(a) Explain with equation where appropriate, the functions of the following substances in the Solvay Process: (i) limestone, (ii) ammonia, (iii) brine. (b) ...

Answer Details

(a) Functions of the substances in the Solvay Process

• (i) Limestone: it is heated (decomposed) to supply carbon (IV) oxide for the process and to produce calcium oxide (quicklime), which is later used to recover ammonia.
  CaCO_3(s) → CaO_(s) + CO_2(g)
• (ii) Ammonia: it makes the brine alkaline (forms ammoniacal brine) so that it can absorb carbon (IV) oxide, and it is recovered and recycled at the end of the process.
  NH_3(g) + H₂O_(l) + CO_2(g) → NH₄HCO_3(aq)
• (iii) Brine: it supplies the sodium ions (as concentrated sodium chloride solution) needed to form sodium hydrogen trioxocarbonate (IV).
  NH₄HCO_3(aq) + NaCl_(aq) → NaHCO_3(s) + NH₄Cl_(aq)

(b) Why the reaction is a neutralization

Aqueous sodium trioxocarbonate (IV) is a base (it is the salt of a strong base and a weak acid and reacts as a base towards acids), while dilute hydrochloric acid is an acid. Their reaction produces a salt and water only, which is the definition of neutralization:

Na₂CO_3(aq) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l) + CO_2(g)

The essential ionic change is H⁺ + OH^- (from the carbonate acting as a base) forming water; hence it is a neutralization.

(c) Mass of Na₂CO₃ from 16.8 g of NaHCO₃

Decomposition equation:

2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(l) + CO_2(g)

Molar mass of NaHCO₃ = 23 + 1 + 12 + 48 = 84 g mol^-1

Moles of NaHCO₃ = 16.8 ÷ 84 = 0.2 mol

From the equation, 2 mol NaHCO₃ give 1 mol Na₂CO₃, so:

Moles of Na₂CO₃ = 0.2 ÷ 2 = 0.1 mol

Molar mass of Na₂CO₃ = 46 + 12 + 48 = 106 g mol^-1

Mass of Na₂CO₃ = 0.1 × 106 = 10.6 g