Multiple Choice Questions (MCQs) and Explanations

Here are 40 MCQs based on the “Chemical Kinetics” notes, with detailed explanations.

Question 1: For a reaction 2A+B→C, if the rate of disappearance of A is 0.2 mol L−1s−1, what is the rate of formation of C? (A) 0.1 mol L−1s−1 (B) 0.2 mol L−1s−1 (C) 0.4 mol L−1s−1 (D) 0.05 mol L−1s−1

Explanation 1:

• Correct Answer: (A)
• Reasoning: Rate =−21​dtd[A]​=+dtd[C]​. Given −dtd[A]​=0.2 mol L−1s−1. So, Rate of formation of C =+dtd[C]​=21​dtd[A]​=21​×0.2=0.1 mol L−1s−1.

Question 2: The unit of rate constant for a zero-order reaction is: (A) mol L−1s−1 (B) s−1 (C) L mol−1s−1 (D) mol s−1

Explanation 2:

• Correct Answer: (A)
• Reasoning: For a zero-order reaction, Rate =k. Since Rate has units of concentration/time (mol L−1s−1), the rate constant k for a zero-order reaction also has the unit mol L−1s−1.

Question 3: What is the molecularity of the reaction H2​+Cl2​hv​2HCl? (A) 0 (B) 1 (C) 2 (D) Cannot be determined

Explanation 3:

• Correct Answer: (C)
• Reasoning: This is an elementary reaction where two molecules (H2​ and Cl2​) collide to form products. Thus, its molecularity is 2 (bimolecular).

Question 4: For a first-order reaction, the half-life period is: (A) Directly proportional to the initial concentration. (B) Inversely proportional to the initial concentration. (C) Independent of the initial concentration. (D) Directly proportional to the square of the initial concentration.

Explanation 4:

• Correct Answer: (C)
• Reasoning: For a first-order reaction, t1/2​=k0.693​. This equation shows that the half-life is constant and independent of the initial concentration of the reactant.

Question 5: Which of the following factors generally increases the rate of a chemical reaction? (A) Decreasing temperature (B) Decreasing concentration of reactants (C) Adding a positive catalyst (D) Decreasing surface area of solid reactants

Explanation 5:

• Correct Answer: (C)
• Reasoning: A positive catalyst increases the rate of reaction by providing an alternative pathway with lower activation energy. The other options would generally decrease the reaction rate.

Question 6: The rate law for a reaction is given as Rate =k[A][B]2. The overall order of the reaction is: (A) 1 (B) 2 (C) 3 (D) 0

Explanation 6:

• Correct Answer: (C)
• Reasoning: The overall order of a reaction is the sum of the powers of the concentration terms in the rate law. Here, order =1(for A)+2(for B)=3.

Question 7: A catalyst increases the rate of reaction by: (A) Increasing the activation energy. (B) Increasing the temperature of the reaction. (C) Providing an alternative pathway with lower activation energy. (D) Increasing the concentration of reactants.

Explanation 7:

• Correct Answer: (C)
• Reasoning: The fundamental role of a catalyst is to provide an alternative reaction mechanism that has a lower activation energy, thereby allowing a larger fraction of molecules to possess the required energy for effective collisions.

Question 8: The slope of a plot of ln[R] vs time for a first-order reaction is: (A) k (B) −k (C) k/2.303 (D) −k/2.303

Explanation 8:

• Correct Answer: (B)
• Reasoning: The integrated rate law for a first-order reaction is ln[R]t​=ln[R]0​−kt. This is in the form of y=c+mx, where y=ln[R]t​, c=ln[R]0​, m=−k, and x=t. So, the slope is −k.

Question 9: Which of the following is a pseudo first-order reaction? (A) Decomposition of N2​O5​ (B) Hydrolysis of ester in acidic medium (C) Photochemical reaction of H2​ and Cl2​ (D) Dissociation of HI

Explanation 9:

• Correct Answer: (B)
• Reasoning: Hydrolysis of an ester, like ethyl acetate, in an acidic medium, involves water as a reactant. Since water is typically present in large excess, its concentration remains nearly constant, making the reaction appear first order with respect to the ester, thus a pseudo first-order reaction.

Question 10: According to Arrhenius equation, the rate constant k is equal to: (A) AeEa​/RT (B) Ae−Ea​/RT (C) Ea​/RT (D) A/Ea​

Explanation 10:

• Correct Answer: (B)
• Reasoning: The Arrhenius equation is k=Ae−Ea​/RT, where A is the pre-exponential factor, Ea​ is the activation energy, R is the gas constant, and T is the temperature.

