AI TutorBetaTopicalsYearlysNotesDashboardPricing
Notes/Chemistry/Paper 1/Nitrogen and Sulfur
CAIEAS Level9701§12.1

Nitrogen and Sulfur

Why nitrogen gas hardly reacts, how ammonia acts as a base and forms the ammonium ion, and how oxides of nitrogen from engines and lightning cause photochemical smog and acid rain, including their role as a catalyst in oxidising sulfur dioxide.

150 min read 8 sub-topics
97
question parts
2021–2025 · 37 papers
3 marks
per paper
≈ 3% of the paper
1.6/3
avg difficulty
moderate
#19
most examined
of 23 topics by marks

In the AS Group 14 note you followed a group of elements down the Periodic Table. This note looks closely at one element, nitrogen, and at how its oxides and sulfur dioxide pollute the air. It uses bond strength, Brønsted–Lowry acids and bases, and oxidation numbers from earlier AS notes.

You start with why nitrogen gas hardly reacts. Next you meet ammonia as a base and the ammonium ion it forms. Then you see how oxides of nitrogen form in car engines and lightning, and how a catalytic converter removes them. Finally you follow these oxides into photochemical smog and acid rain, where they help turn sulfur dioxide into sulfuric acid. Three short sections give background from the older syllabus: uses of ammonia, fertilisers, and where sulfur dioxide comes from.

Before you start you should be able to

  • Brønsted–Lowry acids and bases, and weak bases (AS Equilibria note: “Brønsted–Lowry acids and bases” and “Strong vs weak acids and bases; the pH scale”)

  • Coordinate (dative covalent) bonds and bond energy (AS Chemical Bonding note: “Covalent and coordinate bonding, octet expansion, bond energy and length”)

  • σ and π bonds, and polar and non-polar bonds (AS Chemical Bonding note: “σ and π bonds, and hybridisation” and “Bond polarity and dipole moments”)

  • The shapes and bond angles of NH₃ and NH₄⁺ (AS Chemical Bonding note: “Shapes of molecules: VSEPR theory”)

  • Oxidation numbers, and oxidising and reducing agents (AS Electrochemistry note: “Oxidation numbers: the rules, and how to use them” and “Redox, and oxidising and reducing agents”)

  • Activation energy and what a catalyst does (AS Reaction Kinetics note: “Activation energy and the Boltzmann distribution” and “Catalysts and reaction pathway diagrams”)

  • Relative formula mass and moles from mass (AS Atoms, Molecules and Stoichiometry note: “Relative atomic, isotopic, molecular and formula mass” and “Reacting masses, percentage yield and the limiting reagent”)

By the end of this page you can

  • Explain the lack of reactivity of nitrogen gas in terms of the strength of its triple bond and its lack of polarity

  • Describe and explain the basicity of ammonia using Brønsted–Lowry theory, and the structure of the ammonium ion and its formation by an acid–base reaction

  • Explain why the ammonium ion is a weak Brønsted–Lowry acid, so ammonium salts make water slightly acidic

  • Describe the displacement of ammonia from ammonium salts by a stronger base, and use it as the test for the ammonium ion

  • Compare ammonium-salt fertilisers by their percentage of nitrogen by mass and their effect on soil pH

  • State the natural and man-made sources of the oxides of nitrogen, and describe their catalytic removal from car exhausts with equations and redox roles

  • Explain that NO and NO₂ react with unburnt hydrocarbons to form PAN, a component of photochemical smog

  • Describe the role of NO and NO₂ in acid rain, both directly and as catalysts in the oxidation of atmospheric sulfur dioxide, with equations

  • Optional background: state the main uses of ammonia, describe eutrophication, and explain how burning fossil fuels releases sulfur dioxide

01

Why nitrogen gas is so unreactive

Syllabus requirement · §12.1.1

“

explain the lack of reactivity of nitrogen, with reference to triple bond strength and lack of polarity.

”

A gas that is everywhere and does almost nothing

Nitrogen gas, N2\text{N}_2, is about 78% of the air. It does not burn, it does not help other things burn, and at room temperature it reacts with almost nothing. This section explains why. Later sections show the special conditions, such as the inside of a car engine, where it does react.

Two separate reasons

A full answer needs both of these reasons.

1. The triple bond is very strong. A nitrogen atom (1s22s22p31s^2 2s^2 2p^3) has three unpaired 2p2p electrons. Two nitrogen atoms share three pairs of electrons, which makes a triple bond: one σ\sigma bond and two π\pi bonds. The bond energy of N≡N\text{N}{\equiv}\text{N} is 944 kJ mol−1944\ \text{kJ mol}^{-1} (Data Booklet), one of the largest of any covalent bond.

