Important compounds — NaOH, NaCl, Na₂CO₃ (Solvay), NaHCO₃; CaO, CaCO₃, gypsum, plaster of Paris

1. The two starting rocks/brines

2. Sodium hydroxide — NaOH (caustic soda)

Formula Chlor-alkali electrode reactions (derive from first principles)

Electrolysis of aqueous NaCl. We ask: at each electrode, which species is easiest to discharge?

Cathode (reduction): water is easier to reduce than Na⁺ (Na⁺ has very negative reduction potential), so: $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$
Anode (oxidation): at high Cl⁻ concentration (overpotential favours Cl₂), chloride is discharged: $2Cl^- \rightarrow Cl_2 + 2e^-$
Net: Na⁺ stays in solution with new OH⁻ → NaOH forms. $2NaCl + 2H_2O \xrightarrow{\text{electrolysis}} 2NaOH + Cl_2\uparrow + H_2\uparrow$ Why mercury cathode? It keeps Na (as amalgam) away from OH⁻ so no back-reaction; the amalgam is then treated with water to give pure NaOH.

3. Sodium carbonate — Na₂CO₃ (washing soda) by the Solvay process

Formula Solvay steps (HOW, derived as a logical chain)

Step 1 — make CO₂ and CaO by heating limestone: $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$ Step 2 — saturate brine with NH₃, then CO₂. Ammoniacal brine + CO₂ gives bicarbonate ions: $NH_3 + H_2O + CO_2 \rightarrow NH_4HCO_3$ $NH_4HCO_3 + NaCl \rightarrow NaHCO_3\downarrow + NH_4Cl$ Why does NaHCO₃ precipitate? It is the least soluble species in this cold concentrated mixture (common-ion + low solubility) → drops out, dragging the equilibrium forward. Step 3 — heat NaHCO₃ (calcination): $2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$ Step 4 — recover NH₃ using the CaO from step 1: $2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2NH_3 + 2H_2O$ Overall: $2NaCl + CaCO_3 \rightarrow Na_2CO_3 + CaCl_2$ . K cannot be made this way because KHCO₃ is too soluble to precipitate.

4. Sodium bicarbonate — NaHCO₃ (baking soda)

5. The calcium family — CaO, CaCO₃, gypsum, plaster of Paris

Flashcards

Electrolyte and overall reaction of chlor-alkali process

2NaCl + 2H_2O \to 2NaOH + Cl_2 + H_2

(electrolysis of brine)

Cathode reaction in chlor-alkali cell

2H_2O + 2e^- \to H_2 + 2OH^-

(water reduced, not Na⁺)

Anode reaction in chlor-alkali cell

2Cl^- \to Cl_2 + 2e^-

Why is NaHCO₃ precipitated in Solvay process?

It is the least soluble species in the cold ammoniacal brine, so it crystallises out and drives the equilibrium forward.

Solvay overall reaction

2NaCl + CaCO_3 \to Na_2CO_3 + CaCl_2

(with NH₃ recycled)

How is NH₃ recovered in Solvay process?

2NH_4Cl + Ca(OH)_2 \to CaCl_2 + 2NH_3 + 2H_2O

Why can't K₂CO₃ be made by Solvay?

KHCO₃ is too soluble to precipitate out.

Decomposition of sodium bicarbonate on heating

2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2

Formula and name of washing soda

Na_2CO_3 \cdot 10H_2O

(sodium carbonate decahydrate)

Why is Na₂CO₃ solution alkaline?

CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-

(hydrolysis of weak-acid anion)

Quicklime, slaked lime formulas

CaO (quicklime), Ca(OH)₂ (slaked lime)

Calcination of limestone

CaCO_3 \xrightarrow{\Delta} CaO + CO_2

Why does lime mortar harden in air?

Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O

(re-forms stone)

Formula of gypsum and Plaster of Paris

Gypsum

CaSO_4\cdot2H_2O

; PoP

CaSO_4\cdot\tfrac12H_2O

Temperature to make PoP from gypsum

~393 K (120 °C); above 473 K gives dead-burnt anhydrous CaSO₄ that won't set.

Why does PoP set hard?

It re-absorbs water to reform gypsum

CaSO_4\cdot2H_2O

, an interlocking crystal mass.

Recall Feynman: explain to a 12-year-old

Imagine salty water and white rocks are nature's free LEGO bricks. Run electricity through salty water and out pop three useful things: a strong soapy cleaner (NaOH), a green gas (Cl₂) and a balloon gas (H₂). Now bake the white rock and it puffs out a gas (CO₂) leaving quicklime (CaO) — add water and it hisses and gets hot (slaked lime), and over time it eats the air's CO₂ and turns back into stone — that's how old walls harden. There's another white rock, gypsum: bake it gently and it loses most of its hidden water, becoming a powder. Mix that powder with water and it grabs the water back and turns rock-hard — perfect for casts on broken arms. Bake it too hard, though, and it sulks and won't set anymore.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, is poore topic ka asli funda yeh hai ki industry do sasti cheezon se sab kuch banati hai: NaCl (namak ka paani / brine) aur CaCO₃ (limestone / chuna patthar). Inhi do se baaki saare compounds nikalte hain — sirf paani, CO₂, garmi ya electricity add/remove karke.

NaOH banta hai brine ki electrolysis se (chlor-alkali process). Yaad rakho cathode pe Na⁺ reduce nahi hota, balki paani reduce hota hai (H₂ + OH⁻), aur anode pe Cl⁻ se Cl₂ nikalta hai — to ek saath teen products: NaOH, Cl₂, H₂. Solvay process mein NH₃ ko ek "recyclable carrier" ki tarah use karte hain: ammoniacal brine mein CO₂ daalo, NaHCO₃ precipitate ho jaata hai kyunki wahi sabse kam soluble hai — yahi trick equilibrium ko aage dhakelti hai. Phir NaHCO₃ ko garam karke Na₂CO₃ (soda ash) mil jaata hai. Important: K₂CO₃ Solvay se nahi ban sakta kyunki KHCO₃ bahut zyada soluble hai.

Calcium side pe limestone ko garam karo to CaO (quicklime) + CO₂; CaO mein paani daalo to garmi ke saath Ca(OH)₂ banta hai, aur yeh dheere-dheere hawa ki CO₂ kha kar wapas patthar (CaCO₃) ban jaata hai — isi liye purani deewar ka mortar hard hota hai. Gypsum (CaSO₄·2H₂O) ko halke se 393 K pe garam karo to Plaster of Paris (CaSO₄·½H₂O) banta hai. Paani milao to wapas gypsum ban kar set ho jaata hai — isliye casts aur murti banti hain. Magar zyada garam (473 K se upar) kiya to "dead burnt" CaSO₄ ban jaata hai jo set hi nahi hota. Exam mein bas transformation aur "why" yaad rakho, ratta marne ki zaroorat nahi.