What are the main differences between nitrogen and sulfur in terms of their chemical properties?

Nitrogen and sulfur differ significantly in their chemical properties. Nitrogen is a non-metal with a diatomic molecule (N2) and is known for its ability to form strong triple bonds, making it relatively inert. It is a key component of the atmosphere and is essential for the formation of amino acids and nucleic acids. Sulfur, on the other hand, is also a non-metal but exists in various allotropes, with S8 being the most stable form. It is more reactive than nitrogen and is known for forming compounds like sulfur dioxide (SO2) and sulfuric acid (H2SO4). These differences are crucial for understanding their roles in chemical reactions and biological systems, as covered in the Cambridge International A Levels Chemistry curriculum.

How does the nitrogen cycle contribute to the environment?

The nitrogen cycle is a crucial ecological process that converts nitrogen from the atmosphere into forms usable by living organisms and back again. It involves several stages: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Nitrogen fixation, often facilitated by bacteria, converts atmospheric nitrogen (N2) into ammonia (NH3), which can be used by plants. Nitrification converts ammonia into nitrites and nitrates, which plants absorb as nutrients. Assimilation incorporates these compounds into plant and animal tissues. Ammonification and denitrification return nitrogen to the atmosphere. This cycle is vital for maintaining ecosystem productivity and is a key topic in AS-Level Inorganic Chemistry.

What are the industrial applications of sulfur and its compounds?

Sulfur and its compounds have numerous industrial applications. Sulfuric acid (H2SO4) is one of the most widely produced chemicals globally, used in the manufacture of fertilizers, explosives, and in petroleum refining. Sulfur dioxide (SO2) is used in the production of sulfuric acid and as a preservative in food and wine. Elemental sulfur is used in vulcanizing rubber, producing fungicides, and in the paper industry. Understanding these applications is essential for students studying Chemistry at the Cambridge International A Levels, as it highlights the practical importance of sulfur in various industries.

Why is nitrogen fixation important for agriculture?

Nitrogen fixation is a critical process for agriculture because it converts atmospheric nitrogen, which plants cannot use, into ammonia or related compounds that plants can absorb and utilize for growth. This process is primarily carried out by symbiotic bacteria in the root nodules of legumes and some free-living bacteria. By enriching the soil with nitrogen compounds, nitrogen fixation enhances soil fertility and crop yields. This topic is an integral part of the AS-Level Inorganic Chemistry syllabus, as it connects chemical processes with real-world agricultural practices.

What environmental issues are associated with sulfur compounds?

Sulfur compounds, particularly sulfur dioxide (SO2), are associated with several environmental issues. When released into the atmosphere, SO2 can lead to acid rain, which harms ecosystems, corrodes buildings, and affects water quality. Sulfur compounds also contribute to air pollution and respiratory problems in humans. The study of these environmental impacts is crucial for students in the Cambridge International A Levels Chemistry course, as it emphasizes the importance of understanding the chemical behavior and consequences of sulfur in the environment.

Why is "Giving only one reason for nitrogen's unreactivity." a common mistake in Nitrogen and sulphur?

Why it happens: Stopping at 'strong bond'. How to avoid it: State both the strong triple bond AND the lack of polarity. Source: 9701 Examiner Reports 2022-2024

Why is "Not showing NO₂ regenerated in SO₂ oxidation." a common mistake in Nitrogen and sulphur?

Why it happens: Writing only the first step. How to avoid it: Include the step that re-forms NO₂ to show it is a catalyst.

Why is "Blaming acid rain on carbon dioxide." a common mistake in Nitrogen and sulphur?

Why it happens: Confusing acid rain with general 'pollution'. How to avoid it: Acid rain is from SO₂ and NOₓ (strong acids); CO₂ only makes rain weakly acidic naturally.

Why is "Thinking the catalytic converter removes CO₂." a common mistake in Nitrogen and sulphur?

Why it happens: Lumping all car emissions together. How to avoid it: The converter removes CO, NOₓ and unburnt hydrocarbons — it actually produces CO₂.

