Unit 1: Chemical Kinetics – Previous Year Board Questions
Note for Students: This resource provides very important questions from Unit 3: Chemical Kinetics along with authentic previous years' board questions (PYQs) from CBSE, ISC, NIOS and other Board exams. Use this to master important concepts and practice questions asked in real board exams.
CBSE 2025: Define rate of a reaction and state its SI unit. (1 mark)
The rate of a reaction is the change in concentration of a reactant or product per unit time.
SI unit is mol L−1 s−1.
SI unit is mol L−1 s−1.
CBSE 2025: Write any two factors affecting the rate of a chemical reaction. (2 marks)
Concentration of reactants, temperature, and presence of catalyst affect the rate of reaction.
CBSE 2025: What is the relationship between the order of a reaction and molecularity? (1 mark)
Order is the sum of powers in the rate law and may be fractional, molecularity is the number of molecules involved in an elementary reaction step and is always a whole number.
CBSE 2025: The rate constant of a first-order reaction is 0.03 s−1. Calculate the half-life of the reaction. (2 marks)
Half-life (t1/2 = 0.693/k = 0.693/0.03 = 23.1 seconds.
ISC 2025: A first order reaction is 50% completed in 30 minutes at 300 K and in 10 minutes at 320 K. Calculate the activation energy of the reaction. (4 marks)
Using Arrhenius equation and rate constants at two temperatures, activation energy Ea ≈ 47.8 kJ/mol.
CBSE 2024: Differentiate between average rate and instantaneous rate of reaction. (2 marks)
Average rate is the change in concentration over a finite time interval; instantaneous rate is the rate at a particular instant (slope of concentration-time curve).
CBSE 2024: For a zero-order reaction, the rate constant k = 0.01 mol L−1 s−1. Calculate the time required for the concentration of the reactant to decrease from 0.5 mol L−1 to 0.1 mol L−1. (3 marks)
[A] = [A]o - kt
t = [A]o - [A]/k = (0.5 - 0.1)/0.01 = 40 seconds.
t = [A]o - [A]/k = (0.5 - 0.1)/0.01 = 40 seconds.
CBSE 2024: Write the integrated rate equation for a first-order reaction. (3 marks)
Integrated form: ln[A] = −kt + ln[A]₀
[A] = [A]o e-kt.
[A] = [A]o e-kt.
CBSE 2024: Calculate the half-life of a zero-order reaction if initial concentration is 0.5 mol L−1 and rate constant is 0.01 mol L−1 s−1. (2 marks)
Half-life (t1/2) = [A]o/2k = 25 s.
CBSE 2024: Write the Arrhenius equation and explain each term briefly. (3 marks)
Arrhenius equation: k = A e-Ea/RT, where k is rate constant, A is frequency factor, Ea is activation energy, R is gas constant, and T is temperature in K.
ISC 2024: The initial rate of a reaction doubles when concentration of A is doubled and increases eight fold when concentrations of both A and B are doubled. Find the order with respect to A and B and write the rate law. (3 marks)
Order with respect to A = 1, B = 2. Rate law: rate = k[A]1[B]2.
CBSE 2023: Write two factors affecting the rate of reaction. (1 mark)
Temperature and concentration.
CBSE 2023: Define the specific rate constant. How does it differ from the rate of reaction? (2 marks)
Specific rate constant (k) is a proportionality constant in the rate law, independent of reactant concentration. Rate of reaction depends on both k and concentration of reactants.
CBSE 2023: The concentration of a reactant decreases from 0.5 mol L−1 to 0.125 mol L−1 in 40 minutes for a first-order reaction. Calculate the rate constant. (3 marks)
ln[A]o/[A] = kt
k = ln(0.5/0.125)/(40×60) = 1.386/2400 = 0.0005775 s−1.
k = ln(0.5/0.125)/(40×60) = 1.386/2400 = 0.0005775 s−1.
CBSE 2023: Write the rate law and unit of rate constant for a reaction where rate = k[A]. (2 marks)
Rate = k[A]; unit of k is s−1.
CBSE 2023: Calculate the half-life of a first-order reaction with rate constant 0.03 s−1. (2 marks)
Half-life (t1/2) = 0.693/k = 0.693/0.03 = 23.1 s.
ISC 2023: Write the rate law for reaction order 1, 2, and 0 with respect to reactants A, B, and C respectively. If concentrations of A, B and C are doubled, how many times will the rate increase? (3 marks)
Rate law: rate = k[A]1[B]2[C]0. Rate increase = 2 × 22 × 1 = 8 times.
ISC 2023: Explain the concept of activation energy and write Arrhenius equation. (3 marks)
Activation energy is minimum energy for effective collision to lead to a reaction. Arrhenius equation: k = A e-Ea/RT.