2. The molecule is non-polar. Both atoms are nitrogen, so they have the same electronegativity and share the electrons equally. The molecule has no δ+\delta+ end and no δ−\delta- end, so nothing attracts a charged or polar reagent towards it.

bond energy / kJ mol⁻¹242Cl–Cl350C–C496O=O944N≡NN≡N = 1σ + 2π in one short bond: nearly 3 × C–C and almost 2 × O=O.

The N≡N bond energy (944 kJ mol⁻¹) is much larger than a typical single bond (C–C, 350) and a double bond (O=O, 496).

From bond strength to a slow reaction

To react, N2\text{N}_2 must start breaking its triple bond. Because the bond is so strong, the activation energy is very high. At room temperature almost no collisions have that much energy (AS Reaction Kinetics note, “Activation energy and the Boltzmann distribution”), so the reaction is far too slow to notice.

The mark scheme wording to learn: "N2\text{N}_2 molecules have a strong triple (covalent) bond" and "N2\text{N}_2 molecules are non-polar". One mark each.

Reasons that score nothing

Multiple-choice questions often offer these wrong reasons. Learn why each is wrong.

  • "Nitrogen has a full outer shell." O2\text{O}_2 and Cl2\text{Cl}_2 also have full outer shells in their molecules, and both react readily.
  • "There are no lone pairs in the molecule." Each nitrogen atom in N2\text{N}_2 has one lone pair.
  • "The strong double bond." It is a triple bond.
  • "Strong dipole–dipole forces between the atoms." The atoms are held by a covalent bond, not by intermolecular forces, and N2\text{N}_2 has no permanent dipole.

Comparing two gases

Explain why oxygen gas, O2\text{O}_2, is much more reactive than nitrogen gas, N2\text{N}_2. (Bond energies: O=O\text{O}{=}\text{O} 496 kJ mol−1496\ \text{kJ mol}^{-1}; N≡N\text{N}{\equiv}\text{N} 944 kJ mol−1944\ \text{kJ mol}^{-1}.)

Show full working
  1. 1

    Name each bond. O2\text{O}_2 has a double bond (one σ\sigma + one π\pi). N2\text{N}_2 has a triple bond (one σ\sigma + two π\pi).

    Say what each bond is before you compare them. The comparison then follows from the facts.

  2. 2

    Compare the bond energies. 496 kJ mol−1496\ \text{kJ mol}^{-1} is about half of 944 kJ mol−1944\ \text{kJ mol}^{-1}, so the O=O\text{O}{=}\text{O} bond is much weaker.

    Use the numbers you are given. A comparison backed by data is clearer than just writing "stronger".

  3. 3

    Link bond strength to activation energy. A weaker bond is easier to start breaking, so reactions of O2\text{O}_2 have lower activation energies. More collisions at room temperature have enough energy, so O2\text{O}_2 reacts faster.

    This is the step that turns a fact about bonds into a fact about reactivity. Students often stop after the bond energies.

  4. 4

    Check the second reason. Both molecules are non-polar, so polarity does not explain the difference here. Bond strength alone does.

    Only use a reason when it actually applies. When N2\text{N}_2 is compared with reactive substances in general, polarity is the second mark.

Answer

The O=O\text{O}{=}\text{O} double bond (496 kJ mol−1496\ \text{kJ mol}^{-1}) is much weaker than the N≡N\text{N}{\equiv}\text{N} triple bond (944 kJ mol−1944\ \text{kJ mol}^{-1}), so O2\text{O}_2 reactions have a lower activation energy and more collisions succeed. Both molecules are non-polar, so polarity does not cause the difference.

When two non-polar molecules are compared, bond strength is the only reason. When nitrogen's reactivity is explained on its own, give both bond strength and non-polarity.

Explaining the lack of reactivity of N₂

9701/23 M/J 2024 Q1(a)(i)2 marks

Explain the lack of reactivity of nitrogen gas, N₂(g).

Show full working
  1. 1

    Bond strength. N2\text{N}_2 molecules have a strong triple (covalent) bond.

    Say "triple". Writing only "strong bond" or "strong double bond" loses the mark.

  2. 2

    Polarity. N2\text{N}_2 molecules are non-polar.

    This is a separate mark. A long answer about the triple bond alone scores only 1 of the 2 marks.

Answer

N2\text{N}_2 molecules have a strong triple (covalent) bond, and N2\text{N}_2 molecules are non-polar.

