Nitrogen

Nitrogen (N2) makes up 78% of the atmosphere and is essential for the Haber process, fertiliser production, and understanding air pollution.

Where it appears in 0620

Nitrogen is not tied to one topic — it links three:

• Air quality and climate change: nitrogen is the largest part of clean dry air, and oxides of nitrogen are named atmospheric pollutants.
• Reversible reactions and the Haber process: the synthesis of ammonia is the set-piece equilibrium example, examined for conditions, yield, and Le Chatelier reasoning.
• Fertilisers: ammonia made from nitrogen becomes nitric acid and then ammonium nitrate, a nitrogen fertiliser.

The N≡N triple bond is also the go-to example of a molecule with a multiple covalent bond in simple molecules.

Position in the periodic table

• Symbol: N
• Atomic number: 7
• Group 15 (V), Period 2
• Electron configuration: 2, 5
• Diatomic molecule: N2 (triple bond, N≡N)

Physical properties

Property	Detail
State at room temperature	Gas
Colour	Colourless
Smell	Odourless
Boiling point	-196 C
Solubility	Slightly soluble in water
Reactivity	Very unreactive at room temperature

Composition of air

Gas	Percentage
Nitrogen	78%
Oxygen	21%
Argon	~0.9%
Carbon dioxide	~0.04%

The Haber process

The industrial synthesis of ammonia:

N2(g) + 3H2(g) ⇌ 2NH3(g)

Condition	Detail	Reason
Temperature	450 C	Compromise: higher temp increases rate but shifts equilibrium left
Pressure	200 atm	Higher pressure favours fewer moles (forward reaction, 4 -> 2)
Catalyst	Iron (Fe)	Increases rate of reaction without changing yield

This is a reversible reaction. The conditions are a compromise chosen by applying Le Chatelier’s principle.

Sources: N2 from fractional distillation of liquid air; H2 from natural gas (steam reforming of methane).

Nitrogen oxides as pollutants

Nitrogen oxides (NOx) form when nitrogen and oxygen react at the very high temperatures inside car engines and power stations:

N2(g) + O2(g) -> 2NO(g)

2NO(g) + O2(g) -> 2NO2(g)

See nitrogen oxides for their environmental effects:

• Cause acid rain (NO2 dissolves in rainwater to form nitric acid)
• Cause respiratory problems
• Catalytic converters remove them from exhaust: 2CO(g) + 2NO(g) -> 2CO2(g) + N2(g)

Uses of nitrogen

Use	Reason
Haber process	Raw material for ammonia
Food packaging	Unreactive, prevents oxidation
Cryogenics	Liquid N2 is extremely cold (-196 C)
Modified atmospheres	Keeps food fresh longer

Nitrogen cycle link

Nitrogen is cycled through the environment: N2 in air -> fixed by lightning or bacteria -> nitrates in soil -> taken up by plants -> returned to soil by decomposition -> converted back to N2 by denitrifying bacteria.

Key facts at a glance

Fact	Detail
Symbol	N
Proton number	7
Group / Period	15 (V) / 2
Bonding in the element	Diatomic N2 with a triple bond, N≡N
Key property	Very unreactive at room temperature
Main industrial use	Feedstock for the Haber process (ammonia)

Common exam mistakes

• Writing “N” instead of “N2”. Nitrogen gas is diatomic; every equation must use N2.
• Calling nitrogen a noble gas. It sits in Group 15, not Group 0. Its inertness comes from the strong N≡N triple bond, not a full outer shell.
• Saying the iron catalyst increases the yield of ammonia. A catalyst speeds up how fast equilibrium is reached; it does not shift the position of equilibrium or change the yield.
• Getting the temperature argument backwards. The forward reaction is exothermic, so a lower temperature would give a higher yield — the problem is that the rate becomes too slow, which is why 450 C is a compromise.

Exam-style questions

State the conditions used in the Haber process and explain why a temperature of 450 C is used rather than a lower one. (4 marks)

Mark scheme

• 450 C, 200 atm, iron catalyst [1]
• the forward reaction is exothermic, so a lower temperature would move the equilibrium to the right and give a higher yield [1]
• but at a lower temperature the rate of reaction is too slow / uneconomic [1]
• 450 C is a compromise between an acceptable rate and an acceptable yield [1]

Examiner note: the final mark needs “compromise” linked to both rate and yield — naming only one loses it.

Nitrogen and oxygen do not react in air at room temperature, yet oxides of nitrogen form inside a car engine. Explain how they form and why they cause acid rain. (3 marks)

Mark scheme

• the very high temperature inside the engine gives the molecules enough energy to react: N2(g) + O2(g) -> 2NO(g) [1]
• nitrogen monoxide is further oxidised in air: 2NO(g) + O2(g) -> 2NO2(g) [1]
• nitrogen dioxide dissolves in rainwater to form nitric acid, lowering the pH of the rain [1]

Examiner note: an equation alone will not score the last mark — you must state that NO2 dissolves to form an acid.