Skip to main content
SingaporeChemistrySyllabus dot point

Why is nitrogen so unreactive, and how do its key compounds behave and affect the environment?

Explain the unreactivity of nitrogen, describe the formation and basicity of ammonia, the industrial Haber process and the formation of nitrogen oxides, and discuss the environmental impact of nitrogen oxides and ammonium fertilisers

A focused answer to the H2 Chemistry learning outcome on nitrogen. The inertness of the N triple bond, the basicity of ammonia and its lone pair, the Haber process, the formation of nitrogen oxides in engines, and the environmental impact of NOx and nitrate fertilisers.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to explain the unreactivity of nitrogen, describe the formation and basicity of ammonia, outline the Haber process and the formation of nitrogen oxides, and discuss the environmental impact of nitrogen oxides and nitrogen-based fertilisers. The nitrogen-inertness explanation and the NOx environmental questions are reliable Paper 2 and Paper 3 content.

The answer

Why nitrogen is unreactive

Nitrogen gas, N2\text{N}_2, has a very strong triple bond (one sigma and two pi bonds) with a bond energy of about 945945 kJ per mol. Breaking this bond requires a large amount of energy, giving a high activation energy for most reactions. Nitrogen is also a non-polar molecule with no permanent dipole. Together these make N2\text{N}_2 extremely unreactive, which is why it makes up most of the atmosphere unchanged.

Ammonia: basicity and the lone pair

Ammonia, NH3\text{NH}_3, is trigonal pyramidal with a lone pair on nitrogen. That lone pair is the key to its chemistry:

  • As a Bronsted-Lowry base, it accepts a proton: NH3+H+β†’NH4+\text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+ (a dative bond forms).
  • As a ligand, it donates the lone pair to a metal ion to form complexes such as [Cu(NH3)4(H2O)2]2+[\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}.

Ammonia is a weak base in water: NH3+H2Oβ‡ŒNH4++OHβˆ’\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-.

The Haber process

Ammonia is made industrially by combining nitrogen (from air) and hydrogen (from natural gas) over an iron catalyst:

N2(g)+3H2(g)β‡Œ2NH3(g),Ξ”H<0\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g), \quad \Delta H < 0

Conditions are a compromise: about 200200 atmospheres (high pressure favours fewer gas moles, so more ammonia), about 450450 degrees Celsius (a balance between yield and rate), and an iron catalyst to speed the approach to equilibrium. Most of the ammonia is used to make fertilisers.

Formation of nitrogen oxides

At the high temperatures inside an engine, the normally inert nitrogen reacts with oxygen:

N2+O2β†’2NO,2NO+O2β†’2NO2\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}, \qquad 2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2

These nitrogen oxides (collectively NOx) are pollutants.

Environmental impact

  • Acid rain. NO2\text{NO}_2 dissolves in rainwater to form nitric acid, lowering the pH of rain and damaging buildings, soils and aquatic life.
  • Photochemical smog and respiratory harm. NOx and unburnt hydrocarbons react in sunlight to form smog containing irritants such as ozone.
  • Catalysis of acid rain. NO2\text{NO}_2 catalyses the oxidation of SO2\text{SO}_2 to SO3\text{SO}_3, worsening sulfuric acid formation.
  • Eutrophication from fertilisers. Excess nitrate and ammonium fertilisers wash into waterways, causing algal blooms that deplete oxygen and kill aquatic life.

A catalytic converter reduces NOx to harmless nitrogen while oxidising carbon monoxide: 2NO+2CO→N2+2CO22\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2.

Examples in context

Example 1. Feeding the world and its cost. The Haber process underpins modern agriculture by fixing atmospheric nitrogen into ammonia for fertiliser, supporting a large share of the global food supply. SEAB often pairs this benefit with the environmental cost of fertiliser runoff (eutrophication), asking candidates to evaluate the trade-off, a typical Paper 3 discussion.

Example 2. The catalytic converter as a redox device. In a three-way catalytic converter, nitrogen oxides are reduced to nitrogen while carbon monoxide and hydrocarbons are oxidised, all on a transition-metal surface. This ties nitrogen chemistry to redox and heterogeneous catalysis from the transition-element topic, a common cross-topic exam link.

Try this

Q1. Write the equation for the equilibrium when ammonia dissolves in water and state why ammonia is a weak base. [2 marks]

  • Cue. NH3+H2Oβ‡ŒNH4++OHβˆ’\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-; only partially ionised, so weak.

Q2. State the conditions used in the Haber process and explain why the temperature is a compromise. [3 marks]

  • Cue. About 200200 atm, 450450 degrees Celsius, iron catalyst; high TT favours rate but lowers yield (exothermic), so a moderate TT balances both.

Q3. Explain how nitrogen dioxide contributes to acid rain. [2 marks]

  • Cue. NO2\text{NO}_2 dissolves in rainwater forming nitric acid, lowering the pH of rain; it also catalyses SO2\text{SO}_2 oxidation.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Specimen (9729)3 marksExplain why nitrogen gas is very unreactive, and explain why ammonia is able to act as a base and as a ligand.
Show worked answer β†’

Nitrogen N2 has a triple bond (one sigma and two pi bonds) between the two atoms.

This N to N triple bond is very strong (bond energy about 945 kJ per mol) and requires a large amount of energy to break, giving a high activation energy for most reactions. Nitrogen is also non-polar with no permanent dipole. So N2 is very unreactive.

Ammonia, NH3, has a lone pair of electrons on the nitrogen atom. It can donate this lone pair to a proton (H+) to act as a Bronsted-Lowry base, or to a metal ion to act as a ligand forming a dative bond.

Markers reward the strong triple bond and high activation energy, and the lone pair on nitrogen enabling both base and ligand behaviour.

2023 (style)4 marksDescribe how oxides of nitrogen are formed in a car engine and explain two environmental problems they cause. State how a catalytic converter reduces these emissions.
Show worked answer β†’

In a car engine the high temperature of combustion causes nitrogen and oxygen from the air to react: N2 + O2 -> 2NO. Further oxidation in air gives NO2.

Environmental problems (any two):

  • Nitrogen oxides contribute to acid rain (NO2 dissolves to form nitric acid), damaging buildings, lakes and forests.
  • They contribute to photochemical smog and respiratory irritation.
  • NO2 catalyses the oxidation of SO2 to SO3, worsening acid rain.

A catalytic converter (Pt/Pd/Rh) reduces NOx to harmless nitrogen and oxidises CO to CO2: 2NO + 2CO -> N2 + 2CO2.

Markers reward the high-temperature N2 + O2 reaction, two valid environmental effects, and the catalytic converter reaction.

Related dot points