Nitrogen and sulfur

AS Level Chemistry · Nitrogen and sulfur. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Nitrogen gas is very unreactive. The N≡N triple bond is very strong, so a lot of energy is needed to break it, and the molecule is non-polar, so it does not attract other reactants.

Ammonia is a Brønsted–Lowry base: the lone pair on nitrogen accepts a proton to form the ammonium ion, NH4+. A stronger base such as OH− takes the proton back, so warming an ammonium salt with an alkali releases ammonia.

At very high temperatures nitrogen and oxygen from the air combine to form oxides of nitrogen. These pollutants help to form acid rain and photochemical smog, so car exhausts are fitted with catalytic converters to remove them.

Rules to remember

Common mistake

Students often say that the nitrogen in nitrogen oxides from car engines comes from the fuel. It comes from the air: N2 and O2 react at the very high temperature inside the engine.

Worked example

2.14 g of ammonium chloride, NH4Cl, is warmed with excess aqueous sodium hydroxide. What volume of ammonia is released at room conditions? (Ar: N 14.0, H 1.0, Cl 35.5; molar gas volume 24.0 dm3 mol−1)

  1. Mr of NH4Cl = 14.0 + (4 × 1.0) + 35.5 = 53.5.
  2. Moles of NH4Cl = 2.14 ÷ 53.5 = 0.0400 mol.
  3. NH4Cl + NaOH → NaCl + NH3 + H2O, a 1 : 1 ratio, so moles of NH3 = 0.0400 mol.
  4. Volume = 0.0400 × 24.0 = 0.960 dm3.

Answer: 0.960 dm3 of ammonia.

Practice questions

Try each one, then open the answer.

1. Why does ammonia behave as a Brønsted–Lowry base?

  1. A
    the lone pair on the nitrogen atom accepts a proton
  2. B
    it releases OH− ions from its own molecules
  3. C
    it donates a proton to water
  4. D
    it contains hydrogen atoms
Show answer

Answer: A. A Brønsted–Lowry base is a proton acceptor. The nitrogen lone pair forms a bond to H+, giving NH4+; the OH− in aqueous ammonia comes from water, not from NH3 itself.

2. Why is nitrogen gas so unreactive?

  1. A
    the N≡N triple bond is very strong and the molecule is non-polar, so there is no partial charge to attract reactants
  2. B
    nitrogen atoms are very small
  3. C
    nitrogen has a full outer shell of electrons
  4. D
    nitrogen molecules are very light
Show answer

Answer: A. Breaking the triple bond needs about 945 kJ mol−1, and because the two atoms are identical there is no dipole to start a reaction. Nitrogen atoms do not have a full outer shell; the molecule does.

3. Which reagent, when warmed with solid ammonium chloride, releases ammonia gas?

  1. A
    dilute hydrochloric acid
  2. B
    aqueous silver nitrate
  3. C
    water
  4. D
    aqueous sodium hydroxide
Show answer

Answer: D. OH− is a stronger base than NH3, so it removes the proton from NH4+: NH4+ + OH− → NH3 + H2O. The ammonia turns damp red litmus blue.

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