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

📖

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 ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2021 CBT Test →

← Previous (Q.82) All Chemistry Questions Grid Next (Q.84) →

Spotted a mistake in this solution? Report an error (support@mycompetitiveexams.com)

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2021 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

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?

🔗 Related Questions — Chemical Kinetics

Integrated Rate Law & Stoichiometric Coefficients in Zero-Order Reactions

Arrhenius Plot Intercept & Calculation of Target Temperature

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

Solve 4 NEET Questions a Day — One per Subject

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