Q.83 Chemistry ⭐ 360° Option Elimination Active

NEET 2021 Chemistry Question 83 Solution

Topic: Chemical Kinetics · Official NTA Booklet Code M1

Question

For a reaction A → B, enthalpy of reaction is −4.2 kJ mol⁻¹ and enthalpy of activation is 9.6 kJ mol⁻¹. The correct potential energy profile for the reaction is shown in option.

✓

Verified Against NTA Final Answer Key

Option (2) : Graph (2)

📚 Chemical Kinetics Easy

📋 Given Parameters & Problem Setup

Reaction A -> B

Enthalpy of Reaction (ΔH) -4.2 kJ mol^-1 (< 0, exothermic)

Enthalpy of Activation (Ea) 9.6 kJ mol^-1

⚡

The 30-Second Exam Hall Shortcut: ⚡ ΔH < 0 = Exothermic $\implies$ Products (B) Below Reactants (A)

⏱️ Save ~15 seconds

Enthalpy of reaction is $\Delta H = H_{\text{products}} - H_{\text{reactants}} = -4.2\text{ kJ/mol} < 0$ . Negative $\Delta H$ means the reaction is EXOTHERMIC, which requires products (B) to lie at a LOWER potential energy than reactants (A). In Graph (2), B is lower than A! Match Option (2)!

1 Check the sign of

\Delta H

\Delta H = -4.2\text{ kJ mol}^{-1} < 0 \implies

Exothermic reaction.

2 For an exothermic reaction:

E_{\text{products}} < E_{\text{reactants}}

, meaning point B must be below point A.

3 Inspect the graphs: - Graph (1): B is above A (Endothermic). - Graph (2): B is BELOW A (Exothermic). - Graph (3): B is at the same level as A (

\Delta H = 0

). - Graph (4): B is above A (Endothermic).

4 Match Graph (2).

5 Match Option (2).

🛡️

Why You Can 100% Trust This Shortcut: Canonical reaction energy coordinate profile from NCERT Chemical Kinetics.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Sign of Reaction Enthalpy and Thermochemical Classification

For the general transformation $A \longrightarrow B$ , the standard enthalpy of reaction is defined as the difference between the potential energy of the products and reactants: $\Delta H = H_B - H_A$ Given $\Delta H = -4.2\text{ kJ mol}^{-1} < 0$ , heat is evolved to the surroundings, establishing that the process is exothermic.

Step 2

Relative Potential Energy of Reactants and Products

Because $\Delta H < 0$ : $H_B - H_A < 0 \implies H_B < H_A$ Thus, on the potential energy diagram against reaction coordinate, the baseline energy plateau for product $B$ must lie strictly below that of reactant $A$ .

Step 3

Role of Activation Energy

The forward activation enthalpy is $E_a = 9.6\text{ kJ mol}^{-1}$ . Reactant $A$ must surmount an energy barrier of height $9.6\text{ kJ mol}^{-1}$ to reach the activated transition state ( $A^{\ddagger}$ ) before descending to product $B$ . Graph (2) correctly depicts this energy barrier with the product plateau $B$ lower than $A$ .

⭐

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:

Product B lies at higher potential energy than A, representing an ENDOTHERMIC reaction ( $\Delta H > 0$ ).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing exothermic with endothermic profile.

2 2

✓ 100% Correct

Verdict & Scientific Reason:

Exact profile: potential energy barrier surmounted, with product B lying below reactant A ( $\Delta H < 0$ ).

💡

Why this is airtight: None; rigorous thermochemical energy profile verification.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Reactant A and product B are at identical energy levels, representing $\Delta H = 0$ .

⚠️

Student Fallacy (Why candidates pick this trap): Assuming

\Delta H = 0

4 4

✗ Incorrect

Verdict & Scientific Reason:

Product B lies above reactant A (endothermic reaction profile).

⚠️

Student Fallacy (Why candidates pick this trap): Misreading the energy levels.

⚠️

NTA Trap Alert: Endothermic vs. Exothermic Visual Reading Trap

Students who focus on the activation energy peak ( $E_a = 9.6\text{ kJ/mol}$ ) sometimes forget to compare the final level of $B$ relative to $A$ . A negative $\Delta H$ ALWAYS requires $B$ to be below $A$ .

🛡️ How to avoid the -1 mark: Exothermic (

\Delta H < 0

)

\implies B

is LOWER than

A

. Endothermic (

\Delta H > 0

)

\implies B

is HIGHER than

A

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Core NEET Syllabus Standard

NCERT Chemistry · Chemical Kinetics Standard Benchmark

Core Scientific Principle Tested:

"In an exothermic reaction, the potential energy of products is lower than that of reactants by $\Delta H$ ."

Syllabus Focus: Temperature Dependence of the Rate of a Reaction (Activation Energy and Reaction Profiles)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Chemistry Question 83 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ ΔH < 0 = Exothermic ⟹ \implies⟹ Products (B) Below Reactants (A)

Step-by-Step Rigorous Workflow & Mathematical Proof

Sign of Reaction Enthalpy and Thermochemical Classification

Relative Potential Energy of Reactants and Products

Role of Activation Energy

360° Option Elimination Engine Key Feature

NTA Trap Alert: Endothermic vs. Exothermic Visual Reading Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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The 30-Second Exam Hall Shortcut: ⚡ ΔH < 0 = Exothermic $\implies$ Products (B) Below Reactants (A)