Urea, a commonly used nitrogen-based fertiliser, is prepared by the reaction between ammonia and ______.

carbon dioxide

The correct answer is option 1. carbon dioxide.

The synthesis of urea involves the reaction between ammonia (\(NH_3\)) and carbon dioxide (\(CO_2\)). This chemical process is known as the Bosch–Meiser urea process. Here's an overview of the reaction and the process:

1. Formation of Ammonium Carbamate:

Ammonia gas (\(NH_3\)) is allowed to react with carbon dioxide (\(CO_2\)) in the presence of water (\(H_2O\). The initial reaction involves the formation of ammonium carbamate (\(NH_2COONH_4)).

\[ 2NH_3 + CO_2 + H_2O \rightarrow NH_2COONH_4 \]

2. Formation of Urea:

The ammonium carbamate formed in the first step undergoes further chemical rearrangement to yield urea (\(CO(NH_2)_2\)).

\[ NH_2COONH_4 \rightarrow CO(NH_2)_2 + H_2O \]

3. Overall Reaction

The overall reaction can be represented as follows:

\[ 2NH_3 + CO_2 \rightarrow CO(NH_2)_2 + H_2O \]

4. Process Conditions:

The synthesis typically occurs under high pressure and elevated temperatures to favor the formation of urea. A catalyst, often a compound containing manganese, is used to enhance the reaction rate.

5. Purification:

The urea formed is then purified, often through processes such as crystallization, to obtain the final product in a solid form suitable for use as a fertilizer.

In summary, urea is prepared by reacting ammonia with carbon dioxide in a series of chemical steps known as the Bosch–Meiser urea process. The resulting urea is a valuable nitrogen fertilizer widely used in agriculture.