Chemistry Inorganic - How to Solve: p-Block Elements (Group 13-18) – NEET UG Guide

How to Solve: p-Block Elements (Group 13-18) – NEET UG Guide

Introduction

"Mastering p-Block Elements—Borax, Alum, Silicates, Oxyacids, and Interhalogens—can fetch you 8-10 marks in NEET Chemistry, directly impacting your rank. These compounds appear in every NEET paper, often disguised as reactions, structures, or applications. Let’s break them down so you never lose a mark again."

WHAT YOU NEED TO KNOW FIRST

1. Periodic Table Trends – Atomic size, electronegativity, and oxidation states across groups.
2. Chemical Bonding – Covalent vs. ionic bonds, coordinate bonds, and hydrogen bonding.
3. Acid-Base Chemistry – Bronsted-Lowry and Lewis acid-base concepts.

(If you’re shaky on these, pause and review them first.)

KEY TERMS & FORMULAS

1. Borax (Na₂B₄O₇·10H₂O)

• Formula: Na₂[B₄O₅(OH)₄]·8H₂O (simplified as Na₂B₄O₇·10H₂O)
• Structure: Tetraborate anion [B₄O₅(OH)₄]²⁻ with two Na⁺ ions.
• Reaction with HCl: Na₂B₄O₇ + 2HCl + 5H₂O → 2NaCl + 4H₃BO₃ (Boric acid) MEMORISE THIS – NEET loves this reaction.

2. Alum (K₂SO₄·Al₂(SO₄)₃·24H₂O)

• General Formula: M⁺M³⁺(SO₄)₂·12H₂O (M⁺ = K⁺, Na⁺, NH₄⁺; M³⁺ = Al³⁺, Cr³⁺, Fe³⁺)
• Example: Potash alum = KAl(SO₄)₂·12H₂O
• Uses: Water purification, dyeing, and as a mordant. MEMORISE THIS – NEET asks for the formula and uses.

3. Silicates (SiO₄⁴⁻)

• Basic Unit: [SiO₄]⁴⁻ tetrahedron.
• Types:
• Orthosilicates (Single tetrahedron) – e.g., ZrSiO₄ (Zircon)
• Pyrosilicates (Two tetrahedra sharing one O) – e.g., Sc₂Si₂O₇
• Cyclic Silicates (Ring structure) – e.g., [SiO₃]ₙ²⁻ (n=3,4,6)
• Sheet Silicates (2D layers) – e.g., Mica (KAl₂(AlSi₃O₁₀)(OH)₂)
• 3D Silicates (Quartz, Feldspar) – e.g., SiO₂ (Quartz) MEMORISE THIS – NEET asks for structures and examples.

4. Oxyacids of Halogens (Group 17)

5. Interhalogen Compounds

• General Formula: XYₙ (X = larger halogen, Y = smaller halogen, n = 1,3,5,7)
• Examples:
• XY Type – ClF, BrF, ICl
• XY₃ Type – ClF₃, BrF₃, IF₃
• XY₅ Type – BrF₅, IF₅
• XY₇ Type – IF₇
• Hybridisation & Shape:
• XY₃ → sp³d (T-shaped)
• XY₅ → sp³d² (Square pyramidal)
• XY₇ → sp³d³ (Pentagonal bipyramidal) MEMORISE THIS – NEET asks for shapes and hybridisation.

STEP-BY-STEP METHOD

Step 1: Identify the Compound

• Is it Borax, Alum, Silicate, Oxyacid, or Interhalogen?
• Check the formula or name given in the question.

Step 2: Recall the Structure/Formula

• Borax → Na₂B₄O₇·10H₂O → [B₄O₅(OH)₄]²⁻
• Alum → M⁺M³⁺(SO₄)₂·12H₂O
• Silicates → [SiO₄]⁴⁻ tetrahedron → Identify type (ortho, pyro, cyclic, sheet, 3D)
• Oxyacids → HClO, HClO₂, HClO₃, HClO₄ → Check oxidation state
• Interhalogens → XYₙ → Determine n and shape

Step 3: Write the Reaction (If Asked)

