Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

Ammonia can be dried by (a) conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{P}_{4} \mathrm{O}_{10}\) (c) \(\mathrm{CaO}\) (d) anhydrous \(\mathrm{CaCl}_{2}\)

Step-by-Step Solution

Answer

Ammonia can be dried by (c) CaO.

Step 1: Understanding Chemical Properties

Ammonia (NH_3) is a basic gas that reacts with acids to form ammonium salts. Therefore, it cannot be dried using substances that are acidic or react with basic gases.

Step 2: Acidic Drying Agents

Concentrated H_2SO_4 is an acidic substance. It reacts with NH_3, forming ammonium sulfate. Therefore, H_2SO_4 cannot be used to dry ammonia.

Step 3: Dehydrating But Reactive Compounds

Phosphorus pentoxide (P_4O_{10}) is a powerful dehydrating agent. However, it also reacts vigorously with ammonia, forming ammonium phosphates, making it unsuitable for drying ammonia.

Step 4: Basic Drying Agents

Calcium oxide (CaO) is a basic drying agent. Since it is basic, it does not react with ammonia. Therefore, it can be used to dry ammonia.

Step 5: Non-Reactive with Basic Gases

Anhydrous CaCl_2 is also a drying agent. However, it forms complexes with ammonia, which suggests its ineffectiveness in drying pure ammonia without reactions.

Key Concepts

Chemical Properties of Ammonia

Ammonia, with the chemical formula \( \mathrm{NH}_3 \), is a colorless gas with a distinct odor, often found in nature as part of the nitrogen cycle. A key feature of ammonia is its basic nature. This means it tends to accept protons, or hydrogen ions. When ammonia encounters an acidic substance, it readily reacts to form ammonium salts. For example, when ammonia is exposed to sulfuric acid \( \mathrm{H}_2\mathrm{SO}_4 \), it forms ammonium sulfate \( \mathrm{NH}_4\mathrm{SO}_4 \). This reactivity is crucial when choosing a drying method for ammonia, as it cannot be dried using acidic agents that would react and thus alter its properties.

Acidic Drying Agents

Acidic drying agents are substances that can dehydrate by providing an acidic environment. A common example is concentrated sulfuric acid \( \mathrm{H}_2\mathrm{SO}_4 \), known for its strong dehydrating properties. However, due to its acidity, \( \mathrm{H}_2\mathrm{SO}_4 \) will react with basic substances, like ammonia, leading to the formation of unwanted ammonium salts. Thus, despite its efficacy in removing moisture, \( \mathrm{H}_2\mathrm{SO}_4 \) is unsuitable for drying basic gases such as ammonia.

Basic Drying Agents

Basic drying agents are compounds that do not react with basic gases, making them ideal for drying such substances. Calcium oxide \( \mathrm{CaO} \), also known as quicklime, is a prominent basic drying agent suitable for drying ammonia. Its basicity complements the nature of ammonia, allowing it to absorb moisture without causing any reactions with the gas itself. Thus, \( \mathrm{CaO} \) can effectively dehydrate ammonia without affecting its chemical integrity.

Anhydrous Drying Agents

Anhydrous drying agents are substances that lack water and are used to remove moisture from gases and liquids. Anhydrous calcium chloride \( \mathrm{CaCl}_2 \) is a well-known example, often used to dry gases due to its hygroscopic properties. However, when used with ammonia, \( \mathrm{CaCl}_2 \) can form complexes, disrupting the purity of the gaseous form. This interaction makes \( \mathrm{CaCl}_2 \) less suitable for drying ammonia, as it leads to unintended chemical changes.