Chapter 4 Chemical Kinetics

Here is the complete set of 50 Assertion and Reason questions for Chapter 4: Chemical Kinetics for CBSE Class 12 Chemistry with answers and explanations:

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Options for Assertion-Reason Questions

1. Both A and R are true, and R is the correct explanation of A.
2. Both A and R are true, but R is not the correct explanation of A.
3. A is true, but R is false.
4. A is false, but R is true.

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Assertion and Reason Questions

Basic Concepts of Chemical Kinetics

1. Assertion (A): The rate of a reaction is always positive.
   Reason (R): Reaction rates are expressed as the absolute value of the change in concentration per unit time.
   Answer: 1
   Explanation: Rate is defined as the change in concentration with respect to time and is always positive.

2. Assertion (A): The order of a reaction can be zero or fractional.
   Reason (R): The order of a reaction is determined experimentally and can take any value.
   Answer: 1
   Explanation: Reaction order can be fractional, depending on the reaction mechanism.

3. Assertion (A): The molecularity of a reaction is always an integer.
   Reason (R): Molecularity refers to the number of reactant molecules involved in an elementary step.
   Answer: 1
   Explanation: Molecularity refers to the number of reactant molecules involved in the elementary reaction, which is always a whole number.

4. Assertion (A): The rate constant $k$ of a reaction depends on the concentration of reactants.
   Reason (R): The rate constant is a proportionality factor that relates reaction rate to reactant concentration.
   Answer: 3
   Explanation: The rate constant is independent of reactant concentration and depends only on temperature.

5. Assertion (A): For a zero-order reaction, the rate of reaction is independent of the concentration of reactants.
   Reason (R): The rate law for a zero-order reaction is $R = k$ .
   Answer: 1
   Explanation: In zero-order reactions, the rate remains constant and is independent of reactant concentration.

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Rate Law and Order of Reaction

6. Assertion (A): The rate law of a reaction can be determined from the stoichiometric equation.
   Reason (R): The rate law is based on the mechanism of the reaction, not the stoichiometry.
   Answer: 4
   Explanation: The rate law is determined experimentally and may not follow the stoichiometry.

7. Assertion (A): A first-order reaction has a constant half-life.
   Reason (R): In first-order reactions, the half-life $t_{1/2}$ is independent of initial concentration.
   Answer: 1
   Explanation: The half-life of a first-order reaction is constant, and it does not depend on the concentration of reactants.

8. Assertion (A): For a second-order reaction, the half-life increases as the initial concentration decreases.
   Reason (R): For a second-order reaction, $t_{1/2}$ is inversely proportional to the initial concentration.
   Answer: 1
   Explanation: In second-order reactions, $t_{1/2} = \frac{1}{k[A]_0}$ , meaning the half-life increases as concentration decreases.

9. Assertion (A): The sum of the powers of concentrations in a rate law is the order of the reaction.
   Reason (R): Reaction order is determined experimentally.
   Answer: 2
   Explanation: The order of reaction is determined by experimental data and represents the sum of the exponents in the rate law.

10. Assertion (A): A zero-order reaction has no reactants.
    Reason (R): The rate of reaction is independent of reactant concentration.
    Answer: 4
    Explanation: A zero-order reaction still involves reactants, but their concentration does not affect the rate.

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Integrated Rate Equations

11. Assertion (A): Integrated rate equations can describe the concentration of reactants as a function of time.
    Reason (R): Rate equations are derived from differential rate laws.
    Answer: 1
    Explanation: Integrated rate equations relate concentration to time and are derived from the differential form of the rate law.

12. Assertion (A): The half-life of a zero-order reaction is directly proportional to the initial concentration of reactants.
    Reason (R): The formula for half-life in zero-order reactions is $t_{1/2} = \frac{[A]_0}{2k}$ .
    Answer: 1
    Explanation: The half-life of a zero-order reaction depends directly on the initial concentration of reactants.

13. Assertion (A): For a first-order reaction, a plot of $\ln[A]$ vs. time gives a straight line.
    Reason (R): The integrated rate equation for first-order reactions is $\ln[A] = -kt + \ln[A]_0$ .
    Answer: 1
    Explanation: The integrated rate law for a first-order reaction is linear when $\ln[A]$ is plotted against time.

14. Assertion (A): For a second-order reaction, a plot of $\frac{1}{[A]}$ vs. time gives a straight line.
    Reason (R): The integrated rate equation for second-order reactions is $\frac{1}{[A]} = kt + \frac{1}{[A]_0}$ .
    Answer: 1
    Explanation: The plot of $\frac{1}{[A]}$ vs. time is linear for second-order reactions.

15. Assertion (A): A higher value of $k$ indicates a faster reaction.
    Reason (R): The rate constant $k$ is directly proportional to the reaction rate.
    Answer: 1
    Explanation: A higher value of $k$ corresponds to a faster reaction rate.

