NEET Crash Course Module - 93

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NEET Masterclass • Module 93

Reactivity Trends of p-Block

Strictly aligned with the revised NMC Syllabus. Master the general chemical reactivity trends towards Hydrogen, Oxygen, and Halogens for Groups 15 to 18.

By chemca Academic Team • Updated for NEET 2027

Module Focus: The New Syllabus Boundary

The latest NEET syllabus has removed the preparation and chemical properties of specific compounds (like $HNO_3, H_2SO_4$, and Interhalogens). Instead, examiners will test your ability to predict and compare General Trends in Chemical Reactivity. You must be able to rank hydrides, oxides, and halides by stability, acidic strength, and reducing power across periods and down groups based on bond dissociation enthalpies and Fajans' rules.

1. Reactivity towards Hydrogen (Hydrides)

All p-block elements react with hydrogen to form volatile, covalent hydrides. The central logic governing all their properties is Atomic Size. As you go down a group, the central atom gets larger, making the $E-H$ bond longer and weaker.

Group 15 ($EH_3$)

$NH_3, PH_3, AsH_3, SbH_3, BiH_3$

• Thermal Stability $\downarrow$
  Bi-H bond is longest/weakest.
• Reducing Power $\uparrow$
  $BiH_3$ gives up H most easily.
• Basic Strength $\downarrow$
  Lone pair density is highest on tiny N atom ($NH_3$).

Group 16 ($H_2E$)

$H_2O, H_2S, H_2Se, H_2Te, H_2Po$

• Thermal Stability $\downarrow$
  $H_2Po$ is least stable.
• Acidic Character $\uparrow$
  Bond weakens down group; $H^+$ released easily. ($H_2Te$ is strongest acid).
• Reducing Power $\uparrow$
  $H_2O$ is not reducing.

Group 17 ($HX$)

$HF, HCl, HBr, HI$

• Thermal Stability $\downarrow$
  $HF$ is most stable.
• Acidic Strength $\uparrow$
  HI bond is longest/weakest. $HI$ is the strongest acid.
• Reducing Power $\uparrow$
  $HI$ is the strongest reducing agent.

NEET Mega Trap: Boiling Point Exceptions

Normally, boiling point increases down a group due to increasing molecular mass and stronger van der Waals forces. However, the first members ($NH_3, H_2O, HF$) are massive exceptions due to Intermolecular Hydrogen Bonding.

Group 15:
$PH_3 < AsH_3 < NH_3 < SbH_3 < BiH_3$

Group 16:
$H_2S < H_2Se < H_2Te < H_2O$

Group 17:
$HCl < HBr < HI < HF$

2. Reactivity towards Oxygen (Oxides)

p-Block elements form multiple oxides due to variable oxidation states. The central rule here is governed by Fajans' Rules: Higher oxidation state = more covalent = more acidic.

Trends in Acidic Character of Oxides

1. Along a Period (L $\rightarrow$ R)

Electronegativity increases. Oxides change from basic $\rightarrow$ amphoteric $\rightarrow$ strongly acidic.

$Na_2O \ (\text{Basic}) \rightarrow Al_2O_3 \ (\text{Amph}) \rightarrow Cl_2O_7 \ (\text{Strongly Acidic})$

2. Down a Group (T $\rightarrow$ B)

Metallic character increases. Acidic character of oxides decreases.

Group 15: $N_2O_3 \ (\text{Acidic}) \rightarrow Bi_2O_3 \ (\text{Basic})$

3. Same Element, Different Oxidation States

The oxide with the higher oxidation state is more acidic.

$N_2O \ (\text{Neutral}) < NO \ (\text{Neutral}) < N_2O_3 \ (\text{Acidic}) < N_2O_4 < N_2O_5 \ (\text{Most Acidic})$

3. Reactivity towards Halogens (Halides)

Elements of Groups 15 and 16 form halides, typically $EX_3$ / $EX_5$ and $EX_2$ / $EX_4$ / $EX_6$ respectively.

Group 15 (Tri vs Penta)

• Pentahalides ($EX_5$): Are more covalent than trihalides (Fajans' rule: higher oxidation state +5 polarizes the halide anion more strongly).
• Nitrogen cannot form pentahalides ($NCl_5$) because it has no d-orbitals to expand its octet.
• Bi rarely forms $BiF_5$ due to the Inert Pair Effect making the +5 state highly unstable.

Group 16 ($SF_6$ Exception)

• Hexahalides are formed only with Fluorine (due to small size and high electronegativity).
• Sulfur hexafluoride ($SF_6$): Is exceptionally stable and kinetically inert. Why? Because the central Sulfur atom is completely sterically protected by the six bulky Fluorine atoms, preventing water or nucleophiles from attacking.

4. Group 18: The "Almost" Inert Gases

Noble gases were considered completely inert until Neil Bartlett synthesized the first real noble gas compound ($Xe^+[PtF_6]^-$) in 1962.

Why does Xenon React?

As you go down Group 18, atomic size increases, and Ionization Enthalpy decreases. The $IE$ of Xenon ($1170 \text{ kJ/mol}$) is low enough—and almost identical to that of molecular Oxygen ($O_2$)—that highly electronegative atoms can force it to share electrons.

Xenon reacts exclusively with the two most electronegative elements:
Fluorine (F) and Oxygen (O)

Krypton forms only one known compound ($KrF_2$). Helium, Neon, and Argon form no true chemical compounds.

Target 180/180

NEET Grand Test: p-Block Reactivity

15 High-Yield Questions testing acidic trends, boiling point anomalies, and steric inertness.

All Quizzes NEET Rapid Revision Series Video Lectures

🎯 NEET 2027 Target 180

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