Nitrogen and its compounds

CIE A-Level Chemistry· Unit 11: Nitrogen and sulfur, Topic 1· 25 min read

1. Nitrogen gas: properties and reactivity★★☆☆☆⏱ 7 min

📘 Definition

Nitrogen

N_2

Group 15 non-metal, existing as diatomic N₂ molecules that make up ~78% of Earth's atmosphere by volume

Example:

Biological nitrogen fixation by bacteria converts atmospheric N₂ into usable nitrogen compounds for plants

Nitrogen's low reactivity (inertness) is the most commonly tested property of the element, and it is almost always linked to the strength of the triple bond between the two nitrogen atoms.

📐 Worked Example

Explain why nitrogen reacts with oxygen in car engines but not under standard conditions

1
Step 1: State the bond enthalpy of the N≡N triple bond
$Δ H (N \equiv N) = 945 \text{ kJ mol}^{-1}$
2
Step 2: Relate bond enthalpy to activation energy
3
The high bond enthalpy means the reaction has a very high activation energy, which cannot be overcome at standard temperatures
4
Step 3: Explain the conditions in car engines
5
Temperatures exceed 1000°C inside engine cylinders, providing enough energy to overcome the activation energy, so NO forms

2. Oxides of nitrogen★★★☆☆⏱ 8 min

Nitrogen forms oxides with oxidation states ranging from +1 (N₂O) to +5 (N₂O₅). The most environmentally significant oxides are nitrogen(II) oxide (NO) and nitrogen(IV) oxide (NO₂).

📘 Definition

NOₓ

Mixture of nitrogen oxides (mostly NO and NO₂) formed from high-temperature reaction of N₂ and O₂ in combustion engines and power stations

📐 Worked Example

Write balanced equations to show how NO₂ contributes to acid rain

1
Step 1: Oxidation of NO₂ with water and oxygen forms nitric acid
$4NO_2(g) + 2H_2O(l) + O_2(g) \rightarrow 4HNO_3(aq)$
2
Step 2: Alternate disproportionation reaction of NO₂
$2NO_2(g) + H_2O(l) \rightarrow HNO_3(aq) + HNO_2(aq)$
3
Step 3: Both acids dissociate in water to release H⁺ ions, lowering the pH of rain to below 5.6

3. Ammonia and the Haber process★★★☆☆⏱ 8 min

Ammonia (NH₃, oxidation state of N = -3) is one of the most widely produced industrial chemicals, used to make fertilisers, nitric acid and explosives. It is manufactured via the reversible Haber process.

📘 Definition

Haber process

Industrial synthesis of ammonia from nitrogen (air) and hydrogen (natural gas) using compromise reaction conditions

Example:

The reaction equation is $N_{2} (g) + 3 H_{2} (g) ⇌ 2 N H_{3} (g) Δ H = - 92 kJ mol^{- 1}$

📐 Worked Example

State and explain the typical conditions used in the Haber process

1
Step 1: State the conditions: 450°C, 200 atm, finely divided iron catalyst
2
Step 2: Explain temperature: The forward reaction is exothermic, so low temperature gives higher yield but slow rate. 450°C is a compromise yield and rate
3
Step 3: Explain pressure: Higher pressure shifts equilibrium right (4 moles reactant gas → 2 moles product gas) increasing yield. 200 atm is a compromise between yield, cost and safety
4
Step 4: Explain catalyst: Iron catalyst lowers activation energy, increasing rate, so equilibrium is reached faster, no effect on yield

4. Ammonium salts and nitrates★★☆☆☆⏱ 7 min

Ammonium salts form when ammonia acts as a base and reacts with acids. Most ammonium salts and nitrates are soluble in water, making them ideal for use as nitrogen fertilisers. Thermal decomposition of nitrates follows different patterns based on the metal cation.

📐 Worked Example

Write balanced equations for the thermal decomposition of potassium nitrate and copper(II) nitrate

1
Step 1: Group 1 nitrates (except lithium nitrate) decompose to form nitrite and oxygen
$2KNO_3(s) \rightarrow 2KNO_2(s) + O_2(g)$
2
Step 2: All other nitrates (group 2, transition metals, lithium) decompose to form metal oxide, nitrogen dioxide and oxygen
$2Cu(NO_3)_2(s) \rightarrow 2CuO(s) + 4NO_2(g) + O_2(g)$

5. Common Pitfalls

Wrong move:

Claiming the iron catalyst in the Haber process increases the yield of ammonia

Why:

Catalysts only affect the rate of reaction, not the position of equilibrium, so they do not change yield

Correct move:

State that the catalyst speeds up the rate of reaction, allowing equilibrium to be reached faster, with no effect on yield

Wrong move:

Attributing nitrogen's inertness to having a full outer shell of electrons

Why:

Each nitrogen atom in N₂ has a full outer shell, but the inertness is caused by the strong triple bond, not electron configuration

Correct move:

Explain inertness by the high bond enthalpy of the N≡N triple bond, which requires large energy input to break

Wrong move:

Writing the wrong decomposition product for sodium nitrate, giving oxide, NO₂ and O₂

Why:

Only group 1 nitrates (below lithium) decompose to nitrites, not oxides

Correct move:

Write $2 N a N O_{3} (s) \to 2 N a N O_{2} (s) + O_{2} (g)$ for sodium nitrate decomposition

Wrong move:

Only linking acid rain to sulfur dioxide, forgetting nitrogen oxides

Why:

Students often only associate SO₂ with acid rain, but NOₓ also contributes significantly to acid rain

Correct move:

Recognise that both sulfur dioxide and nitrogen oxides are causes of acid rain in exam answers

6. Quick Reference Cheatsheet

Compound	Formula	Oxidation state of N	Key property
Nitrogen gas	N₂	0	Inert, strong triple bond
Ammonia	NH₃	-3	Weak base, Haber product
Nitrogen monoxide	NO	+2	Formed in car engines
Nitrogen dioxide	NO₂	+4	Causes acid rain
Nitric acid	HNO₃	+5	Strong acid, oxidising agent
Potassium nitrate	KNO₃	+5	Decomposes to nitrite
Copper(II) nitrate	Cu(NO₃)₂	+5	Decomposes to oxide + NO₂

7. Frequently Asked

Why is nitrogen gas relatively unreactive?

Nitrogen contains a strong triple covalent bond ( $N \equiv N$ ) with a very high bond enthalpy (≈945 kJ mol⁻¹), so a large input of energy is required to break the bond for reactions to occur.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 2
Haber process conditions explanation
2022 · 1
Nitrogen oxides environmental impact
2021 · 2
Thermal decomposition of nitrates

Going deeper

syllabusCIE 9701 2025-2026 SyllabusOfficial syllabus document

What's Next

Understanding nitrogen chemistry is critical for both inorganic and environmental chemistry topics in CIE A-Level Chemistry. The Haber process is one of the most important industrial reactions globally, supporting modern agriculture through fertiliser production, and is a common exam question linking equilibrium and rate concepts. Nitrogen oxides and nitrate runoff also have major environmental impacts, connecting to atmospheric chemistry and pollution, which are frequently tested in multiple choice and structured questions. Next you will explore sulfur and its compounds, which also links to acid rain and environmental impacts, building on the concepts you learned here.