Topic 12: Nitrogen and Sulfur

Question 1

Definitions I:
(a) Basicity
(b) Acid rain
(c) Photochemical smog

Answer 1

Definitions I:
(a) Ability of a substance to accept H⁺, e.g., NH₃.
(b) Rain with pH < 5.6 due to atmospheric acids.
(c) Smog from sunlight, NO₂, and hydrocarbons, e.g., PAN.

Question 2

Why is N₂ unreactive in car engines despite high temperatures? [2 Marks]

Answer 2

N≡N triple bond (945 kJ/mol) strong, non-polar (1 mark)
High energy needed, no reaction (1 mark)

Question 3

Write equations for:
(a) NH₃ reacting with HCl [1 Mark]
(b) NH₄Cl with NaOH [1 Mark]

Answer 3

(a) NH₃ + HCl → NH₄Cl (1 mark)
(b) NH₄Cl + NaOH → NaCl + H₂O + NH₃ (1 mark)

Question 4

How does NO in exhaust gases get removed catalytically? Include an equation. [2 Marks]

Answer 4

2NO + 2CO → N₂ + 2CO₂ (catalyst, e.g., Pt) (1 mark)
Reduces NO to N₂ in exhaust (1 mark)

Question 5

Describe the formation of acid rain from NO₂ and SO₂ in the atmosphere. [3 Marks]

Answer 5

NO₂ + H₂O → HNO₃, acidic rain (1 mark)
SO₂ + H₂O → H₂SO₃, NO₂ catalyses to H₂SO₄ (1 mark)
Lower pH in rainwater (1 mark)

Question 6

In a lab, NH₃ neutralises HNO₃. Explain its basicity and product structure. [2 Marks]

Answer 6

NH₃ accepts H⁺ (Brønsted base), forms NH₄⁺ (1 mark)
Tetrahedral, four N-H bonds (1 mark)

Question 7

How does NO₂ contribute to photochemical smog with hydrocarbons? [2 Marks]

Answer 7

NO₂ + sunlight → NO + O, O reacts with hydrocarbons (1 mark)
Forms PAN, smog component (1 mark)

Question 8

Why does SO₂ in the air turn into H₂SO₄ with NO₂ present? Include an equation. [2 Marks]

Answer 8

2SO₂ + O₂ + 2H₂O → 2H₂SO₄ (NO₂ catalyst) (1 mark)
NO₂ oxidises SO₂ to SO₃, then acid (1 mark)

Question 9

Predict the reactivity of a Group 15 element below N based on N₂’s properties. [2 Marks]

Answer 9

Less reactive than N₂, larger atom, weaker bond (1 mark)
Still stable, Group 15 trend (1 mark)

Question 10

In industry, how is NH₃ displaced from NH₄NO₃ in a heated reaction? [2 Marks]

Answer 10

NH₄NO₃ + NaOH → NaNO₃ + H₂O + NH₃ (heat) (1 mark)
Base displaces NH₃ from salt (1 mark)

Question 11

Explain the natural and man-made sources of NO and NO₂ in the atmosphere. [3 Marks]

Answer 11

Natural: lightning (N₂ + O₂ → 2NO) (1 mark)
Man-made: engines (N₂ + O₂ → 2NO, then NO₂) (1 mark)
Both oxidise in air (1 mark)

Question 12

Draw the structure of NH₄⁺ and explain its formation from NH₃. [2 Marks]

Answer 12

Tetrahedral NH₄⁺, four H around N (1 mark)
NH₃ + H⁺, lone pair accepts proton (1 mark)