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Temperature Dependence, Activation Energy, and Collision Theory

16. Assertion (A): The rate of a reaction increases with increasing temperature.
    Reason (R): Higher temperatures provide more kinetic energy to reactant molecules, increasing the frequency of effective collisions.
    Answer: 1
    Explanation: Increasing temperature leads to an increase in the number of molecules with energy greater than the activation energy, thus increasing the reaction rate.

17. Assertion (A): The activation energy is the minimum energy required for a reaction to occur.
    Reason (R): Activation energy is the energy barrier that reactants must overcome for a successful collision.
    Answer: 1
    Explanation: Activation energy represents the energy required for reactants to convert into products through collisions.

18. Assertion (A): The Arrhenius equation shows the relationship between temperature and the rate constant.
    Reason (R): The equation is $k = A e^{-E_a/RT}$ , where $E_a$ is the activation energy, $R$ is the gas constant, and $T$ is the temperature.
    Answer: 1
    Explanation: The Arrhenius equation quantitatively relates the rate constant $k$ to temperature and activation energy.

19. Assertion (A): A catalyst increases the rate of reaction by lowering the activation energy.
    Reason (R): Catalysts provide an alternative reaction pathway with a lower activation energy.
    Answer: 1
    Explanation: Catalysts provide an alternate pathway that requires less energy to initiate the reaction.

20. Assertion (A): A reaction mechanism is the step-by-step sequence of elementary reactions.
    Reason (R): Each elementary reaction in a mechanism corresponds to a single molecular event.
    Answer: 1
    Explanation: Reaction mechanisms are detailed pathways, and each elementary step is a single molecular event.

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Collision Theory

21. Assertion (A): The rate of reaction is proportional to the number of collisions between reactant molecules.
    Reason (R): Only the collisions with energy greater than the activation energy lead to the formation of products.
    Answer: 1
    Explanation: Collisions are necessary, but only those with sufficient energy (activation energy) result in successful reactions.

22. Assertion (A): The frequency of collisions increases with increasing temperature.
    Reason (R): Higher temperatures cause molecules to move faster, increasing the number of collisions per unit time.
    Answer: 1
    Explanation: Higher kinetic energy at higher temperatures leads to more frequent collisions.

23. Assertion (A): An increase in pressure increases the rate of reaction in gaseous reactions.
    Reason (R): Higher pressure decreases the volume of the system, increasing the concentration of reactants and collision frequency.
    Answer: 1
    Explanation: Increased pressure in a gas-phase reaction leads to more collisions between reactant molecules.

24. Assertion (A): For reactions involving gases, increasing the surface area increases the reaction rate.
    Reason (R): A greater surface area provides more collisions between reactant molecules.
    Answer: 1
    Explanation: Increased surface area allows more reactant molecules to collide, increasing the reaction rate.

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Catalysis

25. Assertion (A): Catalysts are consumed during the reaction.
    Reason (R): Catalysts participate in the reaction mechanism but are regenerated at the end.
    Answer: 4
    Explanation: Catalysts are not consumed; they are regenerated by the end of the reaction.

26. Assertion (A): A heterogeneous catalyst operates in a different phase than the reactants.
    Reason (R): Heterogeneous catalysts provide a surface for reactants to adsorb and react.
    Answer: 1
    Explanation: In heterogeneous catalysis, the catalyst is in a different phase (usually solid) than the reactants (often gaseous or liquid).

27. Assertion (A): Catalysts increase the rate of reaction by lowering the activation energy.
    Reason (R): Catalysts provide an alternative reaction pathway with a lower activation energy.
    Answer: 1
    Explanation: Catalysts lower the activation energy, allowing more collisions to be successful at a given temperature.

28. Assertion (A): Catalysts are used in very small amounts because they are not consumed in the reaction.
    Reason (R): Catalysts increase the reaction rate without being used up, allowing them to be reused.
    Answer: 1
    Explanation: Catalysts are not consumed and can be reused multiple times in a reaction.

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Reaction Mechanism

29. Assertion (A): A reaction mechanism involves a series of elementary steps.
    Reason (R): Elementary steps have simple rate laws and can be directly studied in experiments.
    Answer: 1
    Explanation: A reaction mechanism involves several steps, each with its own rate law.

30. Assertion (A): The rate-determining step of a reaction controls the overall rate of the reaction.
    Reason (R): The rate-determining step is the slowest step in the reaction mechanism.
    Answer: 1
    Explanation: The slowest step in the mechanism limits the overall reaction rate.

31. Assertion (A): The reaction rate can be determined by the slowest elementary step.
    Reason (R): The rate of the overall reaction is governed by the rate-determining step.
    Answer: 1
    Explanation: The overall rate is determined by the rate-determining (slowest) step.

32. Assertion (A): The molecularity of a reaction is the number of molecules involved in an elementary step.
    Reason (R): The molecularity can only be determined from the reaction mechanism, not experimentally.
    Answer: 1
    Explanation: Molecularity is based on the elementary step and refers to the number of molecules involved.

