Topic 11: Group 17

Question 1

Definitions I:
(a) Volatility
(b) Oxidising agent
(c) Disproportionation

Answer 1

Definitions I:
(a) Tendency of a substance to vaporise, decreases down Group 17.
(b) Substance that accepts electrons, oxidises another species, e.g., Cl₂.
(c) Reaction where an element is both oxidised and reduced, e.g., Cl₂.

Question 2

Definitions II:
(a) Bond strength
(b) Reducing agent
(c) Instantaneous dipole

Answer 2

Definitions II:
(a) Energy needed to break a bond, e.g., X-X in halogens.
(b) Substance that donates electrons, reduces another species, e.g., I⁻.
(c) Temporary dipole from uneven electron distribution, induces dipoles.

Question 2

Describe the colors and volatility trend of Cl₂, Br₂, and I₂ down Group 17. [3 Marks]

Answer 2

Cl₂: pale green gas, high volatility (1 mark)
Br₂: red-brown liquid, medium volatility (1 mark)
I₂: grey-black solid, low volatility (1 mark)

Question 3

Why does Cl₂ react faster with H₂ than I₂ in sunlight? [2 Marks]

Answer 3

Cl₂ more reactive, stronger oxidising agent (1 mark)
Higher electronegativity, weaker Cl-Cl bond (1 mark)

Question 4

Write equations for:
(a) Cl₂ with cold NaOH [1 Mark]
(b) Br₂ with H₂ [1 Mark]

Answer 4

(a) Cl₂ + 2NaOH → NaCl + NaClO + H₂O (cold) (1 mark)
(b) Br₂ + H₂ → 2HBr (1 mark)

Question 5

How does HI thermally decompose more easily than HCl, and why? [2 Marks]

Answer 5

HI less stable, decomposes to H₂ + I₂ (1 mark)
Weaker H-I bond, lower strength (1 mark)

Question 6

In a test tube, NaCl and NaI react with AgNO₃ then NH₃. Describe the results. [2 Marks]

Answer 6

NaCl + Ag⁺ → AgCl (white, insoluble in NH₃) (1 mark)
NaI + Ag⁺ → AgI (yellow, insoluble in NH₃) (1 mark)

Question 7

Write equations for NaBr and NaI reacting with concentrated H₂SO₄. [2 Marks]

Answer 7

NaBr + H₂SO₄ → NaHSO₄ + HBr (1 mark)
NaI + H₂SO₄ → NaHSO₄ + HI, then 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O (1 mark)

Question 7

In water treatment, how does Cl₂ kill bacteria? Include an equation. [2 Marks]

Answer 7

Cl₂ + H₂O → HCl + HOCl, HOCl kills bacteria (1 mark)
Cl₂ oxidises pathogens, ClO⁻ also active (1 mark)

Question 8

Why is F₂ a stronger oxidising agent than Br₂ in aqueous solution? [2 Marks]

Answer 8

F₂ higher electronegativity, gains electrons easily (1 mark)
Stronger oxidising power than Br₂ (1 mark)

Question 9

Compare the bond strength trend in Cl₂, Br₂, I₂ and its effect on volatility. [3 Marks]

Answer 9

Cl₂ (242 kJ/mol) > Br₂ (193) > I₂ (151), decreases down (1 mark)
Weaker bonds, larger atoms, less volatility (1 mark)
van der Waals’ increase but bond strength dominates (1 mark)

Question 9

Predict the reactivity of astatine (At) with H₂ based on Group 17 trends. [2 Marks]

Answer 9

At less reactive than I₂, weaker oxidising agent (1 mark)
Forms HAt slower, Group 17 trend (1 mark)