AS Chemistry: Nitrogen and Sulfur — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Nitrogen and Sulfur revision notes

---

Questions

1. Explain why nitrogen gas is so unreactive. [2]

2. Ammonia is a base.

(a) Explain, in terms of its structure, why ammonia acts as a base. [2] (b) Write an equation for its reaction with hydrochloric acid. [1] (c) Describe the shape of the ammonium ion and explain how the fourth bond differs from the others. [3]

3. Nitrogen monoxide is formed in car engines.

(a) Explain why nitrogen and oxygen react inside an engine but not in the air normally. [2] (b) Explain how NO contributes to the formation of acid rain. [3] (c) Explain how a catalytic converter removes NO, and write an equation. [3]

4. Explain how sulfur dioxide from burning fossil fuels leads to acid rain, and state two environmental consequences. [4]

5. The Contact process makes sulfur trioxide: 2SO₂ + O₂ ⇌ 2SO₃ ΔH = −196 kJ mol⁻¹

(a) State the catalyst used. [1] (b) Explain why 450 °C is used despite the reaction being exothermic. [3] (c) Explain why only 1–2 atm is used despite the mole ratio favouring high pressure. [2]

6. Nitrogen monoxide can form naturally, without any engine, during a lightning strike. Explain why this happens, given how unreactive nitrogen normally is. [2]

7. Atmospheric NO₂ can increase the rate at which sulfur dioxide contributes to acid rain, even though the NO₂ itself is not used up overall.

(a) Write equations to show how NO₂ converts SO₂ to SO₃, and is then regenerated. [2] (b) State the term for a substance that speeds up a reaction without being consumed overall. [1]

8. NOx and unburned hydrocarbons from vehicle exhaust react in sunlight to form peroxyacetyl nitrate (PAN). State what type of atmospheric pollution this produces, and one of its effects. [2]

---

Answers

1. The N≡N triple bond has a very high bond enthalpy [1], so a large amount of energy is needed to break it and the activation energy for any reaction is very high [1].

2. (a) The nitrogen atom has a lone pair of electrons [1] which can accept a proton [1]. (b) NH₃ + HCl → NH₄Cl [1]. (c) Tetrahedral, 109.5° [1]. The fourth bond is a dative covalent bond [1], in which both electrons come from the nitrogen’s lone pair — though once formed it is identical to the other three [1].

3. (a) The engine reaches a very high temperature [1], supplying enough energy to overcome the high activation energy of breaking the N≡N and O=O bonds [1]. (b) NO is oxidised in air to NO₂ [1], which dissolves in rainwater [1] to form nitric acid, lowering the pH of the rain [1]. (c) The converter provides a surface on which NO is reduced while CO is oxidised [1] [1], both gases adsorbing onto a platinum/rhodium/palladium catalyst: 2NO + 2CO → N₂ + 2CO₂ [1].

4. Sulfur dioxide is released when sulfur-containing fossil fuels burn [1]. It is oxidised to SO₃, either directly by atmospheric oxygen or via the NO₂-catalysed pathway, and dissolves in rainwater [1] to form sulfuric acid [1]. Consequences: damage to aquatic life in acidified lakes, damage to trees and soil, and corrosion of limestone buildings and metal structures — any one [1].

5. (a) Vanadium(V) oxide, V₂O₅ [1].

(b) A lower temperature would give a higher equilibrium yield because the forward reaction is exothermic [1], but the rate would be uneconomically slow [1], so 450 °C is a compromise between yield and rate [1].

(c) The equilibrium yield at 1–2 atm already strongly favours SO₃ under typical operating conditions [1], so the modest further conversion available from raising the pressure does not justify the large cost of high-pressure plant and energy [1]. (The exact percentage depends on temperature, pressure and feed composition — the economic argument only needs “already strongly favours SO₃”, not a specific figure.)

6. A lightning strike releases an enormous amount of energy [1], far more than a car engine or ordinary atmospheric conditions supply, which is enough to overcome the very high activation energy of the N≡N and O=O bonds, driving N₂ + O₂ → 2NO directly, even though these conditions do not exist in ordinary atmospheric chemistry [1].

7. (a) NO₂ + SO₂ → NO + SO₃ [1]; the NO produced is then reoxidised by atmospheric O₂ back to NO₂ — 2NO + O₂ → 2NO₂ — regenerating it for further reaction [1]. (b) Catalyst [1].

8. This produces photochemical smog [1], which acts as an eye and lung irritant, characteristic of sunny, traffic-heavy cities [1].

---

Where marks are usually lost

• Explaining nitrogen’s inertness by “it is a gas” rather than the triple bond.
• Not stating that the dative bond becomes identical once formed.
• Giving “compromise” without saying between what and what.
• Omitting the oxidation step in the acid rain mechanism.
• Treating NO₂’s role in oxidising SO₂ as a one-off reaction rather than a catalytic cycle in which NO₂ is regenerated.
• Confusing acid rain (from SO₂/NOx dissolving to form acids) with photochemical smog (from NOx and hydrocarbons reacting in sunlight) — they are distinct pollution mechanisms with the same pollutant gases.

Work through the Nitrogen and Sulfur revision notes alongside these questions: the notes summarise the structure and bonding arguments in condensed form, while these questions test whether you can apply them to natural formation, catalytic pathways and atmospheric pollution — situations that go beyond the basic definitions.