Class 12 Chemistry – Chemical Kinetics Question Bank
MCQ + Assertion Reason + Fill in the Blanks + Very Short + Conceptual Questions
Section A: Multiple Choice Questions (MCQs)
1. Chemical kinetics deals with the study of:
A. Heat changes during reaction
B. Speed and mechanism of reactions
C. Chemical equilibrium only
D. Energy stored in molecules
Answer: B
2. The unit of reaction rate is:
A. mol L⁻¹
B. mol L⁻¹ s⁻¹
C. s⁻¹
D. L mol⁻¹
Answer: B
3. In a chemical reaction, the concentration of reactants generally:
A. Increases with time
B. Decreases with time
C. Remains constant
D. Becomes zero instantly
Answer: B
4. The negative sign in the rate expression for reactants indicates:
A. Reaction is impossible
B. Concentration decreases with time
C. Rate is negative
D. Reaction is slow
Answer: B
5. The rate measured at a particular instant of time is called:
A. Average rate
B. Initial rate
C. Instantaneous rate
D. Mean rate
Answer: C
6. For the reaction:
2A+B→C
the rate expression is:
A. −dtd[A]
B. −21dtd[A]
C. 21dtd[A]
D. −2dtd[A]
Answer: B
7. The rate law is determined by:
A. Balanced chemical equation
B. Experimental data
C. Atomic masses
D. Molecular formula
Answer: B
8. For the rate equation:
Rate=k[A]2[B]
the order of reaction is:
A. 1
B. 2
C. 3
D. 4
Answer: C
9. The order of reaction can be:
A. Always a whole number
B. Always equal to molecularity
C. Zero or fractional
D. Never zero
Answer: C
10. Molecularity is defined for:
A. All reactions
B. Elementary reactions
C. Complex reactions only
D. Equilibrium reactions
Answer: B
11. Molecularity cannot be:
A. One
B. Two
C. Three
D. Fractional
Answer: D
12. The slowest step in a reaction mechanism is called:
A. Fast step
B. Rate determining step
C. Reverse step
D. Equilibrium step
Answer: B
13. The unit of rate constant for a first-order reaction is:
A. mol L⁻¹ s⁻¹
B. L mol⁻¹ s⁻¹
C. s⁻¹
D. mol² L⁻² s⁻¹
Answer: C
14. For a zero-order reaction:
Rate=
A. k[A]
B. k[A]²
C. k
D. k/[A]
Answer: C
15. The integrated rate equation for zero-order reaction is:
A. [R]=[R]0−kt
B. ln[R]=kt
C. k=t0.693
D. Rate=k[R]
Answer: A
16. Half-life of a first-order reaction is:
A. [R]0/2k
B. 0.693/k
C. k/0.693
D. 2k/[R]0
Answer: B
17. Half-life of a first-order reaction depends on:
A. Initial concentration
B. Rate constant
C. Amount of product
D. Pressure only
Answer: B
18. A reaction having constant half-life is generally:
A. Zero order
B. First order
C. Second order
D. Third order
Answer: B
19. Arrhenius equation relates rate constant with:
A. Pressure
B. Temperature
C. Volume
D. Concentration only
Answer: B
20. Activation energy is:
A. Energy released during reaction
B. Minimum energy required for reaction
C. Energy of products
D. Heat absorbed by catalyst
Answer: B
21. A catalyst increases reaction rate by:
A. Increasing ΔG
B. Increasing activation energy
C. Decreasing activation energy
D. Changing equilibrium constant
Answer: C
22. A catalyst:
A. Is completely consumed
B. Provides alternate pathway
C. Stops reaction
D. Changes products
Answer: B
23. In collision theory, successful collision requires:
A. Only collision
B. Proper energy and orientation
C. High pressure only
D. Large molecules only
Answer: B
24. Hydrolysis of ethyl acetate in excess water is an example of:
A. Zero-order reaction
B. Second-order reaction
C. Pseudo first-order reaction
D. Termolecular reaction
Answer: C
Section B: Assertion–Reason Questions
Choose:
A. Both Assertion and Reason are true and Reason correctly explains Assertion.
B. Both are true but Reason does not explain Assertion.
C. Assertion true, Reason false.
D. Assertion false, Reason true.
1.
