# Nitrogen and its compounds

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u11-nitrogen-and-its-compounds/

This sub-topic covers the chemistry of nitrogen, its oxides, ammonia, ammonium salts and nitrates. We explore industrial importance, environmental impacts and common exam reaction questions.

**Prerequisites:** [Oxidation states and redox reactions](https://www.owlsprep.com/study/cie-9701-u5-redox-reactions-and-electrochemistry/); [Covalent bonding and bond enthalpy](https://www.owlsprep.com/study/cie-9701-u3-chemical-bonding/); [Equilibrium principles](https://www.owlsprep.com/study/cie-9701-u7-equilibria/)

## Learning objectives

- Describe the properties of nitrogen and explain its inertness
- Explain the formation and environmental impact of nitrogen oxides
- Recall Haber process conditions and the chemistry of ammonia
- Distinguish between thermal decomposition reactions of nitrates
- Interpret oxidation states of nitrogen in different compounds

## Nitrogen gas: properties and reactivity

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

*Notation:* N_2

*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

> **Exam tip**
>
> Always link nitrogen's inertness to the high bond enthalpy of the N≡N bond, not just its position in the periodic table

## Oxides of nitrogen

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).

**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

> **info**
>
> Nitrous oxide (N₂O) from agricultural fertiliser use is a potent greenhouse gas that also contributes to ozone layer depletion

## Ammonia and the Haber process

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.

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

*Example:* The reaction equation is $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \ \Delta H = -92 \text{ kJ mol}^{-1}$

**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

> **Exam tip**
>
> Exam questions almost always ask for an explanation of each condition, not just the values, so always link to equilibrium and rate

## Ammonium salts and nitrates

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

   $$2Cu(NO_3)_2(s) \rightarrow 2CuO(s) + 4NO_2(g) + O_2(g)$$

## Common pitfalls

- **Wrong:** Claiming the iron catalyst in the Haber process increases the yield of ammonia
  - Why it fails: Catalysts only affect the rate of reaction, not the position of equilibrium, so they do not change yield
  - Correct: State that the catalyst speeds up the rate of reaction, allowing equilibrium to be reached faster, with no effect on yield
- **Wrong:** Attributing nitrogen's inertness to having a full outer shell of electrons
  - Why it fails: Each nitrogen atom in N₂ has a full outer shell, but the inertness is caused by the strong triple bond, not electron configuration
  - Correct: Explain inertness by the high bond enthalpy of the N≡N triple bond, which requires large energy input to break
- **Wrong:** Writing the wrong decomposition product for sodium nitrate, giving oxide, NO₂ and O₂
  - Why it fails: Only group 1 nitrates (below lithium) decompose to nitrites, not oxides
  - Correct: Write $2NaNO_3(s) \rightarrow 2NaNO_2(s) + O_2(g)$ for sodium nitrate decomposition
- **Wrong:** Only linking acid rain to sulfur dioxide, forgetting nitrogen oxides
  - Why it fails: Students often only associate SO₂ with acid rain, but NOₓ also contributes significantly to acid rain
  - Correct: Recognise that both sulfur dioxide and nitrogen oxides are causes of acid rain in exam answers

## 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 |

## 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.

- [Sulfur and its compounds](https://www.owlsprep.com/study/cie-9701-u11-sulfur-and-its-compounds/)
- [Introduction to organic chemistry](https://www.owlsprep.com/study/cie-9701-u12-overview/)
- [Nomenclature](https://www.owlsprep.com/study/cie-9701-u12-nomenclature/)

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