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JEE Chemistry

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

457 Q 3 Subjects 12th (PCM)
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Difficulty: All Easy Medium Hard 21–30 of 457
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Q.21 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.22 Easy JEE Chemistry Chemical Kinetics
For the reaction A → B, experimental data shows: [A] vs time is a straight line. This indicates:
A First-order reaction
B Zero-order reaction
C Second-order reaction
D Third-order reaction
Correct Answer:  B. Zero-order reaction
EXPLANATION

A straight line in [A] vs time plot indicates [A] = [A]₀ - kt, which is the integrated rate law for zero-order reactions

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Q.23 Medium JEE Chemistry Chemical Kinetics
The temperature coefficient (Q₁₀) for a reaction is 2.5. If the rate at 300 K is r, then the rate at 320 K is approximately:
A 2.5r
B 6.25r
C 5r
D 12.5r
Correct Answer:  B. 6.25r
EXPLANATION

Q₁₀ = rate at (T+10)/rate at T. For 300K to 320K (two 10K intervals), rate = r × 2.5² = 6.25r

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Q.24 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.25 Easy JEE Chemistry Chemical Kinetics
For a reaction with rate law Rate = k[A]⁰[B]¹, if [B] is increased 4 times while [A] remains constant, the rate increases by:
A 2 times
B 4 times
C 8 times
D 16 times
Correct Answer:  B. 4 times
EXPLANATION

Since the reaction is first-order in B and zero-order in A, Rate ∝ [B]. Increasing [B] by 4 times increases Rate by 4 times

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Q.26 Medium JEE Chemistry Chemical Kinetics
In enzyme catalysis, the Michaelis constant (Km) represents:
A Maximum enzyme velocity
B The substrate concentration at half maximum velocity
C The enzyme concentration
D The activation energy of the reaction
Correct Answer:  B. The substrate concentration at half maximum velocity
EXPLANATION

Km is a characteristic constant for an enzyme-substrate pair, representing substrate concentration when v = Vmax/2

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Q.27 Medium JEE Chemistry Chemical Kinetics
The reaction A → B has rate constant k = 4.5 × 10⁻³ min⁻¹. This is a first-order reaction. The percentage of A remaining after 100 minutes is:
A 23.5%
B 35.7%
C 41.2%
D 58.8%
Correct Answer:  C. 41.2%
EXPLANATION

[A]ₜ = [A]₀e^(-kt) = 100 × e^(-0.0045 × 100) = 100 × e^(-0.45) ≈ 41.2%

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Q.28 Easy JEE Chemistry Chemical Kinetics
A catalyst increases the rate of reaction by:
A Increasing activation energy
B Providing an alternative reaction pathway with lower activation energy
C Increasing the frequency factor
D Changing the equilibrium constant
Correct Answer:  B. Providing an alternative reaction pathway with lower activation energy
EXPLANATION

A catalyst works by providing an alternative mechanism with lower Ea, thus increasing reaction rate without being consumed

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Q.29 Medium JEE Chemistry Chemical Kinetics
For a bimolecular reaction between A and B molecules, the collision frequency (Z) depends on:
A Only temperature
B Only molecular size and concentration
C Only collision cross-section
D Temperature, molecular size, concentration, and average relative velocity
Correct Answer:  D. Temperature, molecular size, concentration, and average relative velocity
EXPLANATION

Z ∝ σ × [A] × [B] × √(T/M), where all these factors contribute to collision frequency

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Q.30 Medium JEE Chemistry Chemical Kinetics
In the reaction 2A + B → C, if the concentration of A is doubled and B is tripled, the rate increases by 12 times. 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]²[B]²
Correct Answer:  C. Rate = k[A][B]²
EXPLANATION

If Rate = k[A]^m[B]^n, then 12 = 2^m × 3^n. Testing: 2¹ × 3² = 2 × 9 = 18 (no); 2² × 3¹ = 12 (yes). So m=1, n=2, giving Rate = k[A][B]²

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