ISC 2023: 20% of a first order reaction is completed in five minutes. How much time will the 60% reaction take to complete? Calculate the half-life period (t1/2,) for the above reaction.(3 marks)
The time taken for 60% reaction to complete is approximately 20.53 minutes.
The half-life period (t1/2) for the reaction is approximately 15.54 minutes.
The half-life period (t1/2) for the reaction is approximately 15.54 minutes.
CBSE 2022: Explain the effect of temperature on the rate of reaction with special reference to the Arrhenius equation. (3 marks)
Rate constant increases with temperature exponentially according to k = A e-Ea/RT. Higher temperature decreases negative exponent, increasing k.
CBSE 2022: Explain the term half-life for a first-order reaction and state its significance. (2 marks)
Half-life is the time required for the concentration of a reactant to fall to half its initial value. It is constant and independent of initial concentration for first-order reactions.
CBSE 2022: The half-life of a first-order reaction is 36 minutes. Calculate the rate constant k. (2 marks)
k = 0.693/t1/2 = 0.693/(36×60) = 3.21×10-4 s−1.
CBSE 2022: Differentiate between order and molecularity of a reaction. (3 marks)
Order is the sum of powers of concentration terms in rate law, can be fractional. Molecularity is the number of reacting species in an elementary step, always integer.
ISC 2022: Calculate the time taken for 90% completion of a first order reaction if 25% reaction is completed in 30 minutes. (3 marks)
Using integrated rate law, time for 90% completion ≈ 90 minutes.
ISC 2022: State the elementary idea of collision theory. (2 marks)
Molecules must collide with proper orientation and minimum activation energy for a reaction to occur.
CBSE 2021: What is activation energy? How does it influence reaction rate? (2 marks)
Activation energy is the minimum energy required to initiate a reaction. Larger activation energy slows down the reaction rate.
CBSE 2021: Write the integrated rate equation for a zero-order reaction and explain its graphical representation. (3 marks)
Equation: [A] = [A]o - kt.
Plot of concentration vs time is a straight line with slope −k.
Plot of concentration vs time is a straight line with slope −k.
CBSE 2021: State the elementary idea of collision theory. (2 marks)
Reactions occur when molecules collide with proper orientation and sufficient energy (activation energy).
CBSE 2021: The activation energy for a reaction is 50 kJ mol−1. Calculate the ratio of rate constants at 300 K and 350 K using Arrhenius equation. (4 marks)
k2/k1 = eEa/R[1/T1 - 1/T1]
Ratio is approximately 5.64.
Ratio is approximately 5.64.
ISC 2021: Define molecularity and order of reaction. How are they different? (3 marks)
Molecularity is the number of molecules involved in elementary step (always whole number). Order is sum of powers of concentration in rate law (can be fractional).
ISC 2021: Calculate the ratio of rate constants at 300 K and 350 K for a reaction with activation energy of 50 kJ/mol. (4 marks)
Ratio k350/k300 = eEa/R(1/300) - 1/350) = 5.64.
CBSE 2020: Write the integrated rate law for a zero-order reaction and give the expression for its half-life. (3 marks)
Integrated law: [A]t = [A]o - kt.
Half-life (t1/2) = [A]o/2k.
Half-life (t1/2) = [A]o/2k.
CBSE 2020: A reaction is first order with respect to A. Write the rate equation. What is the unit of rate constant? (2 marks)
Rate = k[A]; unit of rate constant is s−1.
CBSE 2020: List any two assumptions made in the collision theory of reaction rates. (2 marks)
Two assumptions are: molecules must collide to react; only collisions with energy greater than activation energy lead to reaction.
CBSE 2020: For a first-order reaction, how long will it take to reduce the reactant concentration to one-eighth of its initial value if the rate constant is 0.025 s−1? (3 marks)
Time(t) = (1/k) ln[A]o/[A] = (1/0.025)ln8 = 110.9 seconds.
ISC 2020: The concentration of a reactant in a first order reaction falls from 0.5 mol/L to 0.125 mol/L in 40 minutes. Calculate the rate constant k. (3 marks)
ln[A]o/[A] = kt
k = 0.0005775 s−1.
k = 0.0005775 s−1.
ISC 2020: Write the integrated rate law for zero and first order reactions and give their half-life expressions. (4 marks)
Zero order: [A] = [A]o - kt
t1/2 = [A]o/2k.
First order: ln[A] = -kt + ln [A]o
t1/2 = 0.693/k.
t1/2 = [A]o/2k.
First order: ln[A] = -kt + ln [A]o
t1/2 = 0.693/k.