Question 11: The instantaneous rate of reaction is determined by: (A) Measuring the total change in concentration over the entire reaction time. (B) Finding the slope of the tangent at a particular instant on the concentration vs. time graph. (C) Calculating the average rate over a small interval. (D) Observing the color change of the solution.

Explanation 11:

• Correct Answer: (B)
• Reasoning: Instantaneous rate is the rate at a specific moment. Graphically, it’s the slope of the tangent to the concentration-time curve at that instant.

Question 12: For a zero-order reaction, if the initial concentration of the reactant is doubled, its half-life period will: (A) Remain unchanged. (B) Be doubled. (C) Be halved. (D) Be quadrupled.

Explanation 12:

• Correct Answer: (B)
• Reasoning: For a zero-order reaction, t1/2​=2k0​[R]0​​. This shows that t1/2​ is directly proportional to the initial concentration [R]0​. So, if [R]0​ is doubled, t1/2​ will also be doubled.

Question 13: If the units of rate constant are s−1, the order of the reaction is: (A) Zero (B) First (C) Second (D) Third

Explanation 13:

• Correct Answer: (B)
• Reasoning: The general unit for the rate constant is (mol L−1)1−ns−1. For n=1 (first order), the term (mol L−1)1−1=(mol L−1)0=1. So, the unit becomes s−1.

Question 14: Which statement is incorrect regarding molecularity? (A) It can be fractional. (B) It is defined for elementary reactions. (C) It cannot be more than three. (D) It is a theoretical concept.

Explanation 14:

• Correct Answer: (A)
• Reasoning: Molecularity is always a whole number (1, 2, or 3) because it represents the number of molecules participating in a single collision. Order of reaction, however, can be fractional.

Question 15: The slowest step in a reaction mechanism is called the: (A) Elementary step (B) Intermediate step (C) Rate-determining step (D) Transition state

Explanation 15:

• Correct Answer: (C)
• Reasoning: For a complex reaction involving multiple steps, the overall rate of the reaction is determined by the slowest step in the mechanism. This step is called the rate-determining step.

Question 16: A plot of logk vs 1/T gives a straight line. The slope of this line is: (A) Ea​/R (B) −Ea​/R (C) −Ea​/(2.303R) (D) Ea​/(2.303R)

Explanation 16:

• Correct Answer: (C)
• Reasoning: The logarithmic form of the Arrhenius equation is logk=logA−2.303RTEa​​. When plotting logk on the y-axis and 1/T on the x-axis, the slope is −2.303REa​​.

Question 17: The minimum energy required for the reacting molecules to undergo effective collision is called: (A) Kinetic energy (B) Potential energy (C) Activation energy (D) Threshold energy

Explanation 17:

• Correct Answer: (C)
• Reasoning: Activation energy (Ea​) is the minimum excess energy that the reactant molecules must acquire to cross the energy barrier and form products. Threshold energy is the total minimum energy required (including average kinetic energy).

Question 18: For a reaction A→Products, if the concentration of A is doubled and the rate of reaction also doubles, the order of reaction with respect to A is: (A) 0 (B) 1 (C) 2 (D) 3

Explanation 18:

• Correct Answer: (B)
• Reasoning: If Rate =k[A]n, and doubling [A] doubles the rate, then 2×Rate=k[2A]n. Dividing the two equations, 2=2n, which implies n=1. So, the reaction is first order with respect to A.

Question 19: What does the pre-exponential factor (A) in the Arrhenius equation represent? (A) Activation energy (B) Fraction of effective collisions (C) Collision frequency (D) Both collision frequency and proper orientation

Explanation 19:

• Correct Answer: (D)
• Reasoning: The Arrhenius factor (A) accounts for the total number of collisions per unit time (collision frequency) and the fraction of collisions that occur with the correct orientation.

Question 20: Which of the following statements about a catalyst is correct? (A) It changes the equilibrium constant of a reaction. (B) It participates in the reaction but is consumed. (C) It lowers the activation energy of the reaction. (D) It makes an endothermic reaction exothermic.

Explanation 20:

• Correct Answer: (C)
• Reasoning: A catalyst speeds up a reaction by providing an alternative reaction mechanism with a lower activation energy. It does not change the equilibrium constant or the overall enthalpy change of the reaction.

Question 21: The unit of rate constant for a second-order reaction is: (A) s−1 (B) mol L−1s−1 (C) L mol−1s−1 (D) mol2 L−2s−1

Explanation 21:

• Correct Answer: (C)
• Reasoning: For a second-order reaction, the unit of rate constant is L mol−1s−1. Using the general formula (mol L−1)1−ns−1 for n=2, we get (mol L−1)−1s−1=L mol−1s−1.