Nitrogen and sulfur(AS-Level Inorganic Chemistry)Cambridge International A Level Chemistry 9701

Nitrogen and sulphur

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Nitrogen and Sulfur — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Why nitrogen is so unreactive, the basicity of ammonia and the ammonium ion, and the environmental chemistry of the oxides of nitrogen — their formation, catalytic removal, role in photochemical smog and contribution to acid rain.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

12.1.1 — Explain the lack of reactivity of nitrogen, with reference to triple bond strength and lack of polarity.
12.1.2 — Describe and explain: (a) the basicity of ammonia (Brønsted–Lowry); (b) the structure of the ammonium ion and its formation by an acid–base reaction; (c) the displacement of ammonia from ammonium salts by an acid–base reaction.
12.1.3 — State and explain the natural and man-made occurrences of oxides of nitrogen and their catalytic removal from car exhaust gases.
12.1.4 — Understand that atmospheric oxides of nitrogen react with unburned hydrocarbons to form peroxyacetyl nitrate (PAN), a component of photochemical smog.
12.1.5 — Describe the role of NO and NO₂ in acid rain, both directly and in the catalytic oxidation of atmospheric sulfur dioxide.

The unreactivity of nitrogen

A very strong triple bond plus no polarity makes N₂ hard to attack.

Nitrogen gas (N₂) is very unreactive for two reasons:

Strong triple bond: the N≡N bond has a very high bond energy (≈ 994 kJ mol⁻¹). A large amount of energy is needed to break it, so reactions have a high activation energy.
Lack of polarity: N₂ is a non-polar molecule (identical atoms), so it is not attacked by polar reagents or readily attracted to charged species.

This is why nitrogen makes up most of the air yet reacts only under extreme conditions (e.g. high temperature/pressure in the Haber process, or in lightning/engines).

Very strong N≡N triple bond (high bond energy).
Non-polar molecule → not attacked by polar reagents.
High activation energy → unreactive.

Ammonia and the ammonium ion

Ammonia's lone pair makes it a base; donating it to H⁺ forms the tetrahedral NH₄⁺.

Basicity of ammonia. The nitrogen in NH₃ has a lone pair of electrons that can accept a proton (H⁺) — so ammonia is a Brønsted–Lowry base: $\text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+$ (In water: $\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$ — a weak base.)

The ammonium ion, NH₄⁺. The N lone pair forms a coordinate (dative) bond with H⁺. The ion is tetrahedral (109.5°) with four identical N–H bonds (the dative bond is indistinguishable once formed).

Displacement of ammonia from its salts. A base displaces ammonia from an ammonium salt (an acid–base reaction): $\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}$ e.g. warming an ammonium salt with NaOH releases ammonia gas (turns damp red litmus blue) — the test for ammonium ions.

NH₃ lone pair accepts H⁺ → Brønsted–Lowry base.
NH₄⁺: tetrahedral, dative bond, four identical N–H bonds.
NH₄⁺ + OH⁻ → NH₃ + H₂O (test for ammonium ions).

Oxides of nitrogen: formation and catalytic removal

Made by lightning and hot engines; catalytic converters turn them back into N₂.

Formation of oxides of nitrogen (NOₓ):

Natural: lightning provides enough energy for $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ .
Man-made: the high temperature inside car engines makes nitrogen and oxygen from the air combine: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ NO is then oxidised in air: $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$ .

Catalytic removal. A catalytic converter (containing Pt/Pd/Rh) removes these pollutants by reacting NO with carbon monoxide: $2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2$ turning two harmful gases (NO and CO) into harmless N₂ and CO₂.

Lightning and hot engines: N₂ + O₂ → 2NO.
2NO + O₂ → 2NO₂.
Catalytic converter: 2NO + 2CO → N₂ + 2CO₂.

Photochemical smog and acid rain

NOₓ + hydrocarbons → PAN smog; NO₂ makes acid rain and catalyses SO₂ oxidation.

