Modern Chemical Manufactures

Haber's process. Ammonia is made by the direct combination of nitrogen and hydrogen in a 1:3 volume ratio:

$\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}\qquad \Delta H = -92\ \text{kJ mol}^{-1}$

The reaction is exothermic and reduces the number of moles of gas. Using Le Chatelier's principle, the optimum conditions are:

• Pressure: high, about 200 atm (favours the forward reaction).
• Temperature: about 450-500 °C. Lower temperature favours more NH$_3$ but is too slow, so a compromise is used.
• Catalyst: finely divided iron with molybdenum as a promoter.

The gas mixture leaving the converter is cooled so NH$_3$ liquefies and is removed; unreacted N$_2$ and H$_2$ are recycled.

Reactions of ammonia

(i) With copper sulphate solution: a little ammonia first gives a pale blue precipitate of copper(II) hydroxide, which dissolves in excess ammonia to give a deep blue solution of tetraamminecopper(II) sulphate:

$\mathrm{CuSO_4 + 2NH_4OH \rightarrow Cu(OH)_2\downarrow + (NH_4)_2SO_4}$ $\mathrm{Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow [Cu(NH_3)_4]SO_4 + 4H_2O}$

(ii) Over heated copper oxide: ammonia reduces black CuO to reddish-brown copper and is itself oxidised to nitrogen:

$\mathrm{3CuO + 2NH_3 \xrightarrow{\Delta} 3Cu + N_2 + 3H_2O}$

Ammonia to nitric acid (Ostwald process)

1. Catalytic oxidation of NH$_3$ with air over platinum gauze at about 800 °C: $\mathrm{4NH_3 + 5O_2 \xrightarrow{Pt,\ 800^\circ C} 4NO + 6H_2O}$
2. NO is oxidised to nitrogen dioxide on cooling: $\mathrm{2NO + O_2 \rightarrow 2NO_2}$
3. NO$_2$ is absorbed in water in the presence of air: $\mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}$

Haber's process: Ammonia is manufactured by the direct combination of nitrogen and hydrogen:

$\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3 + 92\ kJ}$

The reaction is reversible, exothermic and takes place with a decrease in volume (4 volumes to 2 volumes).

Optimum conditions (Le Chatelier's principle):

• Pressure: about 200 atm (high pressure favours the forward reaction)
• Temperature: about $450$-$500^\circ\mathrm{C}$ (a compromise between yield and rate)
• Catalyst: finely divided iron
• Promoter: $\mathrm{Mo}$ or $\mathrm{Al_2O_3}$/$\mathrm{K_2O}$, which increases the activity of the catalyst

Process: A mixture of $\mathrm{N_2}$ and $\mathrm{H_2}$ in the ratio 1 : 3 is purified, compressed, and passed over the catalyst in a converter. The gases leaving the converter are cooled, so ammonia liquefies and is separated. Unreacted $\mathrm{N_2}$ and $\mathrm{H_2}$ are recycled.

With Nessler's reagent: Ammonia (or ammonium ion) gives a brown precipitate (or brown colouration at low concentration) of the iodide of Millon's base, a test for ammonia:

$\mathrm{2K_2HgI_4 + NH_3 + 3KOH \rightarrow Hg_2NI\cdot H_2O\downarrow + 7KI + 2H_2O}$

Manufacture of H₂SO₄ by the contact process

Principle: sulphur dioxide is oxidised to sulphur trioxide over a solid catalyst (V₂O₅), and the SO₃ is then converted to sulphuric acid.

1. Raw SO₂: burn sulphur (or roast iron pyrites) in excess air. $\mathrm{S + O_2 \rightarrow SO_2} \qquad \mathrm{4FeS_2 + 11O_2 \rightarrow 2Fe_2O_3 + 8SO_2}$
2. Purification: the gas is cleaned of dust and arsenic compounds and dried with conc. H₂SO₄, because impurities poison the catalyst.
3. Catalytic oxidation: the pure SO₂ and air pass over V₂O₅ at about 450 °C and 1-2 atm. $\mathrm{2SO_2 + O_2 \rightleftharpoons 2SO_3} \qquad \Delta H = -197\ \mathrm{kJ}$ The reaction is exothermic and goes with decrease in moles of gas, so moderate temperature (a compromise between rate and yield) and V₂O₅ catalyst are used.
4. Absorption: SO₃ is absorbed in 98% H₂SO₄ to give oleum (a direct absorption in water forms a troublesome acid mist). $\mathrm{SO_3 + H_2SO_4 \rightarrow H_2S_2O_7}$
5. Dilution: oleum is diluted with the required amount of water to get acid of the desired strength. $\mathrm{H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4}$