Question 22: If a reaction is zero order, the concentration of the reactant versus time graph is a: (A) Curve with increasing slope. (B) Curve with decreasing slope. (C) Straight line with positive slope. (D) Straight line with negative slope.

Explanation 22:

• Correct Answer: (D)
• Reasoning: For a zero-order reaction, the integrated rate law is [R]t​=[R]0​−k0​t. This is the equation of a straight line (y=c+mx) with a negative slope (−k0​).

Question 23: The activation energy for a reaction can be determined from the slope of a plot between: (A) logk and logT (B) logk and T (C) logk and 1/T (D) k and T

Explanation 23:

• Correct Answer: (C)
• Reasoning: According to the Arrhenius equation in logarithmic form, logk=logA−2.303RTEa​​. A plot of logk versus 1/T gives a straight line with a slope equal to −2.303REa​​, from which Ea​ can be calculated.

Question 24: Which of the following statements about collision theory is incorrect? (A) All collisions between reactant molecules lead to a reaction. (B) Reactant molecules must collide with sufficient energy. (C) Reactant molecules must collide with proper orientation. (D) The rate of reaction depends on the frequency of effective collisions.

Explanation 24:

• Correct Answer: (A)
• Reasoning: This statement is incorrect. Only effective collisions, where molecules possess sufficient activation energy and proper orientation, lead to a reaction. Most collisions are ineffective.

Question 25: For a first-order reaction, the time taken for 75% completion is twice the time taken for: (A) 25% completion (B) 50% completion (C) 90% completion (D) 100% completion

Explanation 25:

• Correct Answer: (B)
• Reasoning: For a first-order reaction, t75%​=2×t50%​ (since t50%​ is one half-life, and 75% completion means two half-lives have passed: 100t1/2​​50t1/2​​25).

Question 26: The rate of reaction increases with increase in temperature because: (A) Activation energy decreases. (B) Collision frequency decreases. (C) Fraction of molecules exceeding activation energy increases. (D) Reactant molecules become more stable.

Explanation 26:

• Correct Answer: (C)
• Reasoning: The most significant reason for the increase in reaction rate with temperature is the substantial increase in the fraction of molecules that possess kinetic energy equal to or greater than the activation energy, leading to more effective collisions.

Question 27: For a complex reaction, the order of reaction is determined by: (A) The stoichiometry of the overall balanced equation. (B) The molecularity of the slowest step. (C) Experimentally. (D) The molecularity of the fastest step.

Explanation 27:

• Correct Answer: (C)
• Reasoning: The order of reaction is an experimentally determined value. For complex reactions, it is determined by the rate-determining step, but this still requires experimental validation of the rate law.

Question 28: Which of the following reactions is an example of a zero-order reaction? (A) Radioactive decay (B) Hydrolysis of sucrose in acidic solution (C) Decomposition of NH3​ on hot platinum surface (D) 2N2​O5​→4NO2​+O2​

Explanation 28:

• Correct Answer: (C)
• Reasoning: The decomposition of ammonia (NH3​) on a hot platinum surface is a classic example of a zero-order reaction, where the rate is independent of the concentration of ammonia (as the surface becomes saturated). Radioactive decay and N2​O5​ decomposition are first order, and sucrose hydrolysis is pseudo first order.

Question 29: If a reaction has negative activation energy, its rate will: (A) Increase with increasing temperature. (B) Decrease with increasing temperature. (C) Remain constant with temperature. (D) Not possible.

Explanation 29:

• Correct Answer: (B)
• Reasoning: From the Arrhenius equation (k=Ae−Ea​/RT), if Ea​ is negative, then −Ea​/RT becomes positive. As T increases, −Ea​/RT becomes a smaller positive number, meaning e−Ea​/RT decreases, and thus k (and rate) decreases. (Note: Negative activation energy is rare and typically associated with complex mechanisms involving pre-equilibrium steps).

Question 30: The graph showing variation of half-life with initial concentration for a first-order reaction would be: (A) A straight line passing through the origin. (B) A horizontal straight line. (C) A curve showing decrease. (D) A curve showing increase.

Explanation 30:

• Correct Answer: (B)
• Reasoning: For a first-order reaction, t1/2​=k0.693​, which is independent of the initial concentration. Therefore, a plot of half-life vs. initial concentration would be a horizontal straight line.

Question 31: What is the order of reaction if the unit of its rate constant is mol1−n Ln−1 s−1? (A) 0 (B) 1 (C) 2 (D) n

Explanation 31:

• Correct Answer: (D)
• Reasoning: The given unit formula directly implies ‘n’ as the order of the reaction. For example, if n=0, it becomes mol L−1s−1.

