Dinitrogen ($N_2$): Preparation & Properties
The Azide method, Nitrolim, and the unbreakable Triple Bond.
Nitrogen is the most abundant element in our atmosphere, making up roughly $78\%$ of it by volume. It exists naturally as the diatomic molecule Dinitrogen ($N_2$). Despite its abundance, it is remarkably unreactive under normal conditions. This chemical inertness dictates its preparation, its uses, and its properties.
1. Preparation of Dinitrogen
A. Commercial Preparation
Commercially, dinitrogen is produced by the liquefaction and fractional distillation of air. Liquid nitrogen (b.p. $77.2\text{ K}$) distils out first, leaving behind liquid oxygen (b.p. $90\text{ K}$).
B. Laboratory Preparation
In the lab, $N_2$ is prepared by treating an aqueous solution of Ammonium chloride ($NH_4Cl$) with Sodium nitrite ($NaNO_2$).
C. Thermal Decomposition of Ammonium Dichromate
Heating Ammonium dichromate produces a spectacular "volcano" reaction, yielding nitrogen gas, steam, and green Chromium(III) oxide residue.
D. Very Pure Nitrogen (The Azide Method)
This is the most highly tested preparation method in JEE and NEET. Ultra-pure nitrogen is obtained by the thermal decomposition of Sodium or Barium azide.
Similarly, $2NaN_3 \xrightarrow{\Delta} 2Na + 3N_2 \uparrow$. (Sodium azide is famously used in automobile airbags because it rapidly decomposes upon impact to produce a massive volume of $N_2$ gas to inflate the bag).
2. Physical Properties
- It is a colorless, odorless, tasteless, and non-toxic gas.
- It has two stable isotopes: $^{14}N$ and $^{15}N$.
- It has very low solubility in water ($23.2 \text{ cm}^3$ per litre at 273 K).
- It has low freezing point ($63.2\text{ K}$) and boiling point ($77.2\text{ K}$).
3. Chemical Properties & Reactivity
Dinitrogen is rather inert at room temperature. Why? Because of the immensely strong $p\pi-p\pi$ triple bond ($N \equiv N$) holding the two atoms together. The bond enthalpy is staggering: $941.4\text{ kJ mol}^{-1}$. Reactivity only increases rapidly at elevated temperatures when this bond can be broken.
A. Reaction with Metals (Forming Nitrides)
At high temperatures, it directly combines with highly electropositive metals to form predominantly ionic nitrides (containing the $N^{3-}$ ion).
(Lithium uniquely reacts even at room temperature)
B. Reaction with Non-Metals
With Hydrogen (Haber Process): It combines with $H_2$ at $\sim 773\text{ K}$ and high pressure in the presence of an Iron catalyst to form Ammonia.
With Oxygen (Birkeland-Eyde Process): It reacts with $O_2$ only at incredibly high temperatures ($\sim 2000\text{ K}$) to form Nitric Oxide (NO).
C. Reaction with Calcium Carbide: Nitrolim
When nitrogen gas is passed over heated Calcium Carbide ($CaC_2$) at $1000^\circ\text{C}$, it produces a mixture of Calcium Cyanamide ($CaCN_2$) and Carbon.
Uses: The mixture is commercialized as Nitrolim and is used heavily as a nitrogenous fertilizer in agriculture because it slowly hydrolyzes in soil to release ammonia.
4. Uses of Dinitrogen
- Manufacture of Ammonia ($NH_3$) and other industrial chemicals containing nitrogen (e.g., calcium cyanamide).
- Providing an inert atmosphere in metallurgical operations (e.g., iron and steel manufacturing) to prevent oxidation.
- Liquid dinitrogen is used as a highly effective refrigerant to preserve biological materials, food items, and in cryosurgery.
Mastery Check: Dinitrogen
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