Nitrogen and Sulfur

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Explain the lack of reactivity of nitrogen, with reference to triple bond strength and lack of polarity
  2. Describe and explain:
    • (a) the basicity of ammonia, using the Brønsted–Lowry theory
    • (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
  3. State and explain the natural and man-made occurrences of oxides of nitrogen and their catalytic removal from the exhaust gases of internal combustion engines
  4. Understand that atmospheric oxides of nitrogen (NO and NO₂) can react with unburned hydrocarbons to form peroxyacetyl nitrate, PAN, which is a component of photochemical smog
  5. Describe the role of NO and NO₂ in the formation of acid rain both directly and in their catalytic role in the oxidation of atmospheric sulfur dioxide

1. Lack of Reactivity of Nitrogen

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Why is Nitrogen So Unreactive?

Nitrogen gas (N₂) makes up 78% of the air we breathe, yet it hardly reacts with anything. This is unusual because most gases are quite reactive. Two main reasons explain why nitrogen is so unreactive:

Reason 1: Very Strong Triple Bond

The nitrogen molecule contains a triple covalent bond.

  • Each nitrogen atom has 5 electrons in its outer shell
  • A nitrogen atom needs 3 more electrons to fill its outer shell
  • Two nitrogen atoms share 3 pairs of electrons between them, forming a triple bond (written as N≡N)
  • This triple bond is extremely strong — it has a bond enthalpy (bond strength) of about 1000 kJ mol⁻¹
  • This means you need to supply 1000 kJ of energy to break one mole of nitrogen triple bonds

Why does this matter?

  • Because the triple bond is so strong, it's very difficult to break nitrogen molecules apart
  • Most chemical reactions require breaking bonds first, then making new ones
  • The high energy needed to break the N≡N bond means nitrogen won't react under normal conditions
  • Only under extreme conditions (like lightning strikes or very high temperatures in car engines) will nitrogen react

Reason 2: Lack of Polarity

The nitrogen molecule is non-polar.

  • A polar molecule is one where electrons are not shared equally, creating a slight positive charge on one end and a slight negative charge on the other
  • In N₂, both atoms are nitrogen, so they have exactly the same electronegativity (ability to attract electrons)
  • The electrons in the triple bond are shared completely equally between the two nitrogen atoms
  • There is no charge difference anywhere in the molecule — it is non-polar

Why does this matter?

  • Polar molecules are attracted to other molecules (especially polar ones) and tend to react more easily
  • Non-polar molecules like N₂ are not attracted to or likely to react with other molecules
  • This lack of polarity makes nitrogen even more unreactive

Summary

Nitrogen is very unreactive because:

  1. It has a very strong triple bond (1000 kJ mol⁻¹) that is hard to break
  2. It is non-polar, so it's not attracted to other molecules

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