Ch. 6 - Sodium Carbonate

Question 1

Identify the raw materials used in the Solvay process and name the products

Answer 1

Raw materials: Sodium chloride solution (brine), ammonia, calcium carbonate (limestone).

– Note: The input of ammonia is a once-only process as the ammonia is recycled.

Products: Sodium carbonate, calcium chloride.

Question 2

Describe the uses of sodium carbonate

Answer 2

• Glass manufacture for windows and bottles. It is melted with calcium carbonate and sand (silicon dioxide) to form glass, since it lowers the melting point of the mixture.
• Water treatment both industrially and domestically. It softens hard water since the carbonate ions precipitate calcium and magnesium ions

​​CO₃^2- _(aq) + Ca²⁺_(aq) -> CaCO_{3 (s)}

• Remove sulfur dioxide from flue gases from the gaseous combustion in furnaces from power stations. This process has become increasingly common as power stations meet greater emission controls
• Making soaps and detergents, since it is a weaker base and cheaper alternative than sodium hydroxide.

Question 3

Identify the sequences of steps used in the Solvay process.

Answer 3

1. Brine purification
2. Hydrogen carbonate formation
3. Sodium carbonate formation
4. Ammonia recovery

Question 4

Describe step: Brine purification

Answer 4

• A solution of about 30% NaCl in water is required for the Solvay process
• The impurities vary but usually contain calcium, magnesium and iron ions, which need to be removed

1. Sodium carbonate is added to precipitate calcium
2. Sodium hydroxide is then added to precipitate magnesium and heavy metals

Ca²⁺ (aq) + CO₃^2- (aq) –> CaCO₃ (s)

Mg²⁺ (aq) + 2OH^- (aq) –> Mg(OH)₂ (s)

Fe³⁺ (aq) + 3OH^- (aq) –> Fe(OH)₃ (aq)

1. The precipitates are filtered off together with any suspended solids during flocculation

Question 5

Describe hydrogen carbonate formation

Answer 5

1. CaCO₃ (s) –> CO2 (g) + CaO (s)
2. CO2 (g) + H2O (l) ⇌ H2CO3 (aq)
3. H2CO3 (aq) + NH3 (aq) –> NH4+ (aq) + HCO3 (aq)
4. Na+ (aq) + HCO3- (aq) –> NaHCO₃ (s)

Overall in the carbonator:

NaCl (aq) + CO2 (g) + NH3 (aq) + H2O (l) –>

NaHCO3 (s) + NH4Cl (aq)

• Give the 5 reactions, but only use the bolded reactions for calculations (all equimolar ratios)