s-Block Elements — Important Compounds of Na and Ca

• Describe the Castner-Kellner process for NaOH production with electrode reactions and the role of mercury amalgam
• Write all steps of the Solvay process for Na₂CO₃ with balanced equations and explain recovery of NH₃
• Distinguish NaHCO₃ from Na₂CO₃ in properties, uses, and thermal stability
• Explain the chemistry of Plaster of Paris — preparation from gypsum, setting reaction, and uses
• Describe the composition and setting of Portland cement (Ca₃SiO₅, Ca₂SiO₄, Ca₃Al₂O₆)
• Perform stoichiometric calculations on Solvay process yield

Level 1 — Foundations

1. Sodium Hydroxide (NaOH) — Castner-Kellner Process

Industrial production: electrolysis of brine (concentrated NaCl solution) using a mercury cathode cell.

Cell setup:

• Anode: graphite (or titanium) → Cl₂ evolved
• Cathode: mercury (Hg) → Na-Hg amalgam formed (not H₂, despite thermodynamics, because H₂ overpotential on Hg is very high)
• Amalgam flows to a separate "denuder" cell containing water:
  • Na-Hg amalgam + H₂O → NaOH + ½H₂ + Hg (recycled)

Electrode reactions:

At anode (graphite): $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^- \quad \text{(oxidation)}$

At cathode (mercury): $\text{Na}^+ + e^- + \text{Hg} \rightarrow \text{Na-Hg amalgam} \quad \text{(reduction)}$

In denuder (decomposer): $2\text{Na-Hg} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2 \uparrow + 2\text{Hg (recycled)}$

Overall: $2\text{NaCl} + 2\text{H}_2\text{O} \xrightarrow{\text{electrolysis}} 2\text{NaOH} + \text{Cl}_2 + \text{H}_2$

Byproducts: Cl₂ (used for PVC, disinfection) and H₂ (fuel, hydrogenation)

Note: Mercury cell is being phased out due to Hg pollution → replaced by membrane cell (Nafion membrane).

2. Sodium Carbonate (Na₂CO₃) — Solvay Process

Starting materials: NaCl (brine), CaCO₃ (limestone), NH₃ (recycled)

Step-by-step:

Step 1 — Saturate brine with NH₃: $\text{NaCl(aq)} + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl·NaCl (ammoniated brine)}$

Step 2 — Pass CO₂ into ammoniated brine (Solvay tower): $\text{NaCl} + \text{NH}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \underbrace{\text{NaHCO}_3}_{\text{precipitates}} + \text{NH}_4\text{Cl}$

NaHCO₃ precipitates because it is the least soluble of the four possible products.

Step 3 — Filter and heat NaHCO₃: $2\text{NaHCO}_3 \xrightarrow{~300°\text{C}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \uparrow$

CO₂ is recycled back to Step 2.

Step 4 — Recover NH₃ (heat NH₄Cl with Ca(OH)₂): $2\text{NH}_4\text{Cl} + \text{Ca(OH)}_2 \xrightarrow{\Delta} \text{CaCl}_2 + 2\text{NH}_3 \uparrow + 2\text{H}_2\text{O}$

NH₃ is recycled to Step 1. CaCl₂ is the only by-product (limited commercial use).

Step 5 — CaCO₃ is calcined (kiln) to supply CO₂ and CaO: $\text{CaCO}_3 \xrightarrow{900°\text{C}} \text{CaO} + \text{CO}_2 \uparrow$

CaO + H₂O → Ca(OH)₂ (used in Step 4)

Why Solvay cannot make K₂CO₃: KHCO₃ is too soluble to precipitate from brine → cannot be filtered in Step 2.

3. Sodium Hydrogen Carbonate (NaHCO₃) — Baking Soda

NaHCO₃ as antacid: neutralises excess HCl in stomach: $\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \uparrow$

NaHCO₃ is preferred over Na₂CO₃ as antacid — Na₂CO₃ is too strongly alkaline and would damage stomach lining.

4. Plaster of Paris (PoP) — CaSO₄·½H₂O

Source: Gypsum (CaSO₄·2H₂O)

Preparation: Heat gypsum at 120–130°C (controlled — not above 150°C): $\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{120°\text{C}} \text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O}$

Setting reaction (re-absorption of water): $\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O (hard mass)}$

Setting expands slightly (unlike most materials that contract on setting) — this ensures sharp impression in moulds.

