Why conc. H2SO4, Anhydrous CaCl2 or P4div0 cannot be used as dehydrating agents for NH3 ?

When considering dehydrating agents for ammonia (NH3), it's essential to understand the chemical interactions that occur between the dehydrating agent and ammonia. Concentrated sulfuric acid (H2SO4), anhydrous calcium chloride (CaCl2), and phosphorus pentahydride (P4H10) are all strong dehydrating agents, but they are not suitable for use with ammonia for specific reasons.

Concentrated H2SO4 and Ammonia

Concentrated sulfuric acid is a powerful dehydrating agent, but it reacts with ammonia rather than merely removing water. When NH3 comes into contact with H2SO4, it forms ammonium sulfate (NH4)2SO4. This reaction not only consumes the ammonia but also produces a salt, which defeats the purpose of dehydration. The equation for this reaction can be represented as:

• 2 NH3 + H2SO4 → (NH4)2SO4

This means that instead of just dehydrating ammonia, H2SO4 actively participates in a chemical reaction that alters the ammonia itself.

Anhydrous CaCl2 and Its Limitations

Anhydrous calcium chloride is another dehydrating agent that effectively absorbs moisture. However, when it comes to ammonia, it also forms a chemical compound. Calcium chloride reacts with ammonia to form calcium amide (Ca(NH2)2) and releases hydrogen chloride (HCl) gas:

• CaCl2 + 2 NH3 → Ca(NH2)2 + 2 HCl

This reaction indicates that instead of simply dehydrating ammonia, CaCl2 reacts with it, leading to the formation of new compounds, which is not the desired outcome when trying to dehydrate NH3.

P4H10 and Its Reactivity

Phosphorus pentahydride (P4H10) is a less common dehydrating agent, but it also poses issues when used with ammonia. P4H10 can react with ammonia to form phosphine (PH3) and other phosphorus-containing compounds. This reaction can be quite complex and may lead to the production of toxic byproducts:

• P4H10 + NH3 → PH3 + byproducts

Similar to the previous agents, P4H10 does not merely remove water from ammonia; it engages in a chemical reaction that alters the ammonia itself, which is not suitable for dehydration purposes.

Summary of Key Points

In summary, the reason concentrated H2SO4, anhydrous CaCl2, and P4H10 cannot be used as dehydrating agents for ammonia is that they all react chemically with ammonia, leading to the formation of new compounds rather than simply removing water. This reactivity undermines the goal of dehydration, making these agents unsuitable for this specific application.