Chapter 17: Group 2 Elements — Exercise Solutions
Bunyadi Concept (Chapter Ka Overall Theme)
Group 2 elements (Beryllium, Magnesium, Calcium, Strontium, Barium) ko alkaline earth metals kehte hain. Poore chapter ka main theme ye hai ke jaise jaise hum group mein neeche jaate hain (Be→Ba), atomic size barhta hai. Isi ek tabdeeli se bohot saare trends nikalte hain: reactivity, thermal stability, solubility, waghera. Bas ek concept achi tarah samajh lo — size ka farq — aur poora chapter samajh aa jayega.
Q1 — Multiple Choice Questions
I) Sabse zyada alkaline solution konsa oxide banata hai: CaO (Calcium oxide) ✓
- Chota concept: Jaise jaise group mein neeche jaate hain, oxides zyada reactive aur zyada alkaline hote jaate hain (BeO sabse kam alkaline hai, kyunke Be chota hai).
II) Kaunsa element paani/steam se react nahi karta: Beryllium (Be) ✓
- Wajah: Be par ek mazboot protective oxide layer ban jaati hai jo aage reaction hone se rokti hai.
III) Magnesium steam ke saath react kar ke kya banata hai: MgO aur H₂ ✓
- Yaad rakho: Mg cold water ke saath Mg(OH)₂ aur H₂ banata hai, lekin steam ke saath MgO aur H₂ banata hai. Ye farq is question ka trick hai.
IV) Jab CaO paani ke saath react karta hai to kya banta hai: Metal hydroxide ✓
- Basic oxide + water → hydroxide, ye general rule hai.
V) Sabse thermally stable carbonate: CaCO₃ ✓
- Note: Yahan poochha gaya hai in specific options mein se, general trend to BaCO₃ hai (jo sabse stable hai group mein), lekin diye gaye options mein CaCO₃ correct hai.
VI) Group 2 nitrate heat karne se kya products milte hain: Metal oxide, CO₂, O₂ ✓
- (Yahan ek chhoti si baat clear karni hai: asal mein nitrates heat hone se NO₂ aur O₂ banti hai, CO₂ nahi — ye carbonates ka pattern hai. Aapko apni textbook answer key se confirm karna chahiye kyunke is PDF mein diya gaya option (a) format thora ambiguous lag raha hai.)
VII) Nitrates ki thermal stability group mein neeche jaate hue kyun barhti hai: Smaller cations, higher polarizing power ✓
- Concept: Chote cations (jaise Be²⁺) anion ko zyada distort karte hain (polarize karte hain), jisse bond kamzor ho jata hai aur compound kam stable hoti hai, kam temperature par decompose ho jaati hai.
VIII) Sabse kam soluble sulphate: BaSO₄ ✓ (Note: Options mein MgSO₄ ✓ mark tha lekin actual chemistry mein BaSO₄ sabse kam soluble hai — ye apni original book se cross-check kar lein)
IX) Sabse kam soluble kaunsa hai: BaSO₄ ✓
X) Be²⁺, Ba²⁺ se zyada aasani se complexes kyun banata hai: Higher charge density ✓
- Wajah: Be²⁺ bohot chota hota hai isliye uski charge density (charge/size ka ratio) bohot zyada hoti hai, jo isay complexes banane mein madad deti hai.
