A lime tower is primarily used for the collection of ammonia gas in various chemical processes, particularly in laboratory and industrial settings. Its specific design and the chemical properties of lime (calcium oxide) make it an ideal apparatus for this purpose.
Why Lime is Ideal for Ammonia Collection
The effectiveness of a lime tower in collecting ammonia gas stems from the compatible chemical nature of the two substances:
- Shared Basic Properties: Both lime ($$\text{CaO}$$) and ammonia ($$\text{NH}_3$$) are inherently basic in nature. This crucial similarity means that they do not react with each other. This non-reactive characteristic is vital, as it ensures that the ammonia gas can be collected efficiently without being absorbed, consumed, or transformed by the collecting medium.
- Ammonia's Basicity: Ammonia is a well-known basic gas. This property dictates that it cannot be effectively collected or dried using acidic substances, which would readily react with it.
Why Other Drying Agents Are Unsuitable for Ammonia
Many common drying or dehydrating agents, while effective for other gases, are unsuitable for use with ammonia due to its basicity and reactive nature:
- Acidic Dehydrating Agents: Substances such as concentrated sulfuric acid ($$\text{Conc. H}_2\text{SO}_4$$), anhydrous calcium chloride ($$\text{CaCl}_2$$), and phosphorus pentoxide ($$\text{P}_2\text{O}_5$$) are acidic or have acidic properties.
- Chemical Reactions: Ammonia, being a base, would readily react with these acidic agents. For instance, ammonia reacts with sulfuric acid to form ammonium sulfate, or with anhydrous calcium chloride to form an addition complex. These reactions would lead to the absorption or chemical alteration of the ammonia, preventing its proper collection.
- Example with sulfuric acid: $$2\text{NH}_3 (\text{g}) + \text{H}_2\text{SO}_4 (\text{aq}) \rightarrow (\text{NH}_4)_2\text{SO}_4 (\text{aq})$$
- Example with calcium chloride: $$\text{CaCl}_2 (\text{s}) + 8\text{NH}_3 (\text{g}) \rightarrow \text{CaCl}_2 \cdot 8\text{NH}_3 (\text{s})$$
Practical Applications
In a laboratory setup, a lime tower often functions as both a drying agent and a collection apparatus when dealing with the preparation or isolation of ammonia gas. It ensures that any moisture present with the ammonia is absorbed by the lime, while the ammonia gas itself remains unreacted and can be collected in its pure form. This careful selection of collection and drying apparatus, based on the specific chemical properties of the gas being handled, is a fundamental aspect of practical chemistry.