Uses: Medical casts (bone fractures), dental moulds, decorative plaster, fire-resistant boards.

Note: If gypsum is heated above 150°C, dead burnt plaster (CaSO₄, anhydrous) forms — this does NOT set with water and is useless for plasterwork.

5. Cement — Composition and Setting

Portland cement is a complex mixture of calcium silicates and aluminates:

Setting of cement — complex hydration reactions: $\text{Ca}_3\text{SiO}_5 + \text{H}_2\text{O} \rightarrow \text{Ca}_2\text{SiO}_4 + \text{Ca(OH)}_2 + \text{heat}$ $\text{Ca}_2\text{SiO}_4 + \text{H}_2\text{O} \rightarrow \text{C-S-H gel (calcium silicate hydrate)} + \text{Ca(OH)}_2$

C-S-H gel (tobermorite gel) is the main binder — responsible for concrete strength.

Gypsum (3–5%) is added to cement to retard C₃A setting and prevent flash set.

Level 2 — JEE Depth

Why NaHCO₃ Precipitates in Solvay Process (not NaCl or NH₄Cl)

The four ions in solution: Na⁺, NH₄⁺, HCO₃⁻, Cl⁻

Possible salts and solubilities:

NaHCO₃ precipitates first due to common ion effect: Na⁺ from NaCl + HCO₃⁻ from CO₂ absorption → Qsp > Ksp for NaHCO₃ → precipitation.

Membrane Cell vs Mercury Cell (Castner-Kellner)

Thermal Decomposition Temperatures for JEE

Stoichiometry of Solvay Process

The overall (idealised) Solvay reaction: $2\text{NaCl} + \text{CaCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CaCl}_2$

(This is the net reaction — NH₃ and CO₂ are both recovered and recycled, only NaCl and CaCO₃ are consumed.)

Example 1: Write all steps of the Solvay process with balanced equations. Explain why NH₃ is recycled and CaCl₂ is an unavoidable by-product.

Solvay Process — Step by Step:

Step 1: Saturate NaCl solution with NH₃
  NaCl(aq) + NH₃(g) → ammoniated brine

Step 2: Pass CO₂ through ammoniated brine (Solvay tower, ~15°C)
  NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl
  (NaHCO₃ precipitates — least soluble of possible products)

Step 3: Filter and heat NaHCO₃ at ~300°C
  2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑
  (CO₂ recycled to Step 2)

Step 4: Recover NH₃ by treating NH₄Cl with Ca(OH)₂
  2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃↑ + 2H₂O
  (NH₃ recycled to Step 1)

Step 5: Prepare Ca(OH)₂ from CaCO₃
  CaCO₃ → CaO + CO₂ (900°C)
  CaO + H₂O → Ca(OH)₂

Why NH₃ is recycled:
  NH₃ is expensive; recovering it makes the process economically viable.
  Without recycling, NH₃ would be lost as NH₄Cl (which has limited market value).

Why CaCl₂ is unavoidable:
  Ca(OH)₂ must be used to decompose NH₄Cl and release NH₃.
  The Ca²⁺ must pair with the Cl⁻ from NH₄Cl → CaCl₂ is the only by-product.
  CaCl₂ has limited uses (de-icing, drying agent) — this is a drawback of Solvay process.

Net equation:
  2NaCl + CaCO₃ → Na₂CO₃ + CaCl₂

Example 2: Calculate the theoretical mass of Na₂CO₃ that can be obtained from 117 g of NaCl in the Solvay process. What is the % atom efficiency with respect to Na?

Net Solvay equation:
  2NaCl + CaCO₃ → Na₂CO₃ + CaCl₂

Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol
Moles of NaCl = 117 / 58.5 = 2.0 mol

From stoichiometry: 2 mol NaCl → 1 mol Na₂CO₃
Moles of Na₂CO₃ = 2.0 / 2 = 1.0 mol

Molar mass of Na₂CO₃ = 2(23) + 12 + 3(16) = 106 g/mol
Mass of Na₂CO₃ = 1.0 × 106 = 106 g

% Atom efficiency for Na:
  Na in starting material: 2 mol NaCl → 2 mol Na → 2 × 23 = 46 g Na
  Na in product: 1 mol Na₂CO₃ → 2 mol Na → 2 × 23 = 46 g Na
  % efficiency = 46/46 × 100 = 100%

All Na from NaCl ends up in Na₂CO₃ — the process is 100% efficient with respect to Na.
The "waste" atoms are Cl (goes to CaCl₂) and Ca (from CaCO₃, leaves as CaCl₂).

