What is urea?

by BY P GHOSH on December 11, 2020 in INORGANIC

What is urea?

Urea is a very notable organic compound with chemical formula CO (NH2)2. Urea is also known as carbonyldiamide, carbamide, carbonyldiamine etc.

Urea is considered as the first organic compound that synthesized by Friedrich Ohler, in 1828, from inorganic compounds, NH4CNO.

NH4CNO ⟶ CO (NH2)2

It is odorless white solid and highly soluble in water. Solubility in water, 1.08 g/ml at 293K and solubility increase with increasing temperature.

Urea is also soluble in glycerol and ethanol solvent. The solubility of urea in glycerol is 0.5 g/ml and in ethanol is 0.05 g/ml.

The density of urea solution is 1.31 g / ml. Melting point range of urea 406K to 408K.

Urea is neither acidic nor alkaline and practically non-toxic. Urea is used in many biological processes in the body.

Urea plays an important role in the metabolism of nitrogenous compounds by animals. It is the main nitrogenous substance in the urine of mammals.

The liver forms it by combining two ammonia molecules with the carbon dioxide molecule in the urea cycle.

Urea structure and molar mass

Urea is a covalent compound with molecular formula CO (NH2)2. The exact molar mass of urea molecule is 60.

The carbonyl carbon of urea is attached with two amino groups. The carbonyl carbon and oxygen atom of urea are sp2 hybridized.

Hence according to VSEPR theory, the urea molecule is planar in the crystal structure, but the geometry around the nitrogen atoms is pyramidal in the gas-phase minimum-energy structure.

Urea synthesis

Urea is prepared on a large scale by reacting liquid ammonia with gaseous CO2 at 408K under 220 atmosphere pressures. The reaction takes place in two steps.

In the first step ammonium carbamate is produced. Some amount of ammonium carbamate is immediately decomposed to form urea.

The undecomposed ammonium carbamate is processed separately under reduced pressure to give urea. The overall reaction is exothermic.

2 NH3 + CO2 ⇌ H2N-COONH4 (ΔH= -117kJ/mol at 220 atm. and 408K)

H2N-COONH4 ⇌ O=C (NH2)2 + H2O (ΔH= +15.5 kJ/mol)

Use of urea as fertilizer in agriculture

Urea fertilizer is a source of nitrogen and hence it is used predominantly in agriculture for nutrition of plants.

Urea is a better fertilizer than either ammonium sulfate or calcium ammonium nitrate.

This is because the ammonia liberated by hydrolysis is assimilated by the soil through the interaction of nitrifying bacteria as in the case of ammonium sulfate or calcium ammonium nitrate.

O=C(NH3)2 + H2O ➝2 NH3 + CO2

Again, CO2 produced in hydrolysis of urea does not cause any damage to the soil like sulfate or nitrate ions.

Urea is highly soluble in water and so it is very suitable for use in fertilizer solutions (mixed with NH4NO3).

As a fertilizer, urea is much more effective than, ammonium sulphate or calcium ammonium nitrate.

Because of it contains much higher percentage (46.6%) of nitrogen than either (NH4)2SO4 or calcium ammonium nitrate. Thus, it consumes the minimum transport per unit of nitrogen nutrients.

Many soil bacteria contain the enzyme urease, which converts urea to NH3 or ammonium ions (NH4 +) and bicarbonate ions (HCO3⁻). Thus urea fertilizers are rapidly converted to ammonium in the soil.

In grain and cotton crops, urea is often applied at the time of the last cultivation before planting. In sugarcane cultivation, urea is applied side by side after planting and applied to each ratoon crop.

Like most hard fertilizers, storage in a cool, dry, well-ventilated area is recommended. Besides, there is also no risk of urea conservation as it is not related to the risk of explosion.

However, it also has a disadvantage. Urea is not as stable as other nitrogenous fertilizers.

It decomposes even at low temperatures especially in humid environments leaving NH3 and CO2.

Besides, the most common contamination of synthetic urea is burette, which inhibits plant growth. Again, it is harmful to keep urea in large quantities or near seeds.