anhydrous chlorides.

- $MCl_2(l) xrightarrow{\text{electrolysis}} M(l) + Cl_2(g)$

Why not aqueous? — Water reduces preferentially ($E^circ_{H_2O/H_2} > E^circ_{M^{2+}/M}$).
Why not carbon reduction? — Metals are more reactive than carbon or form stable carbides.
Magnesium (Dow's Process): — Seawater $xrightarrow{Ca(OH)_2}$ $Mg(OH)_2 xrightarrow{HCl}$ $MgCl_2 xrightarrow{\text{dehydration}}$ Anhydrous $MgCl_2(l) xrightarrow{\text{electrolysis}}$ $Mg(l)$.
Magnesium (Pidgeon Process): — $2MgO(s) + Si(s) xrightarrow{\text{high temp, vacuum}} 2Mg(g) + SiO_2(s)$.

2-Minute Revision

Alkaline earth metals (Group 2) are highly reactive, so they are always found in nature as stable compounds like carbonates, sulfates, and silicates. Key ores to remember include Beryl for Beryllium, Magnesite, Dolomite, and Carnallite for Magnesium, Limestone and Gypsum for Calcium, Celestite for Strontium, and Barytes for Barium. Radium is uniquely found in uranium ores like Pitchblende.

The primary method for extracting these electropositive metals is the electrolysis of their molten anhydrous chlorides. This is because their high reactivity makes them difficult to reduce chemically, and electrolysis of aqueous solutions would lead to the reduction of water instead of the metal ions.

For example, Calcium is obtained by electrolyzing molten $CaCl_2$. A significant industrial process is Dow's process for Magnesium, which starts from seawater, precipitates $Mg(OH)_2$, converts it to $MgCl_2$, dehydrates it, and then electrolyzes the molten anhydrous $MgCl_2$.

Another method for Magnesium is the Pidgeon process, a thermal reduction of $MgO$ with ferrosilicon under vacuum. Understanding these specific ores and extraction principles is crucial for NEET.

5-Minute Revision

Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) are characterized by their high reactivity, stemming from their $ns^2$ valence electron configuration. This means they readily lose two electrons to form $M^{2+}$ ions, making them highly electropositive. Consequently, they are never found in their elemental state in nature but always as stable compounds in minerals and ores.

Key Ores:

Beryllium: — Beryl ($Be_3Al_2Si_6O_{18}$)
Magnesium: — Magnesite ($MgCO_3$), Dolomite ($CaCO_3 cdot MgCO_3$), Carnallite ($KCl cdot MgCl_2 cdot 6H_2O$), and seawater.
Calcium: — Limestone, Marble, Chalk (all $CaCO_3$), Gypsum ($CaSO_4 cdot 2H_2O$).
Strontium: — Strontianite ($SrCO_3$), Celestite ($SrSO_4$).
Barium: — Barytes ($BaSO_4$), Witherite ($BaCO_3$).
Radium: — Pitchblende (uranium ore, as a decay product).

Extraction Principles:

1
Electrolysis of Molten Anhydrous Chlorides: — This is the most common and effective method. For example, for Calcium:

$CaCl_2(l) xrightarrow{\text{electrolysis}} Ca(l) + Cl_2(g)$ * Why molten? To allow ionic conduction and prevent water electrolysis. * Why not aqueous? $M^{2+}$ ions are harder to reduce than water ($H_2O + 2e^- \rightarrow H_2 + 2OH^-$). * Why not carbon reduction? Alkaline earth metals are stronger reducing agents than carbon at typical temperatures, or they form stable carbides.

1
Thermal Reduction: — Used for some metals like Magnesium, Strontium, and Barium.

* Pidgeon Process (for Mg): $MgO$ is reduced by ferrosilicon ($Si$) at high temperatures under vacuum: $2MgO(s) + Si(s) xrightarrow{\text{high temp, vacuum}} 2Mg(g) + SiO_2(s)$

Dow's Process (for Magnesium from Seawater):

1
Precipitation: $Mg^{2+}(aq) + Ca(OH)_2(s) \rightarrow Mg(OH)_2(s) + Ca^{2+}(aq)$
2
Conversion: $Mg(OH)_2(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)$
3
Dehydration: Evaporation and heating of $MgCl_2$ (often in $HCl$ stream to prevent hydrolysis).
4
Electrolysis: Molten anhydrous $MgCl_2(l) xrightarrow{\text{electrolysis}} Mg(l) + Cl_2(g)$

Remember the specific ores and the underlying reasons for choosing particular extraction methods. Focus on Dow's process steps and the general principle of electrolysis for highly reactive metals.