Photochemical smog. In sunlight, oxides of nitrogen react with unburned hydrocarbons from incomplete combustion to form peroxyacetyl nitrate (PAN) — a key component of photochemical smog, which irritates eyes and lungs.

Acid rain — two roles of NOₓ:

Directly: NO₂ dissolves in rainwater to form nitric acid: $3\text{NO}_2 + \text{H}_2\text{O} \rightarrow 2\text{HNO}_3 + \text{NO}$
As a catalyst for the oxidation of atmospheric sulfur dioxide to sulfur trioxide (which then forms sulfuric acid): $\text{NO}_2 + \text{SO}_2 \rightarrow \text{NO} + \text{SO}_3$ $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2 \;(\text{NO}_2 \text{ regenerated})$ $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$ NO₂ is not used up overall (it is regenerated) — it acts as a homogeneous catalyst for the conversion of SO₂ into sulfuric acid.

NOₓ + unburned hydrocarbons → PAN (photochemical smog).
NO₂ + water → HNO₃ (acid rain directly).
NO₂ catalyses SO₂ → SO₃ → H₂SO₄ (regenerated, homogeneous catalyst).

How it’s examined

Nitrogen's unreactivity, ammonia/ammonium chemistry, and the environmental equations (engine NO, catalytic converter, acid rain) are common Paper 2 marks. Examiner reports flag explaining N₂'s unreactivity by only one factor, and not recognising NO₂ as a regenerated catalyst in SO₂ oxidation.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 12.1; 9701 Examiner Reports 2022-2024; 9701/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Nitrogen and sulphur

Step-by-step solutions to past-paper-style questions on nitrogen and sulphur, written exactly the way a tutor would explain them at the board.

1Why nitrogen is unreactive (2 marks)
Getting started• unreactivity
Question
Explain why nitrogen gas is unreactive.
Step-by-step solution
1. Step 1
  Strong N≡N triple bond with a very high bond energy → hard to break.
2. Step 2
  Non-polar molecule → not attacked by polar reagents.
Answer
Strong triple bond (high bond energy) and non-polar molecule.
Examiner tip
Both reasons are needed for full marks.
2Test for ammonium ions (2 marks)
Getting started• ammonium
Question
Describe how you would test for ammonium ions and give the equation.
Step-by-step solution
1. Step 1
  Warm with NaOH(aq); ammonia gas is released (turns damp red litmus blue).
2. Step 2
  Equation:
  $\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}$
Answer
Warm with NaOH → NH₃ (damp red litmus turns blue); NH₄⁺ + OH⁻ → NH₃ + H₂O.
3Basicity of ammonia (3 marks)
Building confidence• ammonia, base
Question
Explain why ammonia acts as a base and write the equation for its reaction with hydrochloric acid.
Step-by-step solution
1. Step 1
  Nitrogen has a lone pair that can accept a proton (H⁺).
2. Step 2
  Accepting H⁺ makes it a Brønsted–Lowry base (proton acceptor).
3. Step 3
  Reaction with acid:
  $\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}$
Answer
The N lone pair accepts H⁺ (Brønsted–Lowry base); NH₃ + HCl → NH₄Cl.
Examiner tip
The lone pair on nitrogen is the reason ammonia is basic — link it to proton acceptance.
4Catalytic converter reaction (3 marks)
Building confidence• NOx
Question
Nitrogen monoxide forms in car engines. Write the equation for its formation and the equation for its removal in a catalytic converter, and say why removal matters.
Step-by-step solution
1. Step 1
  Formation: the high engine temperature makes N₂ and O₂ react.
  $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$
2. Step 2
  Removal in the converter:
  $2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2$
3. Step 3
  Why: NO causes acid rain and photochemical smog; CO is toxic — both are removed.
Answer
N₂ + O₂ → 2NO (engine); 2NO + 2CO → N₂ + 2CO₂ (converter); removes a smog/acid-rain gas and toxic CO.
Examiner tip
The converter turns two pollutants (NO and CO) into harmless N₂ and CO₂ in one reaction.
5NO₂ catalysing SO₂ oxidation (4 marks)
Stretch• acid rain, catalyst
Question
Show, with equations, how NO₂ acts as a catalyst for the oxidation of SO₂ to sulfuric acid in the atmosphere. (4)
Step-by-step solution
1. Step 1
  NO₂ oxidises SO₂ to SO₃ (NO₂ → NO).
  $\text{NO}_2 + \text{SO}_2 \rightarrow \text{NO} + \text{SO}_3$
2. Step 2
  NO is re-oxidised to NO₂ (regenerated).
  $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$
3. Step 3
  SO₃ + water → sulfuric acid.
  $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$
4. Step 4
  NO₂ is regenerated, so it is a catalyst (used in step 1, reformed in step 2).
Answer
NO₂ + SO₂ → NO + SO₃; 2NO + O₂ → 2NO₂ (regenerated); SO₃ + H₂O → H₂SO₄.
Examiner tip
Show NO₂ being regenerated to demonstrate it is a catalyst.
6Acid rain: causes and control (4 marks)
Stretch• acid rain, desulfurisation
Question
Explain how SO₂ causes acid rain, give one environmental effect, and write an equation for removing SO₂ from flue gases. (4)
Step-by-step solution
1. Step 1
  SO₂ (from burning sulfur-containing fuels) is oxidised and dissolves to form sulfuric/sulfurous acid.
  $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$
2. Step 2
  Effect: acidifies lakes (killing fish), damages forests, and erodes limestone buildings.
3. Step 3
  Control — flue gas desulfurisation: SO₂ is removed with calcium carbonate (or CaO).
  $\text{CaCO}_3 + \text{SO}_2 \rightarrow \text{CaSO}_3 + \text{CO}_2$
Answer
SO₂ → H₂SO₄ acid rain (acidifies lakes/erodes limestone); removed by CaCO₃ + SO₂ → CaSO₃ + CO₂.
Examiner tip
Name a specific effect AND a removal equation — both are needed for full marks.

