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

487 Q 3 Subjects 12th (PCM)

Difficulty: All Easy Medium Hard 41–50 of 487

Subjects in JEE Main

All Mathematics 30 JEE Physics 0 JEE Chemistry 457

Q.41 Medium JEE Chemistry Chemical Kinetics

At 25°C, the rate constant is 3 × 10⁻² s⁻¹ and at 35°C it is 6 × 10⁻² s⁻¹. What is the temperature coefficient (Q₁₀) for this reaction?

A 1.5

B 2

C 3

D 4

Correct Answer: B. 2

EXPLANATION

Q₁₀ = k(T+10)/k(T) = (6 × 10⁻²)/(3 × 10⁻²) = 2. For typical reactions, Q₁₀ = 2-3

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Q.42 Medium JEE Chemistry Chemical Kinetics

The rate law for a reaction is Rate = k[A]¹[B]⁰. If [B] is doubled and [A] is halved, how does the rate change?

A Increases by factor of 2

B Decreases by factor of 2

C Remains same

D Increases by factor of 4

Correct Answer: B. Decreases by factor of 2

EXPLANATION

Rate depends only on [A] (order = 1 w.r.t. A). Halving [A] decreases rate by factor of 2. [B] has no effect

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Q.43 Easy JEE Chemistry Chemical Kinetics

Consider the mechanism: (1) A + B ⇌ C (fast equilibrium), (2) C + D → E + F (slow). Which is the rate-determining step?

A Step 1

B Step 2

C Both steps

D Cannot be determined

Correct Answer: B. Step 2

EXPLANATION

The slowest step in a mechanism is the rate-determining step (RDS), which is step 2

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Q.44 Easy JEE Chemistry Chemical Kinetics

For a zero-order reaction, the integrated rate law is [A] = [A]₀ - kt. If [A]₀ = 0.5 M and k = 0.02 M·s⁻¹, find time when [A] = 0

A 15 s

B 20 s

C 25 s

D 30 s

Correct Answer: C. 25 s

EXPLANATION

0 = 0.5 - 0.02t; t = 0.5/0.02 = 25 s

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Q.45 Easy JEE Chemistry Chemical Kinetics

Which of the following statements about collision theory is INCORRECT?

A Molecules must collide with proper orientation

B All collisions lead to reaction

C Collision frequency increases with temperature

D Only collisions with energy ≥ Eₐ are effective

Correct Answer: B. All collisions lead to reaction

EXPLANATION

Not all collisions are effective; only those with proper orientation and sufficient energy lead to reaction

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Q.46 Medium JEE Chemistry Chemical Kinetics

The rate constant of a reaction increases from 4 × 10⁻³ s⁻¹ to 8 × 10⁻³ s⁻¹ when temperature increases from 300K to 310K. Calculate activation energy (R = 8.314 J/mol·K)

A 50.4 kJ/mol

B 60.8 kJ/mol

C 75.2 kJ/mol

D 85.6 kJ/mol

Correct Answer: A. 50.4 kJ/mol

EXPLANATION

Using ln(k₂/k₁) = (Eₐ/R)(T₂-T₁)/(T₁T₂); ln(2) = (Eₐ/8.314)(10/93000); Eₐ ≈ 50.4 kJ/mol

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Q.47 Easy JEE Chemistry Chemical Kinetics

Half-life of a first-order reaction is 30 minutes. What fraction of reactant remains after 90 minutes?

A 1/2

B 1/4

C 1/8

D 1/16

Correct Answer: C. 1/8

EXPLANATION

After 90 min = 3 half-lives, remaining = (1/2)³ = 1/8

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Q.48 Easy JEE Chemistry Chemical Kinetics

For the reaction 2NO + O₂ → 2NO₂, the rate law is Rate = k[NO]²[O₂]. What is the overall order of reaction?

A First order

B Second order

C Third order

D Zero order

Correct Answer: C. Third order

EXPLANATION

Overall order = sum of exponents in rate law = 2 + 1 = 3 (third order)

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Q.49 Easy JEE Chemistry Chemical Kinetics

For an elementary reaction: A + A + B → Products, the rate law is:

A Rate = k[A][B]

B Rate = k[A]²[B]

C Rate = k[A][B]²

D Rate = k[A]³[B]

Correct Answer: B. Rate = k[A]²[B]

EXPLANATION

For elementary reactions, the rate law is determined by the stoichiometry: Rate = k[A]²[B], as 2 A molecules and 1 B molecule participate

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Q.50 Medium JEE Chemistry Chemical Kinetics

The pre-exponential factor (A) in the Arrhenius equation is related to:

A Only the number of collisions

B Only the proper orientation of reactant molecules

C The number of collisions with proper orientation and energy distribution

D The activation energy only

Correct Answer: C. The number of collisions with proper orientation and energy distribution

EXPLANATION

The pre-exponential factor accounts for collision frequency, proper orientation (steric factor), and the Maxwell-Boltzmann energy distribution of molecules

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