Chemical Kinetics — Previous Year Questions (Class 12 Chemistry)
Chemical Kinetics explores reaction rates, rate laws, and mechanisms. It explains how fast reactions occur and what factors influence reaction speed, essen
TL;DR: Chemical Kinetics explores reaction rates, rate laws, and mechanisms. It explains how fast reactions occur and what factors influence reaction speed,…
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Chemical Kinetics explores reaction rates, rate laws, and mechanisms. It explains how fast reactions occur and what factors influence reaction speed, essen
Chemical Kinetics — Previous Year Questions with Solutions
Q (2023, 2 marks): Define the rate of reaction. For reaction: A → B, if concentration of A decreases by 0.04 mol/L in 4 seconds, find the rate.
Answer: Rate of reaction is the change in concentration of reactants/products per unit time.
Rate = -Δ[A]/Δt or +Δ[B]/Δt
Given: Δ[A] = -0.04 mol/L, Δt = 4 s
Rate = -(-0.04) / 4
Rate = 0.04 / 4
Rate = 0.01 mol/(L·s)
Q (2022, 2 marks): For reaction 2A + B → C, if d[A]/dt = -0.02 mol/(L·s), find d[C]/dt.
Answer: From stoichiometry: 2A + B → C
For every 2 moles of A consumed, 1 mole of C is produced
-d[A]/dt : d[C]/dt = 2 : 1
d[C]/dt = (1/2) × d[A]/dt
d[C]/dt = (1/2) × 0.02
d[C]/dt = 0.01 mol/(L·s)
Q (2023, 1 mark): The rate constant for a reaction is 0.005 min^-1. What is its order?
Answer: The units of rate constant indicate the order:
k has units min^-1 or s^-1
This means: Rate = k[A]^n where n = 1
Therefore, it is a first-order reaction.
(For zero-order: k has units mol/(L·s); for second-order: k has units L/(mol·s); for first-order: k has units s^-1)
Q (2021, 2 marks): For a first-order reaction, if the initial concentration is 0.1 M and rate constant k = 0.693 min^-1, find half-life.
Answer: For first-order reaction: t(1/2) = 0.693 / k
where k = 0.693 min^-1
t(1/2) = 0.693 / 0.693
t(1/2) = 1 minute
Alternatively, using t(1/2) = ln(2) / k = 0.693 / k
Q (2022, 3 marks): Explain the effect of temperature on reaction rate using collision theory.
Answer: Effect of temperature on reaction rate (Collision Theory):
1. Increased molecular kinetic energy: At higher temperature, molecules move faster and have more energy
2. More frequent collisions: Higher velocity increases collision frequency per unit time
3. More effective collisions: A larger fraction of collisions have energy ≥ activation energy (Ea)
4. Exponential increase in rate: For every 10°C increase, reaction rate approximately doubles (rule of thumb)
Mathematically: k = A × e^(-Ea/RT)
where A = pre-exponential factor, R = gas constant, T = temperature
As T increases, k increases exponentially
Q (2023, 2 marks): What is activation energy? How does a catalyst lower it?
Answer: Activation energy (Ea) is the minimum energy required for reactants to form products. It is the energy difference between reactants and the transition state.
How catalyst lowers activation energy:
1. Provides an alternative reaction pathway with lower Ea
2. Forms unstable intermediate complexes that break down easily
3. Does not change the overall energy of reactants and products
4. Increases the rate of both forward and reverse reactions
5. Emerges unchanged at the end of reaction
Example: MnO2 catalyst in decomposition of H2O2 lowers Ea significantly, speeding up reaction
Frequently Asked Questions
What is the difference between rate and rate constant?
Rate is the actual speed of reaction at a given moment (changes with concentration). Rate constant k depends only on temperature, not concentration. Rate = k[A]^n.
How do you determine reaction order from experimental data?
Use initial rate method: vary concentration of one reactant, observe rate change. If doubling concentration doubles rate, it's first-order. If it quadruples, it's second-order. Or use integrated rate law: plot ln[A] vs t (first-order) or 1/[A] vs t (second-order) for straight line.
More Class 12 Chemistry PYQs
- Electrochemistry
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- Coordination Compounds
- Haloalkanes and Haloarenes
- Aldehydes Ketones and Carboxylic Acids
- Some Basic Concepts of Chemistry
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