Assertion: Rate of reaction usually increases with temperature.
Reason: Higher temperature increases the number of molecules having energy greater than activation energy.
Answer: A
2.
Assertion: Order of reaction cannot always be predicted from balanced equation.
Reason: Order depends on experimentally determined rate law.
Answer: A
3.
Assertion: Molecularity may be fractional.
Reason: Molecularity represents number of reacting species.
Answer: D
4.
Assertion: Catalyst increases reaction speed.
Reason: Catalyst lowers activation energy.
Answer: A
5.
Assertion: Half-life of first-order reaction is independent of initial concentration.
Reason: t1/2=0.693/k
Answer: A
Section C: Fill in the Blanks
1. Chemical kinetics deals with the ______ of chemical reactions.
Answer: rate
2. Rate is expressed as change in concentration per unit ______.
Answer: time
3. The rate constant is represented by the symbol ______.
Answer: k
4. The sum of powers of concentration terms in rate law gives ______ of reaction.
Answer: order
5. Molecularity is applicable only to ______ reactions.
Answer: elementary
6. The slowest step of a reaction mechanism is called ______.
Answer: rate determining step
7. For a first-order reaction, unit of rate constant is ______.
Answer: s⁻¹
8. Half-life of first-order reaction is equal to ______.
Answer: 0.693/k
9. Activation energy is represented by ______.
Answer: Ea
10. Arrhenius equation is:
k=Ae−Ea/RT
where A represents ______.
Answer: frequency factor
11. A catalyst provides an alternative pathway with ______ activation energy.
Answer: lower
12. Reaction rate increases when temperature ______.
Answer: increases
Section D: Very Short Answer Questions
1. Define chemical kinetics.
Answer:
Chemical kinetics is the branch of chemistry that studies the rate and mechanism of chemical reactions.
2. What is rate constant?
Answer:
Rate constant is the proportionality constant in the rate equation.
3. What is order of reaction?
Answer:
Order is the sum of powers of concentration terms in the rate law.
4. What is molecularity?
Answer:
Molecularity is the number of reacting species involved in an elementary reaction step.
5. Define activation energy.
Answer:
Activation energy is the minimum energy required for reactant molecules to form products.
6. What happens to rate when temperature increases?
Answer:
Reaction rate generally increases.
7. Give the unit of k for zero-order reaction.
Answer:molL−1s−1
8. Give the unit of k for first-order reaction.
Answer:s−1
Section E: Important 2–3 Mark Questions
1. Differentiate between order and molecularity.
Answer:
| Order | Molecularity |
|---|---|
| Obtained experimentally | Based on reaction mechanism |
| Can be zero/fractional | Always whole number |
| Applies to all reactions | Applies to elementary reactions |
2. Why does a catalyst increase reaction rate?
Answer:
A catalyst lowers activation energy by providing an alternative pathway, increasing the number of successful collisions.
3. Explain pseudo first-order reaction with example.
Answer:
A higher-order reaction that behaves as first order due to excess concentration of one reactant is called pseudo first-order reaction.
Example:
Hydrolysis of ethyl acetate in excess water.
4. Why is molecularity never zero?
Answer:
A reaction step must involve at least one reacting species, so molecularity cannot be zero.
5. Write Arrhenius equation.
Answer:k=Ae−Ea/RT
Section F: Numerical-Based Practice Questions
1. A reaction has rate law:
Rate=k[A]2[B]
Find order.
Answer: 3
2. A first-order reaction has:
k=0.693s−1
Find half-life.
Answer:t1/2=1s
3. For a zero-order reaction:
[R]0=0.5M, k=0.01Ms−1
Find half-life.
Answer:t1/2=25s
4. A first-order reaction has half-life 20 minutes. Find k.
Answer:k=200.693 =0.03465min−1