Modern Chemical Manufactures

1. Introduction

Modern chemical manufactures are the large-scale industrial processes that turn cheap raw materials (air, water, minerals) into valuable chemicals such as ammonia, nitric acid, sulphuric acid and sodium compounds. Each process is designed to give the maximum yield at the lowest cost, applying the principles of chemical equilibrium (Le Chatelier's principle), catalysis and reaction rate. This chapter covers the five key manufactures in the NEB syllabus and the idea of fertilizers.

2. Manufacture of Ammonia by Haber's Process

Ammonia is made by the direct combination of nitrogen (from air) and hydrogen (from natural gas or water gas):

$\ce{N2(g) + 3H2(g) <=> 2NH3(g)} \qquad \Delta H = -92\ \text{kJ mol}^{-1}$

The forward reaction is exothermic and proceeds with a decrease in the number of gas molecules ($4 \to 2$). Applying Le Chatelier's principle, the optimum conditions are:

• High pressure ($\approx 200\ \text{atm}$): favours the side with fewer molecules (more $\ce{NH3}$).
• Moderate temperature ($\approx 450\ ^\circ\text{C}$): low temperature favours the exothermic forward reaction, but too low a temperature makes it too slow, so a compromise temperature is used.
• Catalyst: finely divided iron (with $\ce{Mo}$ or $\ce{K2O}$/$\ce{Al2O3}$ promoters) speeds up equilibrium.

Unreacted $\ce{N2}$ and $\ce{H2}$ are recycled. Ammonia is the starting material for fertilizers and nitric acid.