Q.70 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 70 Solution

Topic: Chemical Kinetics · Official NTA Booklet Code G5

Question

For a certain reaction, the rate = k[A]²[B], when the initial concentration of A is tripled keeping concentration of B constant, the initial rate would

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Verified Against NTA Final Answer Key

Option (4) : increase by a factor of nine.

📚 Chemical Kinetics Easy

📋 Given Parameters & Problem Setup

Differential rate law Rate = k [A]² [B]

Change in [A] [A]' = 3 [A] (tripled)

Change in [B] [B]' = [B] (held constant)

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The 30-Second Exam Hall Shortcut: ⚡ Exponent Scaling Rule: Rate ∝ [A]² ⟹ 3² = 9

⏱️ Save ~25 seconds

The reaction is second-order with respect to A: Rate ∝ [A]². Tripling [A] multiplies the rate by 3² = 9! The initial rate increases by a factor of nine in 5 seconds!

1 Inspect the order with respect to reactant A from the rate law: Rate = k [A]² [B].

2 The exponent on [A] is 2 (second-order with respect to A).

3 When concentration [A] is multiplied by factor 3:

4 Rate' = k (3[A])² [B] = 3² × (k [A]² [B]) = 9 × Rate.

5 The rate increases by a factor of nine.

6 Lock Option (4) increase by a factor of nine immediately!

🛡️

Why You Can 100% Trust This Shortcut: Chemical Kinetics · Rate Law and Reaction Order (For an n-th order reactant, multiplying concentration by factor m scales the rate by mⁿ).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Initial Rate Formulation

According to the given rate law, the initial rate of reaction r₁ at initial reactant concentrations [A] and [B] is:

r_1 = k [A]^2 [B]

Step 2

New Rate Formulation Under Modified Concentration

The initial concentration of A is tripled while [B] remains strictly constant:

[A]' = 3[A], \qquad [B]' = [B]

Step 3

Substitution into the Rate Law

Substituting the new concentrations into the rate equation gives the new initial rate r₂:

r_2 = k [A']^2 [B'] = k (3[A])^2 [B] = k \left(9 [A]^2\right) [B] = 9 \left(k [A]^2 [B]\right)

Step 4

Comparison and Factor Evaluation

Comparing r₂ with r₁:

r_2 = 9\, r_1 \implies \text{The rate increases by a factor of 9}

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

Linear scaling: assumes first-order with respect to A instead of second-order.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate treated the exponent on [A] as 1 rather than 2.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Claims rate decreases despite increasing reactant concentration.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate inverted the rate relationship.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Multiplication slip: candidate computed 3 × 2 = 6 instead of 3² = 9.

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Student Fallacy (Why candidates pick this trap): Candidate multiplied concentration factor by exponent instead of raising to power.

4 (4)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate: Second-order scaling yields 3² = 9 times the initial rate.

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Why this is airtight: None. Candidate correctly squared the tripling factor.

⚠️

NTA Trap Alert: Multiplying by the Exponent (3 × 2 = 6) Instead of Raising to Power (3² = 9)

Students who rush through kinetics frequently compute (3 × 2 = 6) and pick Option (3) "increase by a factor of six". The rate law is an exponential power relationship, NOT a linear multiplication!

🛡️ How to avoid the -1 mark: Remember: It is a POWER: (Factor)^Order = 3² = 9, never 3 × 2.

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Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Chemical Kinetics Standard Benchmark

Core Scientific Principle Tested:

"In rate law r = k[A]^x[B]^y, changing [A] by factor m changes rate by m^x."

Syllabus Focus: Rate Law and Order of Reaction

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 70 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Exponent Scaling Rule: Rate ∝ [A]² ⟹ 3² = 9

Step-by-Step Rigorous Workflow & Mathematical Proof

Initial Rate Formulation

New Rate Formulation Under Modified Concentration

Substitution into the Rate Law

Comparison and Factor Evaluation

360° Option Elimination Engine Key Feature

NTA Trap Alert: Multiplying by the Exponent (3 × 2 = 6) Instead of Raising to Power (3² = 9)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Chemical Kinetics

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Time Required for 99.9% Completion of a First Order Reaction

Time Required for Reactant Concentration Reduction from 7.2 to 0.9 mol L⁻¹

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