• Borax + HCl → 2NaCl + 4H₃BO₃
• Alum hydrolysis → M⁺ + M³⁺ + 2SO₄²⁻ + 12H₂O → Acidic solution (due to Al³⁺ hydrolysis)
• Silicate weathering → CaSiO₃ + CO₂ + H₂O → CaCO₃ + H₄SiO₄
• Oxyacid decomposition → 2HClO₃ → HClO₄ + ClO₂ + H₂O (disproportionation)
• Interhalogen reaction → ClF₃ + H₂O → HCl + HF + O₂

Step 4: Predict Properties

• Borax → White crystalline solid, used in borosilicate glass.
• Alum → Double salt, used in water purification.
• Silicates → Insoluble in water (except alkali silicates), high melting point.
• Oxyacids → Acid strength increases with oxidation state (HClO < HClO₂ < HClO₃ < HClO₄).
• Interhalogens → More reactive than halogens, strong oxidising agents.

Step 5: Solve the Question

• If it’s a reaction, balance it.
• If it’s a structure, draw the shape and label hybridisation.
• If it’s a property, recall trends (e.g., acid strength, solubility).

WORKED EXAMPLES

Example 1 – Basic (Borax Reaction)

Question: What happens when borax is treated with HCl? Write the balanced equation.

Solution:
1. Identify compound → Borax (Na₂B₄O₇·10H₂O).
2. Recall reaction → Borax + HCl → Boric acid + NaCl.
3. Write unbalanced equation: Na₂B₄O₇ + HCl + H₂O → H₃BO₃ + NaCl
4. Balance: - Na: 2 on LHS → 2NaCl on RHS. - B: 4 on LHS → 4H₃BO₃ on RHS. - H & O: Adjust H₂O to balance.
5. Final equation: Na₂B₄O₇ + 2HCl + 5H₂O → 2NaCl + 4H₃BO₃

What we did and why: - We recalled the standard reaction of borax with HCl. - Balanced the equation step-by-step to ensure atom conservation.

Example 2 – Medium (Alum Hydrolysis)

Question: Why does alum solution turn blue litmus red?

Solution:
1. Identify compound → Alum (KAl(SO₄)₂·12H₂O).
2. Recall hydrolysis → Al³⁺ + H₂O → [Al(H₂O)₆]³⁺ → [Al(H₂O)₅(OH)]²⁺ + H⁺
3. Explanation: - Al³⁺ is a small, highly charged ion → Polarises H₂O → Releases H⁺ ions. - H⁺ ions make the solution acidic → Turns blue litmus red.
4. Conclusion: Alum solution is acidic due to Al³⁺ hydrolysis.

What we did and why: - We linked alum’s formula to its hydrolysis behaviour. - Explained why Al³⁺ makes the solution acidic.

Example 3 – Exam-Style (Interhalogen Shape)

Question: Predict the shape and hybridisation of ClF₃.

Solution:
1. Identify compound → ClF₃ (Interhalogen, XY₃ type).
2. Determine central atom → Cl (larger halogen).
3. Count valence electrons: - Cl: 7 - F: 7 × 3 = 21 - Total = 28 electrons (14 pairs).
4. Draw Lewis structure: - Cl is central, bonded to 3 F atoms. - 2 lone pairs on Cl.
5. Hybridisation: - 3 bond pairs + 2 lone pairs = 5 electron pairs → sp³d hybridisation.
6. Shape: - 5 electron pairs → Trigonal bipyramidal electron geometry. - 2 lone pairs occupy equatorial positions → T-shaped molecular geometry.

What we did and why: - We used VSEPR theory to predict shape and hybridisation. - Counted electrons carefully to avoid mistakes.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is your 60-second p-Block survival guide for NEET:
1. Borax = Na₂B₄O₇·10H₂O → Reacts with HCl to give boric acid (H₃BO₃).
2. Alum = M⁺M³⁺(SO₄)₂·12H₂O → Hydrolyses to give acidic solution.
3. Silicates = [SiO₄]⁴⁻ tetrahedra → Ortho (single), pyro (double), cyclic (rings), sheet (layers), 3D (quartz).
4. Oxyacids = HClO (weakest) < HClO₂ < HClO₃ < HClO₄ (strongest).
5. Interhalogens = XYₙ → ClF₃ is T-shaped (sp³d), IF₇ is pentagonal bipyramidal (sp³d³). Memorise these structures, reactions, and trends—you’ll crush it!