Actions of conc. H₂SO₄

• (i) Glucose: it removes the elements of water (dehydration), leaving black carbon. $\mathrm{C_6H_{12}O_6 \xrightarrow{conc.\ H_2SO_4} 6C + 6H_2O}$
• (ii) Phosphorus: it oxidises phosphorus to phosphoric acid and is itself reduced to SO₂. $\mathrm{2P + 5H_2SO_4 \rightarrow 2H_3PO_4 + 5SO_2 + 2H_2O}$
• (iii) Copper: on heating, copper is oxidised to copper sulphate. $\mathrm{Cu + 2H_2SO_4 \rightarrow CuSO_4 + SO_2 + 2H_2O}$

Important use: it is used to manufacture fertilisers such as superphosphate and ammonium sulphate (also as a dehydrating agent and in lead-acid batteries).

Castner-Kellner cell

Principle: electrolysis of brine (NaCl solution) using a flowing mercury cathode. Na⁺ is discharged at the mercury cathode in preference to H⁺ because of the high hydrogen overpotential on mercury, forming sodium amalgam. The amalgam is then decomposed by water to give NaOH.

   Cl2 out          Cl2 out                H2 out
     ^                ^                      ^
 +---|----------------|------+  +------------|---------+
 |  Graphite anodes (+)      |  |  Iron rods (cathode) |
 |   Brine (NaCl soln)       |  |   Water              |
 |  [~~~~~ Hg flowing ~~~~~~~~~~~~~~~~ Hg + Na-amalgam ~] 
 +--- Electrolytic cells -----+  +-- Decomposition cell -+
         (Hg cathode)                  NaOH solution out
                 <-- rocker moves Hg back --

Procedure

• Brine is electrolysed in the inclined cells with graphite anodes; mercury flows along the bottom and acts as cathode.
• Anode: $\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$ (Cl₂ is collected at the top).
• Cathode: $\mathrm{Na^+ + e^- + Hg \rightarrow Na\text{-}Hg}$ (amalgam).
• The amalgam flows into a separate compartment containing water, where it acts as a short-circuited cell with iron rods: $\mathrm{2Na\text{-}Hg + 2H_2O \rightarrow 2NaOH + H_2 + 2Hg}$
• Mercury is pumped back to the electrolytic cell; 20-50% NaOH is collected and evaporated to solid.

Actions of caustic soda

• (i) Chlorine: with cold dilute NaOH, a mixture of hypochlorite and chloride forms; with hot concentrated NaOH, chlorate forms. $\mathrm{Cl_2 + 2NaOH \rightarrow NaCl + NaClO + H_2O}$ $\mathrm{3Cl_2 + 6NaOH \rightarrow 5NaCl + NaClO_3 + 3H_2O}$
• (ii) Zinc: zinc dissolves to give sodium zincate and hydrogen. $\mathrm{Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2}$

Caustic soda to sodium silicate: fuse NaOH with silica (sand). $\mathrm{2NaOH + SiO_2 \rightarrow Na_2SiO_3 + H_2O}$

Diagram (flow sheet of the Solvay process).

 NaCl brine + NH3  -->  [Ammonia absorber]  -->  ammoniated brine
                                                      |
 CO2 (from limekiln) -------------------->  [Carbonating tower]
                                                      |
                                          NaHCO3 (ppt) + NH4Cl solution
                                                      |
                                                 [Filter]
                                             /            \
                                  NaHCO3 (solid)        NH4Cl filtrate
                                        |                     |
                                   [Furnace]          [Ammonia recovery tower]
                                  heat, 2NaHCO3          + Ca(OH)2 from
                                        |                limekiln
                                  Na2CO3 (soda ash)           |
                                        + CO2 (recycled)   NH3 (recycled) + CaCl2

Chemical principle. Sodium hydrogencarbonate is much less soluble in cold ammoniated brine than $\mathrm{NH_4Cl}$, $\mathrm{NaCl}$ and $\mathrm{NH_4HCO_3}$ (aided by the common-ion effect of $\mathrm{Na^+}$). So when $\mathrm{CO_2}$ is passed into cold ammoniated brine, $\mathrm{NaHCO_3}$ precipitates:

$\mathrm{NaCl + NH_3 + CO_2 + H_2O \rightarrow NaHCO_3\downarrow + NH_4Cl}$

The precipitate is filtered and heated (about $150^\circ\mathrm{C}$) to give sodium carbonate:

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

$\mathrm{CO_2}$ comes from heating limestone, $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$. The quicklime is slaked, $\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}$, and used to recover ammonia:

$\mathrm{2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2NH_3 + 2H_2O}$

Overall: $\mathrm{2NaCl + CaCO_3 \rightarrow Na_2CO_3 + CaCl_2}$.