Key Definitions and Keywords — Nitrogen and sulphur

Definitions to memorise and the exact keywords mark schemes credit for nitrogen and sulphur answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Unreactivity of nitrogen
Examiner keyword
Due to the strong N≡N triple bond (high bond energy) and the non-polar nature of the molecule.
Basicity of ammonia
Examiner keyword
Ammonia acts as a Brønsted–Lowry base because the lone pair on nitrogen accepts a proton (H⁺).
Oxides of nitrogen (NOₓ)
Examiner keyword
NO and NO₂, formed in engines/lightning (N₂ + O₂ → 2NO); they cause acid rain and photochemical smog and catalyse SO₂ oxidation.
Acid rain
Examiner keyword
Rain made acidic by dissolved SO₂/SO₃ (→ H₂SO₄) and NOₓ (→ HNO₃); it acidifies water and erodes limestone.
Flue gas desulfurisation
Examiner keyword
Removing SO₂ from power-station gases using calcium carbonate or calcium oxide (CaCO₃ + SO₂ → CaSO₃ + CO₂).

Common Mistakes and Misconceptions — Nitrogen and sulphur

The traps other students keep falling into on nitrogen and sulphur questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Giving only one reason for nitrogen's unreactivity.
9701 Examiner Reports 2022-2024
Why it happens
Stopping at 'strong bond'.
How to avoid it
State both the strong triple bond AND the lack of polarity.
✕Not showing NO₂ regenerated in SO₂ oxidation.
Why it happens
Writing only the first step.
How to avoid it
Include the step that re-forms NO₂ to show it is a catalyst.
✕Blaming acid rain on carbon dioxide.
Why it happens
Confusing acid rain with general 'pollution'.
How to avoid it
Acid rain is from SO₂ and NOₓ (strong acids); CO₂ only makes rain weakly acidic naturally.
✕Thinking the catalytic converter removes CO₂.
Why it happens
Lumping all car emissions together.
How to avoid it
The converter removes CO, NOₓ and unburnt hydrocarbons — it actually produces CO₂.

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