Answer: 106 g Na₂CO₃ from 117 g NaCl; 100% Na atom efficiency.

• Q1: Write the equation for the formation of NaHCO₃ in the Solvay process (Step 2).
• Q2: Why is gypsum added to Portland cement? What happens without it?
• Q3: Write the decomposition reaction of NaHCO₃ on heating. What products are formed?
• Q4: Calculate the moles of H₂ produced when 46 g of Na reacts with water in the Castner-Kellner process.
• Stretch: Q5: The Solvay process cannot be used to make K₂CO₃ even though KCl is abundant. Explain why using solubility arguments (KHCO₃ vs NaHCO₃ solubility at low temperature). Then explain why the Solvay process produces CaCl₂ as an unavoidable by-product, and suggest one use for this by-product that makes it economically attractive.

NCERT Chapter 10 link: The s-Block Elements — Section 10.3 (Sodium Hydroxide), Section 10.4 (Sodium Carbonate), Section 10.5 (Sodium Bicarbonate), Section 10.8 (Calcium Compounds — PoP and Cement)

Exam connections: JEE Mains: Solvay process steps and equations (appears nearly every year), PoP preparation and setting. JEE Advanced: stoichiometric yield calculations, thermodynamics of Solvay precipitation. NEET: PoP vs gypsum, NaOH manufacturing, NaHCO₃ uses. Board: all equations must be balanced; Castner-Kellner electrode reactions.

Study strategy: Solvay process — memorise as a cycle diagram with 5 labelled arrows showing where CO₂ goes (recycled), where NH₃ goes (recycled), what enters (NaCl, CaCO₃), what exits (Na₂CO₃, CaCl₂). For PoP, focus on the temperature window (120–130°C = PoP; >150°C = dead burnt). For cement, 3 components suffice: C₃S, C₂S, C₃A.

• Solvay Process Flow Simulator: Interactive flowchart — students trace NaCl and CaCO₃ through 5 steps; adjust temperature at each step and see product formed; mass balance tracker shows CO₂ and NH₃ recycling in real time.
• Plaster of Paris Setting Lab: Mix virtual PoP + water; observe exothermic setting; compare with dead burnt plaster (no setting); adjust heating temperature of gypsum and see which product forms; link to medical cast applications.
• Cement Hydration World: 3D animation of C₃S hydration producing C-S-H gel and Ca(OH)₂; compare early vs late strength (C₃S vs C₂S); observe role of gypsum in retarding C₃A setting; connect to building construction timelines.

• "In the Solvay process, CO₂ is both produced (from CaCO₃ calcination) and consumed (in the Solvay tower). If CO₂ is recycled, what is the NET input and output of the whole process? Can you write the overall net equation from this analysis?"
• "NaHCO₃ is called 'baking soda' because it releases CO₂ when heated or exposed to acid. In actual baking, both happen — the dough heats up AND the acid from buttermilk or yogurt reacts. Which mechanism releases CO₂ faster and earlier in the baking process?"
• "Plaster of Paris is CaSO₄·½H₂O and gypsum is CaSO₄·2H₂O. The setting of PoP is just the reverse of its preparation — PoP + water → gypsum. So why doesn't gypsum spontaneously turn into PoP in a moist environment? What thermodynamic principle keeps gypsum stable?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• Green cement chemistry: Traditional Portland cement production releases ~0.8 kg CO₂ per kg cement (8% of global CO₂ emissions); startups like CarbonCure and Novacem are developing low-CO₂ cement by using supplementary cementitious materials (slag, fly ash) or carbonated concrete — understanding C₃S/C₂S chemistry is the foundation for this $700B industry transformation.
• NaOH in electrochemical engineering: The chlor-alkali industry (Castner-Kellner/membrane cell) is one of the largest electrochemical industries globally; electrical engineers design electrolysis cells, and chemical engineers optimise membrane selectivity for Cl⁻/OH⁻ — a direct application of electrochemistry + s-block chemistry.
• Solvay process as circular chemistry: The Solvay process was the world's first example of a circular industrial process — NH₃ and CO₂ recycled internally; this "closed-loop" thinking is now the basis of circular economy principles in green chemistry and sustainable manufacturing design.

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.