Advantages. The raw materials (brine and limestone) are cheap and abundant; ammonia and $\mathrm{CO_2}$ are recycled, so only a small make-up of $\mathrm{NH_3}$ is needed; the only waste is $\mathrm{CaCl_2}$; and the process gives fairly pure $\mathrm{Na_2CO_3}$ at low cost and low temperature.

Formulae. (i) Soda ash: $\mathrm{Na_2CO_3}$ (ii) Baking soda: $\mathrm{NaHCO_3}$ (iii) Washing soda: $\mathrm{Na_2CO_3\cdot 10H_2O}$

Manufacture of washing soda by the ammonia-soda (Solvay) process

Principle: When $\mathrm{CO_2}$ is passed into a cold, ammoniacal, saturated solution of sodium chloride (brine), sparingly soluble sodium bicarbonate precipitates out, because $\mathrm{NaHCO_3}$ is much less soluble than $\mathrm{NH_4Cl}$ in the cold brine. Heating the bicarbonate gives sodium carbonate, which crystallises with water as washing soda.

Procedure:

1. Ammoniated brine. Saturated NaCl solution is saturated with ammonia in the ammonia absorber (the ammonia is recovered later).
2. Carbonation. The ammoniated brine runs down the carbonating (Solvay) tower, fitted with perforated plates, while $\mathrm{CO_2}$ is forced up from below (kept cool). $\mathrm{NaHCO_3}$ separates as a precipitate:

$\mathrm{NH_3 + CO_2 + H_2O \rightarrow NH_4HCO_3}$

$\mathrm{NH_4HCO_3 + NaCl \rightarrow NaHCO_3\downarrow + NH_4Cl}$

3. Filtration. The milky suspension is filtered, and the $\mathrm{NaHCO_3}$ is washed.
4. Calcination. The bicarbonate is heated to give soda ash and $\mathrm{CO_2}$, which is recycled to the tower:

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

5. Recovery of ammonia. The filtrate containing $\mathrm{NH_4Cl}$ is heated with slaked lime, and the ammonia released is used again:

$\mathrm{2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2NH_3 + 2H_2O}$

The $\mathrm{CO_2}$ comes from heating limestone, $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$, and the lime formed slakes to $\mathrm{Ca(OH)_2}$.

6. Washing soda. Soda ash is dissolved in water and crystallised to get $\mathrm{Na_2CO_3\cdot10H_2O}$.

Diagram (block form): brine + $\mathrm{NH_3}$ $\rightarrow$ ammonia tower $\rightarrow$ carbonating tower (with $\mathrm{CO_2}$ from lime kiln) $\rightarrow$ filter $\rightarrow$ $\mathrm{NaHCO_3}$ $\rightarrow$ furnace $\rightarrow$ $\mathrm{Na_2CO_3}$; filtrate $\rightarrow$ ammonia recovery tower (+ lime) $\rightarrow$ $\mathrm{NH_3}$ recycled.

Conversion of washing soda

(i) Into soda ash: heating the crystals drives off the water of crystallisation.

$\mathrm{Na_2CO_3\cdot10H_2O \xrightarrow{\Delta} Na_2CO_3 + 10H_2O}$

(ii) Into sodium silicate: fusing with silica (sand):

$\mathrm{Na_2CO_3 + SiO_2 \xrightarrow{fuse} Na_2SiO_3 + CO_2\uparrow}$

Uses of washing soda

• As a cleansing agent in laundries and for washing clothes.
• To remove permanent hardness of water.
• In the manufacture of glass, soap, paper and caustic soda.
• In many industries (textiles, paper, softening) and as a laboratory reagent.

(d) Involves sequence of steps followed in specific order

A batch process is a non-continuous process in which a fixed amount of material is treated by a sequence of steps carried out in a definite order in the same vessel, one batch at a time.