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Activation Energy and Temperature

33. Assertion (A): The rate constant increases with an increase in temperature.
    Reason (R): The Arrhenius equation shows the exponential relationship between rate constant and temperature.
    Answer: 1
    Explanation: According to the Arrhenius equation, the rate constant increases with temperature.

34. Assertion (A): The reaction rate doubles for every 10°C increase in temperature.
    Reason (R): This is the typical result from the Arrhenius equation, as a general rule of thumb.
    Answer: 1
    Explanation: Typically, reaction rates increase by a factor of 2 for each 10°C rise in temperature, depending on the activation energy.

35. Assertion (A): Increasing the temperature leads to a higher number of molecules with sufficient energy to overcome activation energy.
    Reason (R): This leads to more effective collisions and an increased rate of reaction.
    Answer: 1
    Explanation: More molecules have enough energy to overcome the activation energy at higher temperatures, increasing the reaction rate.

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Important Mechanisms and Rates

36. Assertion (A): A reaction with a high activation energy has a lower rate constant at a given temperature.
    Reason (R): Higher activation energy means fewer molecules have enough energy to react.
    Answer: 1
    Explanation: A high activation energy reduces the number of molecules with enough energy to react, decreasing the rate constant.

37. Assertion (A): A reaction with a lower activation energy will have a faster rate at a given temperature.
    Reason (R): Lower activation energy means more molecules have sufficient energy to react.
    Answer: 1
    Explanation: A lower activation energy allows more molecules to participate in effective collisions.

38. Assertion (A): Reactions that proceed through a single elementary step are generally of first-order.
    Reason (R): The rate law for elementary reactions is determined by the molecularity of the step.
    Answer: 1
    Explanation: First-order reactions often occur in a single elementary step, with a rate law that corresponds to the molecularity of that step.

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Factors Affecting Reaction Rate

39. Assertion (A): The concentration of reactants directly affects the rate of the reaction.
    Reason (R): Higher reactant concentration increases the number of collisions, thus increasing the reaction rate.
    Answer: 1
    Explanation: More reactant molecules lead to more collisions, increasing the reaction rate.

40. Assertion (A): Catalysts are always present in small amounts in a reaction.
    Reason (R): Catalysts are not consumed in a reaction; they are regenerated.
    Answer: 1
    Explanation: Catalysts participate in the reaction but are not consumed, and they remain available for future reactions.

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Summary of Reaction Rate

41. Assertion (A): The rate of a reaction increases with increasing pressure.
    Reason (R): Increasing pressure increases the frequency of collisions in gaseous reactions.
    Answer: 1
    Explanation: An increase in pressure compresses the gas, leading to more collisions between reactant molecules.

42. Assertion (A): The rate of reaction increases with an increase in temperature.
    Reason (R): Higher temperatures provide more kinetic energy to molecules, leading to more frequent collisions.
    Answer: 1
    Explanation: Higher temperatures increase the energy of the molecules, making more collisions effective.

43. Assertion (A): The presence of a catalyst reduces the activation energy of the reaction.
    Reason (R): Catalysts provide an alternative pathway with lower activation energy.
    Answer: 1
    Explanation: Catalysts lower the activation energy, making it easier for reactants to form products.

44. Assertion (A): In a reaction mechanism, the rate-determining step governs the overall reaction rate.
    Reason (R): The rate-determining step is the slowest step in the mechanism.
    Answer: 1
    Explanation: The slowest step limits the overall reaction rate.

45. Assertion (A): A first-order reaction has a constant half-life.
    Reason (R): The half-life for a first-order reaction is independent of concentration.
    Answer: 1
    Explanation: The half-life in first-order reactions is constant and does not depend on the initial concentration of reactants.

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Conclusion

46. Assertion (A): Reactions with a low activation energy are faster.
    Reason (R): Lower activation energy means more molecules can successfully collide.
    Answer: 1
    Explanation: A lower activation energy increases the rate of reaction by allowing more molecules to collide effectively.

47. Assertion (A): The activation energy of a reaction can be lowered by the use of a catalyst.
    Reason (R): Catalysts provide an alternative mechanism with a lower energy requirement.
    Answer: 1
    Explanation: Catalysts lower the activation energy, increasing the rate of the reaction.

48. Assertion (A): The rate of reaction is directly proportional to the concentration of reactants for first-order reactions.
    Reason (R): For first-order reactions, the rate law is $\text{Rate} = k[A]$ .
    Answer: 1
    Explanation: The rate of reaction for first-order reactions is directly proportional to the concentration of the reactant.

49. Assertion (A): The rate constant decreases with increasing temperature.
    Reason (R): Rate constants are typically higher at elevated temperatures, not lower.
    Answer: 4
    Explanation: Rate constants generally increase with temperature, following the Arrhenius equation.

50. Assertion (A): A catalyst increases the rate of reaction without changing the overall energy balance of the reaction.
    Reason (R): Catalysts lower the activation energy but do not affect the overall energy change.
    Answer: 1
    Explanation: Catalysts accelerate the reaction by lowering activation energy but do not alter the overall energy change of the reaction.