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JEE Chemistry

JEE Main MCQ questions — Mathematics, Physics, Chemistry for engineering entrance.

89 Q 3 Subjects 12th (PCM)

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All Mathematics 30 JEE Physics 0 JEE Chemistry 457

Q.1 Hard JEE Chemistry Chemical Kinetics

A reaction has Eₐ = 60 kJ/mol. How many times faster will it be at 327°C compared to 27°C? (R = 8.314 J/mol·K, assume A constant)

A 100 times

B 256 times

C 500 times

D 1000 times

Correct Answer: C. 500 times

EXPLANATION

Using ln(k₂/k₁) = (Eₐ/R)[(T₂-T₁)/(T₁T₂)]; ln(k₂/k₁) = (60000/8.314)[(300)/(600×300)] ≈ 6.2; k₂/k₁ ≈ 500

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Q.2 Hard JEE Chemistry Chemical Kinetics

For the reaction: (1) Cl₂ ⇌ 2Cl (fast), (2) Cl + H₂ → HCl + H (slow), (3) H + Cl₂ → HCl + Cl (fast). The overall reaction is:

A Cl₂ + H₂ → 2HCl

B 2Cl₂ + H₂ → 2HCl + Cl

C H₂ + Cl₂ → 2HCl

D Cl + H₂ → HCl + H

Correct Answer: A. Cl₂ + H₂ → 2HCl

EXPLANATION

Adding all steps and canceling intermediates (Cl, H): Cl₂ + H₂ → 2HCl

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Q.3 Hard JEE Chemistry Chemical Kinetics

For a reaction with Eₐ = 50 kJ/mol and A = 2 × 10¹³ s⁻¹ (Arrhenius pre-exponential factor), calculate k at 300K (R = 8.314 J/mol·K)

A 2.3 × 10⁻² s⁻¹

B 1.8 × 10⁻¹ s⁻¹

C 3.4 × 10⁻³ s⁻¹

D 5.1 × 10⁻² s⁻¹

Correct Answer: C. 3.4 × 10⁻³ s⁻¹

EXPLANATION

k = A·exp(-Eₐ/RT) = 2 × 10¹³ × exp(-50000/8.314×300) = 2 × 10¹³ × exp(-20.03) ≈ 3.4 × 10⁻³ s⁻¹

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Q.4 Hard JEE Chemistry Chemical Kinetics

In the complex reaction: A + B → I (fast equilibrium), I + B → C (slow), the rate law derived from this mechanism is:

A Rate = k[A][B]

B Rate = k[A][B]²

C Rate = k[I][B]

D Rate = k[A][B]²[I]⁻¹

Correct Answer: B. Rate = k[A][B]²

EXPLANATION

Rate = k₂[I][B]. From equilibrium: [I] = K₁[A][B]. Therefore: Rate = k₂K₁[A][B]², which is second-order in B

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Q.5 Hard JEE Chemistry Chemical Kinetics

In the reaction mechanism: (1) A + B ⇌ AB (fast), (2) AB + C → ABC (slow), the order predicted from this mechanism is:

A First order overall

B Second order overall

C Third order overall

D Fourth order overall

Correct Answer: C. Third order overall

EXPLANATION

Rate = k₂[AB][C]. From fast equilibrium: [AB] = K₁[A][B]. So Rate = k₂K₁[A][B][C], making it third-order overall

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Q.6 Hard JEE Chemistry Chemical Kinetics

The rate constant for a reaction at 298 K is 2 × 10⁻⁵ s⁻¹ with Ea = 80 kJ/mol. What is the frequency factor (A) if rate = Ae^(-Ea/RT)?

A 1.5 × 10⁻² s⁻¹

B 8.0 × 10⁸ s⁻¹

C 3.2 × 10¹¹ s⁻¹

D 5.6 × 10¹³ s⁻¹

Correct Answer: D. 5.6 × 10¹³ s⁻¹

EXPLANATION

A = k/e^(-Ea/RT) = (2 × 10⁻⁵)/e^(-80000/2.478) ≈ 5.6 × 10¹³ s⁻¹

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Q.7 Hard JEE Chemistry Chemical Kinetics

For a pseudo-first-order reaction where [B]₀ >> [A]₀, the rate law simplifies to first-order even though the actual order is higher. This is because:

A [B] remains essentially constant throughout the reaction

B [A] is in large excess

C The reaction temperature is constant

D The rate constant becomes independent of [B]

Correct Answer: A. [B] remains essentially constant throughout the reaction

EXPLANATION

When [B]₀ >> [A]₀, the concentration of B doesn't change significantly during the reaction, so it can be incorporated into the rate constant, making the reaction appear first-order in A only.

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Q.8 Hard JEE Chemistry Chemical Kinetics

In the Lindemann mechanism for unimolecular reactions, A* represents an activated molecule. The rate-determining step is:

A A + A ⇌ A* + A (fast equilibrium)

B A* → products (slow)

C A → A* (slow)

D A* + A → products (slow)

Correct Answer: B. A* → products (slow)

EXPLANATION

In the Lindemann mechanism: Step 1 (fast equilibrium): A + A ⇌ A* + A, Step 2 (slow): A* → products. The slow step is rate-determining.

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Q.9 Hard JEE Chemistry Chemical Kinetics

For the consecutive reaction A → B → C, if the rate constants are k₁ = 0.1 s⁻¹ and k₂ = 0.05 s⁻¹, and k₁ > k₂, which statement is true?

A B is consumed faster than it is formed

B B accumulates and then decreases

C B is immediately consumed after formation

D No B is formed in the reaction

Correct Answer: B. B accumulates and then decreases

EXPLANATION

Since k₁ > k₂, A converts to B faster than B converts to C, so B accumulates initially and then decreases as it slowly converts to C.

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Q.10 Hard JEE Chemistry Chemical Kinetics

In a reaction, the rate increases by a factor of 8 when [A] doubles and by a factor of 2 when [B] doubles. What is the overall order of the reaction?

A 2

B 3

C 4

D 5

Correct Answer: C. 4

EXPLANATION

When [A] doubles, rate increases by 8 = 2³, so order w.r.t. A = 3. When [B] doubles, rate increases by 2 = 2¹, so order w.r.t. B = 1. Overall order = 3 + 1 = 4.

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