(b) NH$_2$COONH$_4$

In the ammonia–carbon dioxide technology, ammonia and carbon dioxide first combine to form the intermediate ammonium carbamate, NH$_2$COONH$_4$, which is then dehydrated to give urea:

$\mathrm{2NH_{3} + CO_{2} \rightarrow NH_{2}COONH_{4} \rightarrow NH_{2}CONH_{2} + H_{2}O}$

(b) CO$_2$ and NH$_3$

In the Solvay process, the CO$_2$ released by heating limestone (CaCO$_3$ → CaO + CO$_2$) and the NH$_3$ recovered from the reaction of NH$_4$Cl with slaked lime (2NH$_4$Cl + Ca(OH)$_2$ → 2NH$_3$ + CaCl$_2$ + 2H$_2$O) are both fed back into the process, so only these two are recycled.

Flow sheet diagram — Manufacture of ammonia by Haber–Bosch process

        N₂ (from air)                H₂ (from water gas / natural gas)
             │                                    │
             └──────────────┬─────────────────────┘
                            ▼
                 ┌─────────────────────┐
                 │  Purification of    │  (removal of dust, H₂S, CO, CO₂,
                 │  N₂ + H₂ gas mixture│   moisture — ZnO, NaOH, etc.)
                 └──────────┬──────────┘
                            ▼
                 ┌─────────────────────┐
                 │   Compression to    │
                 │   200–300 atm       │
                 └──────────┬──────────┘
                            ▼
        ┌───────────────────────────────────────┐
        │      Catalytic Converter (NH₃ reactor) │
        │   Fe catalyst + Mo promoter            │
        │   Temperature 400–500 °C               │
        │   Pressure 200–300 atm                 │
        │   N₂ + 3H₂ ⇌ 2NH₃  (exothermic)        │
        └───────────────────┬───────────────────┘
                            ▼
                 ┌─────────────────────┐
                 │  Cooling / Condensation│
                 │  (liquefaction of NH₃) │
                 └──────────┬──────────┘
                            ▼
                 ┌─────────────────────┐
                 │  Liquid ammonia     │
                 │  stored in cylinders│
                 └─────────────────────┘
                            ▲
                            │
        Unreacted N₂ + H₂ ──┘  (recycled back to the compressor)

Key points shown in the diagram

• Nitrogen from air and hydrogen from water gas (or natural gas) are mixed in the ratio 1 : 3 by volume.
• The mixture is purified to remove impurities (H₂S, CO, CO₂, moisture) which would poison the catalyst.
• The purified gases are compressed to about 200–300 atm.
• The compressed gases enter the catalytic converter packed with finely divided iron (with molybdenum as promoter) kept at 400–500 °C.
• Ammonia formed is liquefied by cooling and collected; unreacted gases are recycled to improve yield (Le Chatelier's principle).

Major challenge: shortage of capital and modern technology (together with limited skilled manpower and few developed raw-material sources), so large chemical plants are costly to build and run in a landlocked country like Nepal.

Strategies to overcome it:

• attract foreign investment and technical collaboration (joint ventures, technology transfer);
• train skilled manpower in chemical engineering and technology within Nepal;
• use local raw materials such as limestone, water, forest and agricultural products;
• provide government support through subsidies, tax incentives, power supply and infrastructure.

Four steps of the contact process starting from iron sulfide (FeS₂):

Step 1 — Roasting of iron sulfide (production of SO₂): Iron sulfide is roasted in excess air to give sulfur dioxide.

$4\text{FeS}_2 + 11\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + 8\text{SO}_2$

Step 2 — Oxidation of SO₂ to SO₃ (catalytic oxidation): Sulfur dioxide is oxidised by air in the presence of vanadium pentoxide (V₂O₅) catalyst at about 400–450 °C and 1–2 atm.

$2\text{SO}_2 + \text{O}_2 \xrightleftharpoons[\text{V}_2\text{O}_5]{400-450\,^\circ\text{C}} 2\text{SO}_3 + \text{heat}$

Step 3 — Absorption of SO₃ in concentrated H₂SO₄ (production of oleum): Sulfur trioxide is dissolved in 98% concentrated sulfuric acid to form oleum (pyrosulfuric acid).

$\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7 \quad (\text{oleum})$

Step 4 — Dilution of oleum to give sulfuric acid: Oleum is diluted with water to obtain sulfuric acid of the required concentration.

$\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4$

(Note: SO₃ is not absorbed directly in water because the reaction is highly exothermic and produces a dense mist of acid droplets that is difficult to condense.)