Q2 — Short Answer Questions
(a) Ca(OH)₂ vs Ba(OH)₂ — kis ka pH zyada hoga:
- Concept: Group 2 hydroxides ki solubility neeche jaate hue barhti hai (Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂)
- Isliye Ba(OH)₂ zyada ghulnasheel (soluble) hai, isliye zyada OH⁻ ions release karega
- Answer: Ba(OH)₂ ka pH zyada hoga
(b) Calcium dichromate vs strontium dichromate — kaunsa zyada soluble:
- Concept: Sulphates jaise large anions ke liye, solubility group mein neeche jaate hue kam hoti hai (kyunke hydration enthalpy, lattice enthalpy se zyada tezi se gharti hai)
- Dichromate ion bhi bara anion hai (sulphate se bhi bara), isliye same trend follow hoga
- Answer: Calcium dichromate (chota cation) zyada soluble hoga
(c) Ba ka cold water ke saath equation:
Ba(s)+2H2O(l)→Ba(OH)2(aq)+H2(g)Ba(s) + 2H_2O(l) \rightarrow Ba(OH)_2(aq) + H_2(g)
(d) Be metal protective layer kyun banata hai:
- Concept: Be²⁺ bohot chota hai aur uski charge density bohot zyada hai, isliye O²⁻ ke saath bohot mazboot bond banata hai
- Ye ek patli, dense, chipakne wali BeO layer banata hai jo metal ko surface par protect kar leti hai
- Baaki bare metals mein M²⁺-O²⁻ bond kamzor hota hai (kyunke size bara hai), isliye unki oxide layer utni protective nahi hoti
(e) Barium nitrate ka thermal decomposition:
- Products: Barium oxide, brown NO₂ gas, aur O₂ gas 2Ba(NO3)2(s)→2BaO(s)+4NO2(g)+O2(g)2Ba(NO_3)_2(s) \rightarrow 2BaO(s) + 4NO_2(g) + O_2(g)
(f) Hydroxides ki solubility ∆H_hyd aur ∆H_lattice ke terms mein kyun barhti hai:
- Group mein neeche jaate hue dono (∆H_hyd aur ∆H_lattice) kam hote hain jaise jaise cation bara hota hai
- Lekin chunke hydroxide ion chota hai, lattice enthalpy zyada tezi se girti hai hydration enthalpy ke muqablay mein
- Isliye overall dissolution zyada favourable ho jata hai, aur solubility barhti hai
(g) Dow Process mein Mg precipitate karne ka equation:
Mg2+(aq)+Ca(OH)2(s)→Mg(OH)2(s)+Ca2+(aq)Mg^{2+}(aq) + Ca(OH)_2(s) \rightarrow Mg(OH)_2(s) + Ca^{2+}(aq)
(h) BaSO₄ aur Mg(OH)₂ ka medical use:
- BaSO₄: ‘Barium meal’ mein use hota hai X-ray imaging ke liye (digestive system ka), kyunke ye highly insoluble (isliye safe) aur X-rays ke liye opaque hai
- Mg(OH)₂ (Milk of Magnesia): Stomach ke acid ko neutralize karta hai, aur mild laxative ki tarah bhi kaam karta hai
Q3 — Constructed Response Questions
(a) Beryllium ka melting point aur density baaki Group 2 elements se zyada kyun hai:
- Be ka atomic radius sabse chota hai, isliye uske atoms bohot qareeb qareeb packed hote hain aur valence electrons nucleus se strongly attract hote hain
- Isse group mein sabse mazboot metallic bonding banti hai → sabse zyada melting point (1287°C)
- Density ke liye: Be se Ca tak density kam hoti hai (radius mass se tezi se barhta hai), lekin Sr aur Ba ke liye dobara barhti hai (kyunke unka mass, volume ke increase se zyada hawi ho jata hai)
(b) Group 1 metals, Group 2 se zyada reactive kyun hote hain (same period mein):
- Group 1 metals ko sirf ek electron nikalna hota hai stable configuration ke liye, aur ye kam ionization energy se nikal jata hai
- Group 2 metals ko do electrons nikalne parte hain (first + second ionization energy dono ki total zyada energy chahiye)
- Isliye Group 1 metals zyada aasani se electrons khote hain, is wajah se zyada reactive hote hain
(c) Calcium hydroxide agriculture mein kyun use hota hai:
- Ca(OH)₂ ek base hai. Acidic soil mein daalne se ye extra acidity ko neutralize kar deta hai aur soil ka pH crops ke liye suitable bana deta hai
- Ye calcium bhi supply karta hai, jo plant cell walls banane ke liye zaroori nutrient hai
Q4 — Group 2 Elements Ki Water Aur Dilute HCl Ke Saath Reactivity
Water ke saath:
- Be paani ya steam se bilkul react nahi karta (protective oxide layer ki wajah se)
- Mg cold water ke saath bohot dheere react karta hai: Mg(OH)₂ aur H₂ banta hai. Steam ke saath tezi se react karta hai: MgO aur H₂ banta hai
- Ca, Mg se zyada vigorously react karta hai cold water ke saath, aur Ca(OH)₂ ek safed precipitate ki soorat mein nazar aata hai (kyunke ye sparingly soluble hai)
Dilute HCl ke saath:
- Sab metals general equation follow karte hain: M + 2HCl → MCl₂ + H₂
- Be dheere react karta hai (pehle oxide layer todni parti hai)
- Mg aasani se react karta hai
- Ca vigorously react karta hai
Overall trend: Reactivity group mein neeche jaate hue barhti hai (Be<Mg<Ca<Sr<Ba), kyunke ionization energy kam hoti hai aur atomic size barhta hai, jisse metal ke liye 2 valence electrons chhodna aasan ho jata hai.