Learn the two short phrases, not a paragraph: strong triple bond; non-polar molecule.

Why nitrogen and oxygen do not react in the air

9701/21 M/J 2020 Q3(c)2 marks

Nitrogen and oxygen do not react at normal atmospheric temperatures.

Explain why.

Show full working
  1. 1

    Name the obstacle. Nitrogen has a (strong) triple bond.

    The first mark is for the triple bond itself.

  2. 2

    Link it to temperature. A lot of energy is needed to break this bond. At normal temperatures there is not enough energy to overcome the activation energy.

    Here the second mark is about energy, not polarity, because the question is about temperature. Read what the question is steering you towards.

Answer

Nitrogen has a triple bond; high energy is needed to break it, and at normal temperatures there is not enough energy to overcome the activation energy.

"Why is N₂ unreactive?" → triple bond + non-polar. "Why don't N₂ and O₂ react at room temperature?" → triple bond + not enough energy to overcome the activation energy.

Your turn

  1. 19701/22 F/M 2024 Q3(a)2 marks

    Give two reasons to explain the lack of reactivity of nitrogen.

    Stuck? Show hint

    One reason is about the bond itself; the other is about how the electrons are shared across the molecule.

    Show solution
    1. 1

      Nitrogen has a strong triple bond (a high triple bond enthalpy).

      The word "triple" is needed.

    2. 2

      The N2\text{N}_2 molecule is non-polar.

      Identical atoms share the electrons equally, so there is no δ+ or δ− end.

    Answer

    Strong triple bond / high triple bond enthalpy; non-polar molecule.

  2. 29701/22 O/N 2024 Q3(f)(ii)1 mark

    At very high temperatures, phosphorus can form P2\text{P}_2 molecules. P2\text{P}_2 contains a triple bond, P≡P\text{P}{\equiv}\text{P}.

    The bond energy of P≡P\text{P}{\equiv}\text{P} is 485 kJ mol−1485\ \text{kJ mol}^{-1}. The bond energy of N≡N\text{N}{\equiv}\text{N} is 944 kJ mol−1944\ \text{kJ mol}^{-1}.

    Compare the reactivity of P2\text{P}_2 and N2\text{N}_2. Explain your answer.

    Stuck? Show hint

    Both molecules are non-polar, so only one factor differs.

    Show solution
    1. 1

      Compare the bonds: 485 kJ mol−1485\ \text{kJ mol}^{-1} is much less than 944 kJ mol−1944\ \text{kJ mol}^{-1}, so the P≡P\text{P}{\equiv}\text{P} bond is much weaker.

      Both are triple bonds between identical atoms, so bond energy is the only difference.

    2. 2

      A weaker bond needs less energy to break, so P2\text{P}_2 is more reactive than N2\text{N}_2.

      The mark needs both the comparison of bond strength and the conclusion about reactivity.

    Answer

    P≡P\text{P}{\equiv}\text{P} is much weaker than N≡N\text{N}{\equiv}\text{N}, so P2\text{P}_2 is more reactive than N2\text{N}_2.

Paper 1 · MCQReal past-paper questions · Unreactivity of nitrogenPaper 2 · TheoryReal past-paper questions · Unreactivity of nitrogen

The rest of this note

Checking your access…

Can you do all of these?

  • Explain the lack of reactivity of N₂ with both reasons: a strong triple bond and a non-polar molecule

  • Describe ammonia as a weak Brønsted–Lowry base, with the equilibrium equation and the reversible arrow

  • Describe the formation of NH₄⁺ by a coordinate bond, and its tetrahedral shape (109.5°) with four identical N–H bonds

  • Explain that NH₄⁺ is a weak acid, so ammonium salts are slightly acidic and lower soil pH

  • Describe the displacement of NH₃ from an ammonium salt by a stronger base, and the test for NH₄⁺

  • Calculate the percentage of nitrogen by mass in a fertiliser

  • State lightning and internal combustion engines as the sources of NOₓ, and describe the catalytic converter reactions with oxidation number changes

  • State that NO and NO₂ react with unburnt hydrocarbons to form PAN in photochemical smog

  • Write the equations for NO₂ forming acid rain directly and for NO₂ catalysing the oxidation of SO₂, and state effects of acid rain

  • Optional: uses of ammonia, the eutrophication chain, and SO₂ from sulfur impurities in fossil fuels

Paper 1 · MCQ
Real past-paper questions, sorted by difficulty, with mark schemes
Paper 2 · Theory
Real past-paper questions, sorted by difficulty, with mark schemes