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Questions 5.3(b)

The drying process in Step I makes use of a liquid. Suggest the identity of the liquid.
Name the two elements that react with each other in Step II.
Write an equation for the direct combination of (a) sulphur (VI) oxide with concentrated sulphuric acid in Step V to form oleum (b) dilution of oleum using water, in Step VI to form concentrated sulphuric acid.
Identify two reactive components of the gaseous mixture, R, and explain why the mixture is recycled rather than allowing all of it to flow into the absorption tower.
Explain why use of elemental sulphur (S), where it is available, is preferred to metal sulphide ores in the sulphuric (VI) acid manufacturing process.
The catalyst often employed is vanadium (V) oxide (V₂O₅) rather than platinum (Pt). Give a reason for this (Search in other sources).
A student argued that "dissolving sulphur (VI) oxide in concentrated sulphuric acid, then diluting the product to give back the acid" does not make sense, because it assumes that the acid is already manufactured.
1. Suggest a possible source of the sulphuric (VI) acid used to dissolve sulphur (VI) oxide.
2. Why might this possible source be unsuitable for large scale production of the acid?
Identify any four factors you would consider before constructing a sulphuric (VI) acid manufacturing plant.
Like in other industrial plants, it is difficult to completely convert the two oxides of sulphur into sulphuric (VI) acid; yet they are harmful to the environment. Suggest what can be done to the residual gases.

Answers to Questions 5.3(b)

Concentrated sulphuric (VI) acid
Sulphur and oxygen
(a) H ₂ SO ₄ (l) + SO ₃ (g) → H ₂ S ₂ O ₇
(b) H ₂ S ₂ O ₇ (l) + H ₂ O(l) →2H ₂ SO ₄ (l)
Sulphur (VI) oxide (SO ₃ ), sulphur (IV) oxide (SO ₂ ), oxygen (O ₂ )
The mixture is recycled to convert more of the unreacted sulphur (IV) oxide to sulphur (VI) oxide to maximize the yield of sulphuric (VI) acid. Moreover, this would prevent contamination of the product with the unreacted sulphur (IV) oxide.
Though it is a more effective catalyst, vanadium (V) oxide is easily poisoned by impurities such as dust, making the process more expensive and less efficient in the long run.
(a) Dissolving some sulphur (VI) oxide directly in water.
(b) This source is highly exothermic and therefore potentially explosive and expensive.
Availability of sulphur, or sulphur ores, availability and cost of transportation, source of clean water, reliable source of heat or electrical energy, disposal of industrial waste and safety of the environment, market for the product and by- products.
Scrubbing: converting unreacted oxides to other useful products such as calcium hydrogensulphite used as a bleaching agent in paper industry.