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Difficulty: MediumHalogens: Chlorine, Hydrogen Chloride, and Oxoacids

During the laboratory preparation of dry hydrogen chloride gas, concentrated tetraoxosulfate(VI) acid (H2SO4\text{H}_2\text{SO}_4) is used as the drying agent rather than quicklime (CaO\text{CaO}). Which of the following explains why quicklime cannot be used to dry hydrogen chloride gas?

  1. Quicklime is a basic oxide that chemically reacts with acidic hydrogen chloride gas to form calcium chloride and water.Answer
  2. B
    Quicklime acts as a strong oxidizing agent that oxidizes hydrogen chloride gas into chlorine gas.
  3. C
    Quicklime is an acidic oxide that increases the pH of the solution by releasing hydrogen ions.
  4. D
    Quicklime physically absorbs hydrogen chloride gas without chemical change, altering the molar volume of the gas at STP.

Answer

Quicklime is a basic oxide that chemically reacts with acidic hydrogen chloride gas to form calcium chloride and water.
Drying agents must not react chemically with the gas being dried. Because hydrogen chloride (HCl\text{HCl}) is an acidic gas and quicklime (CaO\text{CaO}) is a basic oxide, they undergo a neutralization reaction forming calcium chloride and water. Concentrated tetraoxosulfate(VI) acid (H2SO4\text{H}_2\text{SO}_4) is acidic and does not react with HCl\text{HCl}, making it the correct drying agent.

Step-by-Step Solution

1
Identify the chemical nature of hydrogen chloride gas.
Hydrogen chloride (HCl\text{HCl}) is an acidic gas.
It dissolves in water to form hydrochloric acid and reacts readily with bases.
2
Identify the chemical nature of quicklime (CaO\text{CaO}).
Calcium oxide (CaO\text{CaO}) is a basic oxide.
Group 2 metal oxides are basic oxides.
3
Determine the interaction between an acidic gas and a basic drying agent.
They undergo an acid-base neutralization reaction: CaO(s)+2HCl(g)→CaCl2(s)+H2O(l)\text{CaO(s)} + 2\text{HCl(g)} \rightarrow \text{CaCl}_2\text{(s)} + \text{H}_2\text{O(l)}.
A drying agent must be chemically unreactive towards the gas being dried.

Key Concept

Chemical compatibility of drying agents with gases
Estimated Time:1m 0s
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Topics

Halogens: Chlorine, Hydrogen Chloride, and Oxoacids

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