Modern Chemical Manufactures

Some modern chemical manufactures are described below.

Manufacture of ammonia by Haber’s process

Principle: When a mixture of nitrogen and hydrogen in the ratio of 1:3 by volume is heated to about 450°C under a pressure of 200-900 atm in the presence of finely divided iron as a catalyst and some molybdenum as the promoter, ammonia is formed.

N_{2} + 3H_{2}\ \underset{Fe/Mo}{\stackrel{450 \degree C\ 200-900\ atm}{\rightleftharpoons}}\ 2NH_{3} + 45kcal

Conditions for high yield of ammonia:

Since the reaction is exothermic, reversible and occurs by decrease in volume, it is favoured by (according to Le-Chatlier’s principle) :

1. Optimum temperature: Since the reaction is exothermic, low temperature favours the forward reaction. But if the temperature is too low, the reaction will be slow. So optimum temperature of 450°C is used.

2. High pressure: The formation of ammonia occurs by decreasing volume or decreasing the number of moles of product. So, a high pressure of 200-900 atm is required.

3. High concentration: High concentration of nitrogen and hydrogen favours the formation of ammonia.

4. Use of catalyst: To speed up the rate of reaction, finely divided iron (catalyst) with molybdenum (promoter) is used.

5. Purity of N₂ and H₂: The reaction mixture should be free from impurities.

Manufacture of nitric acid by Ostwald’s process

Principle:

1. Catalytic Oxidation of ammonia: Ammonia is oxidized in the air in the presence of platinum as a catalyst at 850 °C to give nitric oxide.

NH_{3} + 5O_{2}\ \xrightarrow{Pt,\ 850\ \degree C,\ 5\ atm}\ 4NO + 6H_{2}O

2. Oxidation of nitric oxide: Nitric oxide is oxidized into nitrogen dioxide in the presence of oxygen.

2NO + O_{2} \rightarrow 2NO_{2}

3. Absorption of nitrogen dioxide: Nitrogen dioxide is absorbed in water in the presence of air to give nitric acid.

4NO_{2} + 2H_{2}O + O_{2} \rightarrow \underset{Nitric\ acid}{4HNO_{3}}

Manufacture of sulphuric acid by contact process

Principle:

1. Production of sulphur dioxide: Sulphur dioxide gas can be prepared either by burning sulphur or by the roasting of iron pyrite.

S + O_{2} \rightarrow SO_{2}\\ \underset{Iron\ pyrite}{4FeS_{2}} + 11O_{2} \rightarrow 2Fe_{2}O_{3} + 8SO_{2}

2. Oxidation of sulphur dioxide: Sulphur dioxide is oxidized to sulphur trioxide in presence of vanadium pentoxide as a catalyst at about 450°C and 2 atm pressure.

2SO_{2} + O_{2}\ \underset{2\ atm}{\stackrel{V_{2}O_{5},\ 450 \degree C}{\rightleftharpoons}}\ 2SO_{3} + 45kcal

3. Absorption of sulphur trioxide: Sulphur trioxide is absorbed by conc. sulphuric acid to form pyrosulphuric acid called oleum.

SO_{3} + H_{2}SO_{4}\ \rightarrow\ \underset{Oleum}{H_{2}S_{2}O_{7}}

4. Dilution of oleum: Oleum is added with a calculated amount of water to produce sulphuric acid.

H_{2}S_{2}O_{7} + H_{2}O\ \rightarrow\ \underset{Sulphuric\ acid}{2H_{2}SO_{4}}

Condition for high yield of sulphuric acid

Production of SO₂ to SO₃ is one of the most important steps in the manufacture of sulphuric acid. This is a reversible reaction. A higher yield of SO₃ leads to a high yield of H₂SO₄. Hence, according to Le-Chatlier’s principle, it is favoured by:

1. Low temperature (optimum): Since the reaction is exothermic, the lower the temperature, the greater is the yield. However, a too-low temperature makes the reaction too slow to attain equilibrium. That is why an optimum temperature of 450°C is supplied.

2. High pressure: Since the reaction takes place by the decrease in volume, a high pressure of 2 atm favours the forward direction

3. High concentration: High concentration of SO₂ and O₂ are used for more production of SO₃.

4. Use of catalyst: The rate of reaction is increased by the use of vanadium pentoxide as a catalyst.

5. Purity of gas: SO₂ and O₂ should be free from impurities.

Manufacture of NaOH by Diaphragm cell

A diaphragm cell is an electrolytic cell consisting of a steel tank with two compartments separated by a diaphragm of asbestos or metal oxide with polymer. Graphite or titanium rod acts as an anode. Steel mesh acts as a cathode.
Sodium hydroxide can be manufactured by electrolysis of saturated brine solution using a diaphragm cell.

NaCl(aq)\ \rightarrow\ Na^{+} +\ Cl^{-}\\ H_{2}O\ \rightleftharpoons\ H^{+} +\ OH^{-}\\ At\ anode: 2Cl^{-}\ \rightarrow\ Cl_{2}\ +\ 2e^{-}\\ At\ cathode: 2H^{+} +\ 2e^{-}\ \rightarrow\ H_{2}

Na⁺ and OH^– ions present in solution combine with each other to form NaOH.

Manufacture of washing soda by Solvay or ammonia soda process

Principle: Brine solution saturated with ammonia gas reacts with carbon dioxide to form sodium bicarbonate.

NH_{3} + H_{2}O + CO_{2}\ \rightarrow\ NH_{4}HCO_{3}\\ NaCl + NH_{4}HCO_{3}\ \rightarrow\ NaHCO_{3} + NH_{4}Cl

Sodium bicarbonate is sparingly soluble in water due to unreacted NaCl.The filtered product on heating gives Na₂CO₃.

2NaHCO_{3}\ \overset{\Delta}{\rightarrow}\ Na_{2}CO_{3} + CO_{2} + H_{2}O

Sodium carbonate solution is crystallized to get washing soda.

Na_{2}CO_{3}\ \xrightarrow{crystallization}\ Na_{2}CO_{3}.10H_{2}O

Some Important Questions

1. Write the principle and flow sheet diagram for the manufacture of
   a. ammonia by Haber’s process
   b. nitric acid by Ostwald’s process
   c. sulphuric acid by contact process
   d. sodium hydroxide by Diaphragm cell
   e. washing soda by Solvay process