Unit 12: Nitrogen and its Compounds
From the inert gas in our atmosphere to the essential compounds that feed the world.
12.31 Nitrogen (N₂) (Preparation, Uses)
Nitrogen ($N_2$) is a diatomic gas that makes up approximately 78% of the Earth's atmosphere. The two nitrogen atoms are joined by a very strong triple covalent bond ($N \equiv N$).
This strong triple bond requires a large amount of energy to break, which makes nitrogen gas very unreactive or inert under normal conditions. Industrially, it is obtained by the fractional distillation of liquid air. Its main use is as a raw material for the Haber Process, but its inertness also makes it useful for providing a protective atmosphere for food packaging and in chemical processes.
Solved Examples:
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Why is nitrogen gas so unreactive?
Solution: The two nitrogen atoms in an $N_2$ molecule are held together by a very strong triple covalent bond. A large amount of activation energy is required to break this bond before nitrogen can react. -
Why are bags of potato chips often filled with nitrogen gas?
Solution: Nitrogen is used to provide an inert (unreactive) atmosphere inside the bag. This displaces oxygen, preventing the fats in the chips from reacting with oxygen and becoming rancid, thus keeping them fresh for longer.
12.32 The Haber Process (Manufacture of Ammonia)
The Haber Process is a vital industrial process that "fixes" atmospheric
nitrogen by reacting it with hydrogen to produce ammonia ($NH_3$). This is one of the most
important industrial reactions in the world, as the ammonia produced is primarily used to
make fertilizers.
$$ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \, kJ/mol $$
The reaction is reversible and exothermic. To achieve an economically viable rate and yield, a set of compromise conditions is used:
- Pressure: A high pressure of 200 atmospheres is used. According to Le Chatelier's principle, high pressure favors the product side, as there are fewer moles of gas (2 moles of $NH_3$ vs. 4 moles of reactants).
- Temperature: A compromise temperature of 400-450 °C is used. A low temperature would favor the exothermic forward reaction, but the rate would be too slow. This temperature provides a fast enough rate with an acceptable equilibrium yield.
- Catalyst: An iron catalyst is used to increase the rate of reaction, allowing the process to be run at a lower temperature than would otherwise be possible.
Unreacted nitrogen and hydrogen are recycled back into the reactor to maximize the yield.
Solved Examples:
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Why is high pressure used in the Haber Process?
Solution: The forward reaction involves a decrease in the number of moles of gas (from 4 moles to 2 moles). According to Le Chatelier's principle, increasing the pressure will shift the equilibrium position to the right, favoring the formation of ammonia and increasing the yield. -
What is the purpose of the iron catalyst in the Haber Process?
Solution: The iron catalyst increases the rate at which the reaction reaches equilibrium. It provides an alternative reaction pathway with a lower activation energy, but it does not change the position of the equilibrium itself.
12.33 Ammonia (NH₃) & Ammonium Ions (NH₄⁺)
Ammonia (NH₃)
Ammonia is a colorless gas with a characteristic sharp, pungent smell. It is a weak
base. It is very soluble in water, where it reacts to form a weakly alkaline
solution.
$NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$
The test for ammonia gas is its ability to turn damp red litmus paper blue.
Ammonium Ions (NH₄⁺)
The ammonium ion is formed when ammonia acts as a base. As the conjugate acid of a weak
base, the ammonium ion is a weak acid. The test for the ammonium ion in
solution is to add a strong base (like NaOH) and gently warm the mixture. This reverses the
equilibrium, releasing ammonia gas, which can be detected by its smell and with damp red
litmus paper.
$NH_4^+(aq) + OH^-(aq) \xrightarrow{\Delta} NH_3(g) + H_2O(l)$
Solved Examples:
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Is an aqueous solution of ammonium chloride ($NH_4Cl$) acidic, basic, or
neutral? Explain.
Solution: It is acidic. The ammonium ion ($NH_4^+$) is the conjugate acid of the weak base ammonia, so it will react with water to produce H⁺ ions ($NH_4^+ \rightleftharpoons NH_3 + H^+$). The chloride ion is the conjugate base of a strong acid (HCl) and does not react. -
Describe the test for the ammonium ion.
Solution: Add sodium hydroxide solution to the sample and warm it gently. Test the gas produced with damp red litmus paper. If the paper turns blue, the ammonium ion was present.
12.34 Nitric Acid (HNO₃) & Nitrate(V) Ions (NO₃⁻)
Nitric acid ($HNO_3$) is a strong acid, meaning it fully dissociates in water to form hydrogen ions ($H^+$) and nitrate ions ($NO_3^-$). It is a key industrial chemical, primarily used to manufacture fertilizers (like ammonium nitrate, $NH_4NO_3$) and explosives.
The nitrate ion ($NO_3^-$) is the anion of nitric acid. According to the solubility rules, all nitrate salts are soluble in water. The test for the nitrate ion involves reducing it to ammonia. This is done by adding aluminum powder (a reducing agent) and sodium hydroxide to the sample and warming. If nitrate ions are present, ammonia gas will be produced, which can be identified with damp red litmus paper.
Solved Examples:
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What is the main use of nitric acid?
Solution: The manufacture of nitrogen-based fertilizers, such as ammonium nitrate. -
A student adds aluminum powder and NaOH to a solution and warms it. A gas is
produced that turns damp red litmus blue. What ion was present in the original
solution?
Solution: The nitrate ion ($NO_3^-$).
12.35 Thermal Decomposition of Nitrates
Most metal nitrates decompose when heated strongly, but the products depend on the reactivity of the metal.
- Reactive Metals (e.g., Na, K): Nitrates of the most reactive metals
decompose to form the metal nitrite and oxygen gas.
$$ 2NaNO_3(s) \rightarrow 2NaNO_2(s) + O_2(g) $$ - Less Reactive Metals (e.g., Ca, Mg, Zn, Cu): Nitrates of most other
metals decompose to form the metal oxide, brown nitrogen dioxide gas, and oxygen gas.
$$ 2Cu(NO_3)_2(s) \rightarrow 2CuO(s) + 4NO_2(g) + O_2(g) $$ - Very Unreactive Metals (e.g., Ag): Nitrates of very unreactive metals decompose completely to the metal, nitrogen dioxide, and oxygen.
Solved Examples:
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What are the products when solid zinc nitrate is heated?
Solution: Zinc is a moderately reactive metal. The products will be zinc oxide (ZnO), nitrogen dioxide ($NO_2$), and oxygen ($O_2$). -
A white solid is heated, and it melts and produces a gas that relights a
glowing splint. No brown gas is seen. Identify the original solid.
Solution: The gas is oxygen. The absence of brown $NO_2$ gas indicates the solid is the nitrate of a very reactive metal, such as sodium nitrate ($NaNO_3$) or potassium nitrate ($KNO_3$).