24.2.

In order to understand the Solvay process, it is necessary to understand the chemistries of carbon dioxide and ammonia. We first encountered carbon dioxide (CO2) way back in Chapter 1 as a product of the combustion of charcoal. We learned in Chapter 4 that yeasts fart carbon dioxide when they consume honey. Chapter 18 characterized carbon dioxide as a mildly acidic gas which reacts with bases to form carbonate or bicarbonate[1] salts. Carbon dioxide may be liberated from these salts either by heating the bejeezus out of them or by reacting them with acids. The properties of carbon dioxide are summarized in Equation 24-1.

Equation 24-1. Reactions Involving Carbon Dioxide

Equation 24-2. Reactions Involving Ammonia

We met ammonia (NH3) in Chapter 12 as a mild alkali suitable for dissolving indigo dye. Ammonia reappeared in Chapter 22 as a by-product of the distillation of coal tar. The "ammonia" sold at the grocery store is actually a solution of ammonia in water. Just as carbon dioxide combines with water to form carbonic acid, ammonia combines with water to form the alkali, ammonium hydroxide (NH4OH). Ammonium hydroxide participates in the usual acid-base reactions to form ammonium salts. Ammonia may be liberated from these salts either by heating the bejeezus out of them or by reacting them with stronger alkalis like sodium or calcium hydroxide. The properties of ammonia are compared to those of carbon dioxide in Equation 24-2.

Carbonic acid reacts with a base to form a bicarbonate salt and water; ammonium hydroxide reacts with an acid to form an ammonium salt and water. It is only natural that carbonic acid should react with ammonium hydroxide to produce ammonium bicarbonate and water:

H2CO3(aq) + NH4OH(aq) = NH4HCO3(aq) + H2O(l)

When a solution of ammonium bicarbonate evaporates, it leaves behind solid ammonium bicarbonate, a salt with the properties of both bicarbonate salts and ammonium salts. Like other bicarbonate salts, heating the bejeezus out of it liberates carbon dioxide; like other ammonium salts, heating the bejeezus out of it liberates ammonia. The curious thing about ammonium bicarbonate it that when you liberate bejeesical carbon dioxide and bejeesical ammonia, the only thing left is bejeesical water. Thus ammonium bicarbonate is entirely bejeesical in nature; heat it and it just disappears. This property makes it a useful ingredient of baking powders because in the oven it is able to leaven breads and cakes without leaving anything behind.

Ammonium bicarbonate is crucial to the Solvay process because it undergoes a metathesis reaction with sodium chloride to produce sodium bicarbonate and ammonium chloride:

NaCl(aq) + NH4HCO3(aq) = NaHCO3(s) + NH4Cl(aq)

You are probably thinking that sodium bicarbonate is soluble in water. That is so, but it is less soluble than sodium chloride, ammonium bicarbonate, or ammonium chloride. Thus in a very concentrated solution, sodium bicarbonate will be the first to precipitate. This metathesis reaction is the key to the Solvay process.

Figure 24-1. The Solvay Soda Process

The three critical reactions of the Solvay process occur simultaneously in reactor (e) of Figure 24-1. Remember that in a process schematic, reactants enter a reactor from the left and products leave to the right. An aqueous solution of ammonium hydroxide and sodium chloride enters reactor (e), an absorber, where it meets a stream of carbon dioxide gas. The carbon dioxide dissolves to produce carbonic acid, which reacts with ammonium hydroxide to produce ammonium bicarbonate. If there were plenty of water, everything would remain in solution—sodium ion, chloride ion, ammonium ion, and bicarbonate ion. But if water is limited, that is, if the solutions are concentrated, then sodium bicarbonate precipitates and ammonium chloride remains in solution. The liquid returns to reactor (c) and the solid goes to furnace (f), where bejeesical carbon dioxide is driven off. The sodium carbonate which remains is the intended product of the whole process.

Reactors (e) and (f) are the stars of the Solvay process; all of the other reactors play supporting roles. Absorber (e) requires ammonium hydroxide from another absorber, (d). Absorber (d) requires gaseous ammonia from a still, (c). Still (c) requires ammonium chloride recycled from absorber (e) and slaked lime from a slaker, (b). Finally, slaker (b) requires lime from a furnace, (a). As raw materials, the whole process requires limestone, water, and salt. It produces waste calcium chloride and a single product, sodium carbonate. If you add up all of the reactions from all of the reactors, canceling compounds that appear on both sides of the equal sign, the result is remarkably simple:

CaCO3(s) + 2 NaCl(aq) = CaCl2(aq) + Na2CO3(s).

This reaction may seem familiar to you, since it was the second reaction discussed in Section 7.2. Our solubility rules tell us, however, that this reaction goes the other way. Soluble calcium chloride and sodium carbonate will react in aqueous solution to produce insoluble calcium carbonate and soluble sodium chloride. The Solvay process, with its elegant scheme for recycling carbon dioxide and ammonia, is simply an elaborate means for pushing this classic metathesis reaction backwards.

Compare the Solvay process to the Leblanc process, Figure 20-2, and you will immediately see why the one superseded the other. Both processes require limestone, salt, and water as raw materials, but while the Leblanc process additionally requires sulfuric acid, the Solvay process recycles its ammonia. Both processes produce the same product, sodium carbonate, but while the Leblanc process produces two hazardous waste products, the Solvay process produces only one relatively harmless waste product. In fact, calcium chloride is useful as a road de-icer, though the market is not large enough to absorb the huge amounts produced each year. So while the waste produced by a Solvay plant is not nearly as hazardous as those produced by a Leblanc plant, calcium chloride remains a disposal problem for the soda industry.

WarningMaterial Safety
 

Locate MSDS's for ammonia (CAS 7664-41-7), carbon dioxide (CAS 124-38-9), and ammonium bicarbonate (CAS 1066-33-7). Summarize the hazardous properties in your notebook, including the identity of the company which produced each MSDS and the NFPA diamond for each material.[2]

Your most likely exposure will be to ammonia fumes. If a persistent cough develops, seek medical attention.

You should wear safety glasses while working on this project. All activities should be performed in a fume hood or with adequate ventilation. Leftover materials can be flushed down the drain with plenty of water.

NoteResearch and Development
 

You are probably wondering what will be on the quiz.

  • You should know the meanings of all of the words important enough to be included in the index or glossary.

  • You should know the Research and Development points from Chapter 18 and Chapter 20.

  • You should know the reactions of Figure 24-1.

  • You should know the hazardous properties of ammonia, carbon dioxide, and ammonium bicarbonate.

  • You should be able to reproduce Figure 24-1 and to explain this schematic in your own words.

  • You should be able to explain the advantages of the Solvay process over the Leblanc process.

  • You should be able to tell the story of Earnest Solvay.

Notes

[1]

The modern name for the bicarbonate ion is the hydrogen carbonate ion. The older name, however, continues to be widely used.

[2]

The NFPA diamond was introduced in Section 15.2. You may substitute HMIS or Saf-T-Data ratings at your convenience.