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Difficulty: MediumTetraoxosulfate(VI) Acid: Contact Process and Properties

In the industrial manufacture of tetraoxosulfate(VI) acid by the Contact Process, sulfur(VI) oxide gas (SO3SO_3) is absorbed in concentrated H2SO4H_2SO_4 to form oleum (H2S2O7H_2S_2O_7) rather than being dissolved directly in water. Which of the following best explains the primary reason for this operational step?

  1. Direct dissolution in water is an extremely exothermic reaction that produces a dense acid mist which is difficult to condense.Answer
  2. B
    Direct dissolution in water decomposes tetraoxosulfate(VI) acid into sulfur(IV) oxide gas and oxygen.
  3. C
    Direct dissolution in water causes the vanadium(V) oxide catalyst to poison and deactivate rapidly.
  4. D
    Sulfur(VI) oxide is insoluble in pure water under atmospheric temperature and pressure.

Answer

Direct dissolution in water is an extremely exothermic reaction that produces a dense acid mist which is difficult to condense.
Dissolving sulfur(VI) oxide directly in water releases an extreme amount of heat, causing the liquid to boil instantly and forming a fog-like mist of liquid tetraoxosulfate(VI) acid droplets suspended in steam. This mist is difficult to condense and collect industrially. Absorbing the gas in 98% concentrated tetraoxosulfate(VI) acid forms oleum smoothly without misting.

Step-by-Step Solution

1
Examine the thermochemistry of the direct hydration reaction SO3(g)+H2O(l)→H2SO4(aq)SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq).
The direct combination of sulfur(VI) oxide gas with liquid water is highly exothermic.
A massive amount of heat energy is liberated instantly during the hydration process.
2
Analyze the physical impact of the heat generated during direct absorption.
The intense heat vaporizes water rapidly, converting the acid into tiny aerosol droplets (dense mist).
This fine acid mist resists condensation and escapes into the atmosphere with the exhaust gases.
3
Identify the industrial workaround used in the Contact Process.
SO3SO_3 gas is absorbed smoothly into 98% concentrated H2SO4H_2SO_4 to yield oleum (H2S2O7H_2S_2O_7), which is subsequently diluted with a controlled volume of water to yield concentrated acid.
Dissolving SO3SO_3 in concentrated acid avoids the violent mist-producing reaction while yielding high-purity tetraoxosulfate(VI) acid.

Key Concept

Absorption of sulfur(VI) oxide in the Contact Process
Estimated Time:1m 0s
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Topics

Tetraoxosulfate(VI) Acid: Contact Process and Properties

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