Q5 — Carbonates Aur Nitrates Ka Thermal Decomposition
Carbonates: MCO₃ heat hone se MO + CO₂ banta hai
Nitrates: 2M(NO₃)₂ heat hone se 2MO + 4NO₂ + O₂ banta hai
Trend Ki Wajah — Polarizing Power Ka Concept:
- Chote, zyada charged cations (jaise Be²⁺) mein zyada polarizing power hoti hai
- Ye large anion (carbonate/nitrate) ko zyada distort karte hain, jisse anion ka internal bond kamzor ho jata hai
- Isliye ye compound kam temperature par hi decompose ho jata hai
Jaise jaise group mein cation ka size barhta hai, polarizing power kam hoti jaati hai, distortion kam hota hai, aur compound zyada thermally stable ho jata hai.
Order (barhta hua thermal stability):
BeCO3<MgCO3<CaCO3<SrCO3<BaCO3BeCO_3 < MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3
Same order nitrates ke liye bhi hai.
Q6 — Sulphates Aur Hydroxides Ki Solubility
Bunyadi concept: Solubility do energies ke balance par depend karti hai:
- Lattice enthalpy (endothermic — solid ko ions mein todne ke liye energy chahiye)
- Hydration enthalpy (exothermic — ions ko paani ke molecules gher lete hain, energy release hoti hai)
Hydroxides: Solubility group mein neeche jaate hue barhti hai (Mg(OH)₂ kam soluble, Ba(OH)₂ zyada soluble)
- Wajah: Hydroxide ion chota hai, isliye cation bara hone par lattice enthalpy tezi se girti hai hydration enthalpy ke muqablay mein
Sulphates: Solubility group mein neeche jaate hue kam hoti hai (MgSO₄ soluble, BaSO₄ tقریبا insoluble)
- Wajah: Sulphate ion bara hai, isliye cation bara hone par hydration enthalpy tezi se girti hai lattice enthalpy ke muqablay mein
Farq ki wajah: Anion ka size decide karta hai ke lattice enthalpy zyada gire ga ya hydration enthalpy — chota anion (hydroxide) → lattice enthalpy hawi; bara anion (sulphate) → hydration enthalpy hawi.
Trends:
- Hydroxide: Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂ (increasing)
- Sulphate: MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄ (decreasing)
Q7 — Dow’s Process (Magnesium Ki Extraction)
Ye samandar ke paani (seawater) se magnesium nikalne ka industrial tareeqa hai. 4 steps yaad karo:
Step 1 — Precipitation: Seawater mein Ca(OH)₂ milaya jata hai, jo Mg²⁺ ko insoluble Mg(OH)₂ ki soorat mein precipitate kar deta hai:
Mg2+(aq)+Ca(OH)2(s)→Mg(OH)2(s)+Ca2+(aq)Mg^{2+}(aq) + Ca(OH)_2(s) \rightarrow Mg(OH)_2(s) + Ca^{2+}(aq)
Step 2 — Chloride mein convert karna: Mg(OH)₂ ko filter kar ke HCl ke saath react karaya jata hai:
Mg(OH)2(s)+2HCl(aq)→MgCl2(aq)+2H2O(l)Mg(OH)_2(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)
Step 3 — Concentration aur drying: MgCl₂ solution ko concentrate kar ke solid/molten MgCl₂ banaya jata hai
Step 4 — Electrolysis: Molten MgCl₂ ko electrolyse kiya jata hai:
MgCl2(l)→Mg(l)+Cl2(g)MgCl_2(l) \rightarrow Mg(l) + Cl_2(g)
- Cathode par: pure Mg metal milta hai
- Anode par: Cl₂ gas milti hai
Ahmiyat: Ye process industrially bohot important hai kyunke ye seawater jaisi abundant lekin low-concentration source se magnesium ko commercial scale par efficiently nikalta hai.
Sabse Zaroori Concept Jo Poore Chapter Ko Jorta Hai:
Group mein neeche jaate hue (Be→Ba):
- Atomic size barhta hai → ye sab kuch decide karta hai
- Reactivity barhti hai (water/acid ke saath)
- Carbonate/nitrate ki thermal stability barhti hai (polarizing power kam hone ki wajah se)
- Hydroxide solubility barhti hai, lekin sulphate solubility ghatti hai (anion size ke farq ki wajah se)