Q.79 Chemistry · Section A ⭐ 360° Option Elimination Active
NEET 2024 Chemistry Question 79 Solution
Topic: Equilibrium · Official NTA Booklet Code T1
Question
In which of the following equilibria, Kp and Kc are NOT equal?
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Verified Against NTA Final Answer Key
Option (3) : PCl5(g) ⇌ PCl3(g) + Cl2(g)
📚 Equilibrium Easy
📋 Given Parameters & Problem Setup
Core relationship K_p = K_c (RT)^Δn_g
Target condition Equilibria in which K_p and K_c are NOT equal (Δn_g ≠ 0)
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⏱️ Save ~35 seconds The 30-Second Exam Hall Shortcut: ⚡ Δng ≠ 0 Instant Filter
K_p = K_c when Δn_g = 0. In PCl5(g) ⇌ PCl3(g) + Cl2(g), Δn_g = (1 + 1) - 1 = +1 ≠ 0 -> K_p ≠ K_c!
1 Formula: K_p = K_c · (RT)^Δn_g, where Δn_g = (moles of gaseous products) - (moles of gaseous reactants).
2 K_p equals K_c ONLY when Δn_g = 0.
3 Check Option (1): CO(g) + H2O(g) ⇌ CO2(g) + H2(g) -> Δn_g = 2 - 2 = 0 (K_p = K_c).
4 Check Option (2): 2 BrCl(g) ⇌ Br2(g) + Cl2(g) -> Δn_g = 2 - 2 = 0 (K_p = K_c).
5 Check Option (3): PCl5(g) ⇌ PCl3(g) + Cl2(g) -> Δn_g = (1 + 1) - 1 = +1 ≠ 0 (K_p ≠ K_c).
6 Check Option (4): H2(g) + I2(g) ⇌ 2 HI(g) -> Δn_g = 2 - 2 = 0 (K_p = K_c).
7 Option (3) is the only equilibrium where K_p ≠ K_c in 5 seconds!
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Why You Can 100% Trust This Shortcut: NCERT Chemistry · Equilibrium (Relationship between equilibrium constant in terms of partial pressure Kp and molar concentration Kc).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Thermodynamic Derivation of Kp = Kc(RT)^Δng
For a general gas-phase equilibrium aA + bB ⇌ cC + dD, partial pressures relate to molar concentrations by the ideal gas law: p_i = [i]RT. Substituting this into the Kp expression yields:
Step 2
Condition for Equality of Kp and Kc
For Kp to equal Kc identically (independent of temperature): (RT)^Δng = 1 ⟹ Δng = 0. Thus, Kp = Kc if and only if the total stoichiometric sum of gaseous product coefficients equals that of the gaseous reactants.
Step 3
Evaluation of Δng for Each Given Equilibrium
Calculating Δng for all four reactions: 1. CO(g) + H2O(g) ⇌ CO2(g) + H2(g): Δng = (1 + 1) - (1 + 1) = 0 ⟹ Kp = Kc. 2. 2 BrCl(g) ⇌ Br2(g) + Cl2(g): Δng = (1 + 1) - 2 = 0 ⟹ Kp = Kc. 3. PCl5(g) ⇌ PCl3(g) + Cl2(g): Δng = (1 + 1) - 1 = +1 ⟹ Kp = Kc(RT) ≠ Kc! 4. H2(g) + I2(g) ⇌ 2 HI(g): Δng = 2 - (1 + 1) = 0 ⟹ Kp = Kc.
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 (1)
✗ Incorrect Verdict & Scientific Reason:
Δng = 0, so Kp = Kc.
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Student Fallacy (Why candidates pick this trap): Miscounting gaseous moles.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
Δng = 0, so Kp = Kc.
⚠️
Student Fallacy (Why candidates pick this trap): Forgetting coefficient of 2 for BrCl.
3 (3)
✓ 100% Correct Verdict & Scientific Reason:
Δng = +1 ≠ 0, making Kp and Kc strictly unequal.
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Why this is airtight: None.
4 (4)
✗ Incorrect Verdict & Scientific Reason:
Δng = 0, so Kp = Kc.
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Student Fallacy (Why candidates pick this trap): Thinking HI formation involves pressure change.
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NTA Trap Alert: Missing the Word "NOT"
The question asks where Kp and Kc are NOT equal. Three options have Kp = Kc, and only Option (3) has Kp ≠ Kc.
🛡️ How to avoid the -1 mark: Highlight the negative constraint "NOT equal" and immediately look for Δng ≠ 0.
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Core NEET Syllabus Standard
Chemistry Core Physical Chemistry · Equilibrium Standard Benchmark
Core Scientific Principle Tested:
"Kp equals Kc only when Δng = 0; when Δng ≠ 0 (as in PCl5 decomposition where Δng = +1), Kp ≠ Kc."
Syllabus Focus: Relationship Between Kp and Kc
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
When gas moles on left and right agree, Kp equals Kc easily! When moles expand (1 to 2), Kp and Kc part ways anew!
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