Green Chemistry and the Ten Commandments

MassCa(OH) 2 = 0.0422 L HCl ×

0.112 mol HCl

×

1 mol Ca(OH) 2

×

74.1 g Ca(OH) 2

1 L HCl 2 mol HCl 1 mol Ca(OH) 2

(4.11.15)

MassCa(OH)

2

= 0.175 g

PercentCa(OH) 2 = mass Ca(OH)^2 × 100 = 0.175 g × 100 =13.9%

mass sample 1.26 g (4.11.16)

Exercise: A 0.638 g sample consisting of oxalic acid, H 2 C 2 O 4 , and sodium

oxalate, Na 2 C 2 O 4 was dissolved and titrated with 0.116 mol/L sodium hydroxide,

of which 47.6 mL (0.0476 L) was required. Each molecule of H 2 C 2 O 4 releases 2

H

+

ions. Calculate the percentage of oxalic acid in the sample.

Answer: 38.9%

4.12. Industrial Chemical Reactions: The Solvay Process

Literally thousands of chemical reactions are used to make important industrial
products. Most of these involve organic chemicals, which are addressed in Chapter 9
and later chapters of this book. Some are used to make inorganic chemicals in large
quantities. One such synthesis operation is the Solvay process, long used to make sodium
bicarbonate and sodium carbonate, industrial chemicals required for glass making,
cleaning formulations, and many other applications. The Solvay process is examined
in some detail in this section because it illustrates some important inorganic chemical
reactions and can be used for the discussion of green chemistry in industry.
The key reaction in Solvay synthesis is,

NaCl + NH 3 + CO 2 + H 2 O → NaHCO 3 (s) + NH 4 Cl (4.12.1)

in which a sodium chloride solution (brine) is saturated with ammonia gas (NH 3 ), then
with carbon dioxide, and finally cooled. This is a precipitation reaction in which solid
sodium bicarbonate, NaHCO 3 , comes out of solution. When heated, the solid NaHCO 3
yields solid sodium carbonate, Na 2 CO 3 , water vapor, and carbon dioxide gas:

2NaHCO 3 + heat → Na 2 CO 3 + H 2 O(g) + CO 2 (g) (4.12.2)

In keeping with the practice of green chemistry (although Solvay developed the process
long before anyone ever thought of green chemistry), the CO 2 from Reaction 4.12.2 is
recycled back into Reaction 4.12.1.
The raw materials for the Solvay process are cheap. The NaCl solution can be
pumped from the ground from brine deposits in some locations, or fresh water can be
pumped into a salt formation to dissolve NaCl and the resulting brine pumped to the
surface. The most expensive raw material is ammonia, which is made by the reaction of
elemental hydrogen and nitrogen over an iron-based catalyst,

3H 2 + N 2 → 2NH 3 (4.12.3)

102 Green Chemistry, 2nd ed