Chemistry
Srishti needs to dry ammonia gas in the laboratory. She knows the following drying agents:
- Solid calcium chloride
- Concentrated sulphuric acid
- Solid calcium oxide
- Phosphorus pentoxide
However, she chooses to use solid calcium oxide.
Answer the following questions:
(a) Why is solid calcium oxide preferred over other drying agents for drying ammonia gas?
(b) Why is concentrated sulphuric acid not used to dry ammonia gas?
(c) Concentrated sulphuric acid is a good dehydrating agent. Explain.
(d) State one difference between a drying agent and a dehydrating agent.
Water
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Answer
(a) Calcium oxide is preferred because it removes moisture without reacting with ammonia gas. It acts as a suitable drying agent for ammonia. Other drying agents like concentrated acids react with ammonia, so they cannot be used.
(b) Concentrated sulphuric acid is not used to dry ammonia gas because it is acidic and reacts with basic ammonia gas to form ammonium sulphate. Thus, ammonia gets chemically absorbed instead of only being dried.
2NH3 + H2SO4 ⟶ (NH4)2SO4
(c) Concentrated sulphuric acid is a good dehydrating agent because it can remove chemically combined water molecules from compounds. For example, it removes water of crystallisation from blue vitriol, CuSO4.5H2O, forming anhydrous CuSO4.
CuSO4.5H2O ⟶ CuSO4 + 5H2O
(d) A drying agent removes moisture from substances without chemically reacting with them, whereas a dehydrating agent removes chemically combined water molecules from compounds, usually causing a chemical change.
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