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

158 Q 3 Subjects 12th (PCM)
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Q.31 Easy JEE Chemistry Chemical Kinetics
Which of the following statements about activation energy (Eₐ) is correct?
A It is always positive and independent of temperature
B It depends on the direction of the reaction
C It is the minimum energy required for reactants to form products
D It is the total energy of reactants
Correct Answer:  C. It is the minimum energy required for reactants to form products
EXPLANATION

Activation energy is the minimum energy required for reactants to overcome the energy barrier and form products. It is always positive and independent of temperature.

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Q.32 Easy JEE Chemistry Chemical Kinetics
A reaction has a rate constant of k = 2.5 × 10⁻⁴ s⁻¹ at 298 K. What is the order of this reaction if the units of k are s⁻¹?
A Zero order
B First order
C Second order
D Third order
Correct Answer:  B. First order
EXPLANATION

The units s⁻¹ or time⁻¹ indicate a first-order reaction. For first-order reactions, the rate constant has units of time⁻¹.

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Q.33 Easy JEE Chemistry Chemical Kinetics
The half-life of a zero-order reaction is:
A Independent of initial concentration
B Directly proportional to initial concentration
C Inversely proportional to initial concentration
D Logarithmic with respect to initial concentration
Correct Answer:  B. Directly proportional to initial concentration
EXPLANATION

For a zero-order reaction, t₁/₂ = [A]₀/(2k), which is directly proportional to initial concentration.

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Q.34 Easy JEE Chemistry Chemical Kinetics
The rate of a chemical reaction is defined as the change in concentration of reactants or products per unit time. Which of the following is NOT a factor affecting reaction rate?
A Temperature of the system
B Nature of reactants and products
C Color of the reaction vessel
D Concentration of reactants
Correct Answer:  C. Color of the reaction vessel
EXPLANATION

The color of the reaction vessel does not affect the reaction rate. Temperature, nature of reactants, concentration, pressure, and catalyst are actual factors affecting reaction rate.

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Q.35 Easy JEE Chemistry Electrochemistry
Which factor does NOT significantly affect the conductance of an electrolytic solution?
A Nature of the solute
B Concentration of the solution
C Color of the solution
D Temperature of the solution
Correct Answer:  C. Color of the solution
EXPLANATION

Color of a solution is a physical property unrelated to ionic conductance. Conductance depends on nature of solute, concentration, temperature, and solvent.

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Q.36 Easy JEE Chemistry Electrochemistry
The electrochemical series is based on which property?
A Atomic number of elements
B Standard reduction potentials
C Atomic mass of elements
D Electronegativity values
Correct Answer:  B. Standard reduction potentials
EXPLANATION

The electrochemical series arranges elements in order of their standard reduction potentials, with more positive values indicating stronger oxidizing agents.

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Q.37 Easy JEE Chemistry Electrochemistry
What is the effect of temperature on the conductivity of ionic solutions?
A Conductivity decreases with increase in temperature
B Conductivity increases with increase in temperature
C Conductivity remains constant with temperature
D Temperature has no effect on ionic mobility
Correct Answer:  B. Conductivity increases with increase in temperature
EXPLANATION

Conductivity increases with temperature because ionic mobility increases due to decreased viscosity of the medium.

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Q.38 Easy JEE Chemistry Electrochemistry
The standard reduction potential of H⁺/H₂ electrode is taken as 0.00 V because:
A Hydrogen is the most abundant element
B It serves as a reference electrode for all other half-cells
C Hydrogen has zero atomic number
D It's the easiest element to oxidize
Correct Answer:  B. It serves as a reference electrode for all other half-cells
EXPLANATION

SHE (Standard Hydrogen Electrode) is the arbitrary reference against which all other reduction potentials are measured.

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Q.39 Easy JEE Chemistry Electrochemistry
In a Daniel cell, the mass of zinc electrode decreases and copper electrode increases. This indicates:
A Zn acts as anode and Cu as cathode
B Both electrodes are anodes
C Both electrodes are cathodes
D Cu acts as anode and Zn as cathode
Correct Answer:  A. Zn acts as anode and Cu as cathode
EXPLANATION

Anode undergoes oxidation (Zn → Zn²⁺ + 2e⁻), losing mass. Cathode undergoes reduction (Cu²⁺ + 2e⁻ → Cu), gaining mass.

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Q.40 Easy JEE Chemistry Electrochemistry
In the electrolysis of molten NaCl using inert electrodes, what is produced at the cathode?
A Cl₂ gas
B Na metal
C O₂ gas
D NaOH solution
Correct Answer:  B. Na metal
EXPLANATION

At cathode: Na⁺ + e⁻ → Na (reduction). At anode: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

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