Chapter 3 — p-Block Elements II Exercise Solutions
This chapter has many facts and reactions. Use tables for trends; group equations by element.
Key Concepts Quick Recap
| Concept | Key Fact |
|---|---|
| Group 15 elements | N, P, As, Sb, Bi — valence config ns²np³ |
| Group 16 elements | O, S, Se, Te, Po — ns²np⁴ |
| Group 17 (Halogens) | F, Cl, Br, I, At — ns²np⁵; strongest oxidising agents |
| Group 18 (Noble gases) | He, Ne, Ar, Kr, Xe, Rn — completely filled shells |
| Dinitrogen stability | Triple bond (N≡N) — bond energy 941 kJ/mol |
| Ozone (O₃) | Allotrope of oxygen; angular; strong oxidising agent |
| Interhalogen compounds | Formed between two different halogens — e.g. ClF₃, BrF₅ |
| Fluorine anomaly | Highest electronegativity, no d orbitals, only −1 oxidation state |
| Xenon compounds | XeF₂, XeF₄, XeF₆ — formed with F₂ under UV/heat |
| Haber process | N₂ + 3H₂ ⇌ 2NH₃ at 450 °C, 200 atm, Fe catalyst |
| Contact process | 2SO₂ + O₂ ⇌ 2SO₃ using V₂O₅ catalyst |
Important Areas
| Group | Focus Topics |
|---|---|
| Group 15 | Ammonia, nitric acid, oxoacids of P, allotropes of P |
| Group 16 | Oxygen allotropes, ozone, sulphuric acid (contact process) |
| Group 17 | Reactivity order, oxoacids of Cl, interhalogens |
| Group 18 | Xenon fluorides, uses of noble gases |
I. Choose the Best Answer (MCQs)
1. Which process is used for industrial manufacture of ammonia? (a) Ostwald process (b) Haber process (c) Contact process (d) Solvay process
2. The oxidation state of nitrogen in N₂O₅ is: (a) +3 (b) +4 (c) +5 (d) +2
3. Which halogen has the highest electronegativity? (a) Cl (b) Br (c) I (d) F
4. The shape of XeF₄ is: (a) Tetrahedral (b) Square planar (c) Pyramidal (d) See-saw
5. Ozone is used as a disinfectant because it is: (a) A reducing agent (b) A strong oxidising agent (c) Paramagnetic (d) A noble gas
6. The bleaching action of SO₂ is due to: (a) Oxidation (b) Reduction (c) Hydration (d) Substitution
7. Which noble gas is used in filling airships (and is non-flammable)? (a) He (b) Ne (c) Ar (d) Kr
II. Short Answer Questions
Q1. Why does nitrogen form multiple oxides?
Q2. Compare the oxidising power of F₂ and Cl₂.
Q3. Why does fluorine not exhibit positive oxidation states?
Q4. What are interhalogen compounds? Give two examples and their shapes.
Q5. Write the structure and basicity of H₃PO₃ and H₃PO₄.
Worked Examples
Example 1 — Haber process conditions and yield
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol
Conditions: 450 °C, 200 atm, Fe catalyst (promoters: Al₂O₃, K₂O)
- High pressure favours NH₃ (fewer moles on product side).
- Moderate temperature balances rate and yield.
- Catalyst increases rate without changing equilibrium position.
- ~15% yield per pass; unconverted gases are recycled.
Example 2 — Contact process for H₂SO₄
Step 1: S + O₂ → SO₂
Step 2: 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅, 450 °C, 1–2 atm)
Step 3: SO₃ + H₂SO₄ → H₂S₂O₇ (oleum / fuming H₂SO₄)
Step 4: H₂S₂O₇ + H₂O → 2H₂SO₄
Note: SO₃ is NOT dissolved directly in water — it is explosive.
Example 3 — Halogen displacement reactions
Reactivity: F₂ > Cl₂ > Br₂ > I₂
A more reactive halogen displaces a less reactive one from its salt:
Cl₂ + 2KBr → 2KCl + Br₂ (Cl₂ displaces Br⁻)
Br₂ + 2KI → 2KBr + I₂ (Br₂ displaces I⁻)
F₂ + 2NaCl → 2NaF + Cl₂ (F₂ displaces Cl⁻)
I₂ cannot displace any other halide (least reactive).
Example 4 — Xenon fluorides
Xe + F₂ →(UV light, 2:1) XeF₂ (linear)
Xe + 2F₂ →(400 °C, 1 atm) XeF₄ (square planar)
Xe + 3F₂ →(400 °C, 60 atm) XeF₆ (distorted octahedral)
Example 5 — Oxoacids of chlorine: oxidation states and acid strength
| Compound | Formula | Cl Oxidation State | Acid Strength |
|---|---|---|---|
| Hypochlorous acid | HOCl | +1 | Weakest |
| Chlorous acid | HClO₂ | +3 | ↑ |
| Chloric acid | HClO₃ | +5 | ↑ |
| Perchloric acid | HClO₄ | +7 | Strongest |
Higher oxidation state of Cl → more O atoms → greater electron withdrawal from O−H → stronger acid.
Example 6 — Ozone vs oxygen
| Property | O₂ | O₃ |
|---|---|---|
| Colour | Colourless | Pale blue |
| Smell | Odourless | Pungent |
| Stability | Stable | Decomposes to O₂ |
| Magnetic nature | Paramagnetic | Diamagnetic |
| Bond angle | 180° (linear) | 117° (angular) |
| Reaction with Hg | No | Hg turns dull (tailing) |
| Bond length | 121 pm | 128 pm |
Common Mistakes
- Basicity of H₃PO₃ is 2, not 3 — only P−OH counts, not P−H.
- Wrong halogen reactivity order — always F > Cl > Br > I.
- Saying noble gases cannot form any compounds — Xe does form XeF₂, XeF₄, XeF₆.
- Mixing contact process (H₂SO₄) with Ostwald process (HNO₃).
- Forgetting that SO₂ bleaches by reduction (temporary), while Cl₂ bleaches by oxidation (permanent).
Exam Checklist
- Formula of each product is correct.
- Oxidation states are verified (sum = charge).
- Trend order (reactivity, acidity, electronegativity) is in correct direction.
- Reason is included alongside the fact.
- Industrial process conditions (temperature, pressure, catalyst) are stated.
- Equations are balanced.
Frequently Asked Questions
Oxoacids, halogen trends, interhalogen compounds, noble gas compounds, and preparation/properties of important compounds are scoring areas.