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

238 Q 3 Subjects 12th (PCM)
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Q.21 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.22 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.23 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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Q.24 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.25 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.26 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.27 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.28 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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Q.29 Medium JEE Chemistry Chemical Kinetics
The half-life of a first-order reaction is independent of the initial concentration. If t₁/₂ = 30 minutes for a reaction, the time for the concentration to reduce to 1/4th of initial value is:
A 30 minutes
B 45 minutes
C 60 minutes
D 90 minutes
Correct Answer:  C. 60 minutes
EXPLANATION

For first-order reaction, [A]ₜ = [A]₀(1/2)^(t/t₁/₂). For [A]ₜ = 1/4[A]₀, we need (1/2)^(t/30) = 1/4, so t/30 = 2, giving t = 60 minutes

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Q.30 Medium JEE Chemistry Chemical Kinetics
In the decomposition of N₂O₅, the rate constant at 320 K is 1.7 × 10⁻⁵ s⁻¹ and at 330 K is 5.0 × 10⁻⁵ s⁻¹. The activation energy is approximately:
A 50 kJ/mol
B 100 kJ/mol
C 150 kJ/mol
D 200 kJ/mol
Correct Answer:  A. 50 kJ/mol
EXPLANATION

Using ln(k₂/k₁) = (Ea/R)(1/T₁ - 1/T₂): ln(5.0/1.7) = (Ea/8.314)(1/320 - 1/330), solving gives Ea ≈ 50 kJ/mol

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