Question 32: In a reaction, the concentration of reactant decreases from 0.6 M to 0.3 M in 10 minutes. If the reaction is first order, the rate constant (k) is approximately: (A) 0.0693 min−1 (B) 0.693 min−1 (C) 0.03 min−1 (D) 0.00693 min−1

Explanation 32:

• Correct Answer: (A)
• Reasoning: Since the concentration reduces to half (0.6→0.3), the time taken (10 min) is the half-life (t1/2​). For a first-order reaction, k=t1/2​0.693​=10 min0.693​=0.0693 min−1.

Question 33: The activation energy of a reaction is 100 kJ mol−1. In the presence of a catalyst, it decreases to 60 kJ mol−1. By what factor does the rate constant increase at 300 K? (Given: e16≈8.8×106) (A) 104 (B) 106 (C) 108 (D) 1010

Explanation 33:

• Correct Answer: (B)
• Reasoning: The ratio of rate constants with and without catalyst is given by: lnkun​kcat​​=RTEa(un)​−Ea(cat)​​ Ea(un)​−Ea(cat)​=100−60=40 kJ mol−1=40000 J mol−1. R=8.314 J K−1 mol−1, T=300 K. lnkun​kcat​​=8.314×30040000​=2494.240000​≈16.03 kun​kcat​​=e16.03≈e16=8.8×106. So, the rate constant increases by a factor of approximately 106.

Question 34: Which of the following is an example of a unimolecular reaction? (A) 2HI→H2​+I2​ (B) H2​+I2​→2HI (C) NH4​NO2​→N2​+2H2​O (D) 2NO+O2​→2NO2​

Explanation 34:

• Correct Answer: (C)
• Reasoning: In the decomposition of ammonium nitrite (NH4​NO2​), a single molecule undergoes rearrangement to form products. Thus, its molecularity is 1 (unimolecular). The others are bimolecular or trimolecular.

Question 35: If the concentration of reactants is expressed in moles per litre, and time in seconds, the unit of rate of reaction is: (A) mol s (B) mol L s (C) mol L−1s−1 (D) s−1

Explanation 35:

• Correct Answer: (C)
• Reasoning: Rate of reaction is defined as change in concentration per unit time. Concentration units are mol L−1 and time is s. So, the unit of rate is mol L−1s−1.

Question 36: The term ‘activated complex’ refers to: (A) The stable intermediate formed during a reaction. (B) The unstable state of maximum potential energy during a reaction. (C) The reactant molecules that have sufficient energy to react. (D) The product molecules formed after a reaction.

Explanation 36:

• Correct Answer: (B)
• Reasoning: The activated complex (or transition state) is a short-lived, high-energy, unstable intermediate configuration of atoms that exists at the peak of the potential energy barrier during an effective collision.

Question 37: Which of the following is true for a zero-order reaction? (A) The rate of reaction decreases with time. (B) The rate of reaction is independent of reactant concentration. (C) The half-life is constant. (D) The plot of ln[R] vs t is a straight line.

Explanation 37:

• Correct Answer: (B)
• Reasoning: By definition, for a zero-order reaction, the rate of reaction does not depend on the concentration of the reactants. This means the rate remains constant throughout the reaction until the reactant is consumed. (A) is false as rate is constant. (C) is false as t1/2​ depends on initial concentration. (D) is for first-order reaction.

Question 38: The time taken for the completion of a first-order reaction is: (A) 2×t1/2​ (B) 3×t1/2​ (C) Infinite (D) Dependent on initial concentration

Explanation 38:

• Correct Answer: (C)
• Reasoning: For a first-order reaction, the concentration theoretically approaches zero only after an infinite amount of time. Thus, a first-order reaction never truly “completes.”

Question 39: For a reaction to be effective, collisions must have: (A) Sufficient energy (B) Proper orientation (C) Both (A) and (B) (D) High frequency

Explanation 39:

• Correct Answer: (C)
• Reasoning: According to collision theory, for a collision to be effective and lead to a reaction, the colliding molecules must possess both sufficient energy (activation energy) and be oriented properly.

Question 40: What is the effect of increasing temperature by 10∘C on the rate of reaction for many reactions? (A) Rate decreases by half. (B) Rate doubles or triples. (C) Rate remains unchanged. (D) Rate decreases significantly.

Explanation 40:

• Correct Answer: (B)
• Reasoning: For many reactions, especially those occurring at or near room temperature, the rate of reaction approximately doubles or even triples for every 10∘C rise in temperature. This is related to the temperature coefficient.