Prelims Revision Notes

Occurrence and Extraction of Alkaline Earth Metals (Group 2)

1. General Occurrence:

Highly reactive elements (Be, Mg, Ca, Sr, Ba, Ra) due to $ns^2$ configuration.
Always found in combined forms, never free metals.
Common forms: carbonates, sulfates, silicates, halides.

2. Specific Ores/Sources:

Beryllium (Be): — Beryl ($Be_3Al_2Si_6O_{18}$)
Magnesium (Mg):

* Magnesite ($MgCO_3$) * Dolomite ($CaCO_3 cdot MgCO_3$) * Carnallite ($KCl cdot MgCl_2 cdot 6H_2O$) * Seawater (major source of $Mg^{2+}$ ions)

Calcium (Ca):

* Limestone, Marble, Chalk (all $CaCO_3$) * Gypsum ($CaSO_4 cdot 2H_2O$) * Fluorapatite ($3Ca_3(PO_4)_2 cdot CaF_2$)

Strontium (Sr):

* Strontianite ($SrCO_3$) * Celestite ($SrSO_4$)

Barium (Ba):

* Barytes ($BaSO_4$) * Witherite ($BaCO_3$)

Radium (Ra): — Pitchblende (Uranium ore, as a decay product)

3. General Extraction Principles:

Primary Method: — Electrolysis of molten anhydrous chlorides ($MCl_2$).

* Cathode: $M^{2+} + 2e^- \rightarrow M(l)$ * Anode: $2Cl^- \rightarrow Cl_2(g) + 2e^-$

Why molten? — To ensure ionic conduction and prevent water electrolysis.
Why not aqueous electrolysis? — Water has a higher reduction potential than $M^{2+}$ ions; $H_2$ gas would be produced instead of metal.
Why not carbon reduction? — Alkaline earth metals are more electropositive than carbon, or form stable carbides, making carbon an ineffective reducing agent.
Thermal Reduction: — Used for some elements (e.g., Mg, Sr, Ba) with stronger reducing agents at high temperatures under vacuum.

4. Key Processes:

Beryllium: — Electrolysis of molten $BeCl_2$ (with $NaCl$) or reduction of $BeF_2$ with $Mg$.
Magnesium (Dow's Process from Seawater):

1. $Mg^{2+}(aq) + Ca(OH)_2(s) \rightarrow Mg(OH)_2(s)$ (Precipitation) 2. $Mg(OH)_2(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)$ (Conversion) 3. Dehydration of $MgCl_2$ (to anhydrous form, often in $HCl$ stream). 4. Electrolysis of molten anhydrous $MgCl_2$.

Magnesium (Pidgeon Process): — Thermal reduction of $MgO$ (from dolomite/magnesite) with ferrosilicon ($Si$) at high temp/vacuum:

$2MgO(s) + Si(s) xrightarrow{\text{high temp, vacuum}} 2Mg(g) + SiO_2(s)$

Calcium, Strontium, Barium: — Electrolysis of their molten anhydrous chlorides ($CaCl_2$, $SrCl_2$, $BaCl_2$). Fluxes like $CaF_2$ are added to lower melting points.

5. Refining: Distillation (for Mg, Be) or electrolytic refining.

Vyyuha Quick Recall

Be My Cute Sister But Remember:

Beryl (Be) Magnesite, Dolomite, Carnallite (Mg) Limestone, Gypsum (Ca) Strontianite, Celestite (Sr) Barytes, Witherite (Ba) Pitchblende (Ra)

Extraction is Electrolysis of Molten Chlorides (EMC) - Every Metal Can be extracted this way!