HS 2nd year Chemistry Unit 3: Chemical Kinetics

 

🔹 Topic: Introduction to Chemical Kinetics

Q1. What does chemical kinetics study?
Ans. Chemical kinetics studies the rate of chemical reactions, factors affecting them, and the mechanism by which they occur.

Keywords:

• Chemical kinetics – রাসায়নিক গতি বিদ্যা

• Reaction rate – বিক্রিয়া হার

• Mechanism – প্রক্রিয়া

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🔹 Topic: Average and Instantaneous Rate of Reaction

Q2. Define average rate of a chemical reaction.
Ans. Average rate is the change in concentration of reactants or products divided by the time interval during which the change occurs.
$\text{Average Rate} = \frac{\Delta [R]}{\Delta t}$

Q3. Define instantaneous rate of reaction.
Ans. It is the rate of change of concentration of a reactant or product at a specific instant of time.
$\text{Instantaneous Rate} = \frac{d[R]}{dt}$

Keywords:

• Average rate – গড় হার

• Instantaneous rate – তাৎক্ষণিক হার

• Concentration – ঘনত্ব

• Time interval – সময় ব্যবধান

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🔹 Topic: Rate of Reaction and Stoichiometry

Q4. Why do we divide the rate of disappearance or appearance by stoichiometric coefficients?
Ans. Because in reactions where coefficients are not 1, rate is proportionally related and must be normalized to compare rates of different species.

Keywords:

• Stoichiometry – স্টয়কিওমেট্রি

• Disappearance – অদৃশ্য হওয়া

• Appearance – আবির্ভাব

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🔹 Topic: Rate Law and Rate Constant

Q5. What is rate law?
Ans. It expresses the rate of a reaction as a function of the concentration of reactants raised to some powers.
Example: $\text{Rate} = k[A]^x[B]^y$

Q6. What is rate constant?
Ans. Rate constant $k$ is the proportionality factor in the rate law.

Keywords:

• Rate law – হার সূত্র

• Rate constant – হার ধ্রুবক

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🔹 Topic: Order and Molecularity of Reaction

Q7. What is the order of a reaction?
Ans. The sum of the powers of concentration terms in the rate law expression.

Q8. What is molecularity?
Ans. The number of reacting species involved in an elementary step of a reaction.

Keywords:

• Order – ক্রম

• Molecularity – অণুসংখ্যা

• Elementary reaction – মৌলিক বিক্রিয়া

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🔹 Topic: Integrated Rate Equations

Q9. Derive the integrated rate law for a first-order reaction.
Ans. For a first-order reaction:
$\ln[R] = -kt + \ln[R_0]$
or
$k = \frac{2.303}{t} \log \frac{[R_0]}{[R]}$

Keywords:

• Integrated rate equation – সমন্বিত হার সমীকরণ

• First order – প্রথম ক্রম

• Logarithm – লগারিদম

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🔹 Topic: Half-life of Reactions

Q10. Define half-life of a reaction.
Ans. It is the time taken for the concentration of a reactant to reduce to half of its initial value.

• For first-order: $t_{1/2} = \frac{0.693}{k}$

Keywords:

• Half-life – অর্ধ-আয়ু

• Initial concentration – প্রাথমিক ঘনত্ব

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🔹 Topic: Temperature Dependence – Arrhenius Equation

Q11. Write and explain the Arrhenius equation.
Ans.
$k = A e^{-E_a/RT}$
It shows the effect of temperature on the rate constant. $E_a$ is the activation energy.

Keywords:

• Arrhenius equation – অ্যারেনিয়াস সমীকরণ

• Activation energy – সক্রিয়ন শক্তি

• Frequency factor – বারম্বারতা ধ্রুবক

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🔹 Topic: Catalyst and Collision Theory

Q12. What is the role of a catalyst in a reaction?
Ans. A catalyst increases the rate of reaction by providing an alternate path with lower activation energy.

Q13. What are effective collisions?
Ans. Collisions in which molecules have sufficient energy and proper orientation to form products.

Keywords:

• Catalyst – অনুঘটক

• Collision – সংঘর্ষ

• Effective collision – কার্যকর সংঘর্ষ

• Orientation – বিন্যাস

🔬 Class 12 Chemistry – Chapter 3: Chemical Kinetics

📝 Worksheet (With Bengali Keywords)

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Section A: Very Short Answer (1 mark each)

Write the answer in one sentence.

1. What is chemical kinetics?

2. What is the formula for average rate of a chemical reaction?

3. Write the expression for the instantaneous rate of a reaction.

4. What is meant by ‘rate constant’?

5. Define molecularity.

6. What is the half-life of a reaction?

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Section B: Short Answer (2-3 marks each)

Answer briefly.

7. What is the difference between average rate and instantaneous rate?

8. Why is the rate of disappearance or appearance divided by stoichiometric coefficients?

9. What is a rate law? Give an example.

10. Explain the order of a reaction with an example.

11. Write the integrated rate equation for a first-order reaction.

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Section C: Long Answer (5 marks each)

Answer in detail.

12. Derive the expression for the integrated rate law of a first-order reaction.

13. Explain the Arrhenius equation. How does temperature affect the rate of a reaction?

14. Describe the role of a catalyst in a chemical reaction. How does it affect activation energy?

15. What are effective collisions? Explain with an example.

16. Differentiate between order and molecularity of a reaction.

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Section D: Numerical Questions

17. For a first-order reaction, calculate the half-life when rate constant $k = 2.31 \times 10^{-3}\ s^{-1}$.

18. A reaction follows first-order kinetics. If 80% of the reaction is complete in 20 minutes, calculate the rate constant.

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Section E: Bengali Keyword Matching

Match the English keyword with its Bengali meaning:

English Term	Bengali Meaning
1. Reaction rate	A. অনুঘটক
2. Catalyst	B. বিক্রিয়া হার
3. Molecularity	C. লগারিদম
4. Logarithm	D. সক্রিয়ন শক্তি
5. Activation energy	E. অণুসংখ্যা

Section F: Application-Based Questions

19. In the reaction A + B → C, if the rate = k[A][B]², how will the rate change if:
    a) [B] is doubled
    b) Both [A] and [B] are halved

20. Explain how reaction mechanism relates to the rate determining step with an example.