Exam Notes
Chemical Kinetics Class 12 Exam Prep Revision — Grandmaster Guide
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Last Updated: 2026-04-20
- Rate of reaction = ΔC/Δt
- Rate constant (k) = (2.303/t) log([A]₀/[A]ₜ)
- First order reaction: rate = k[A]
- Second order reaction: rate = k[A]²
- Half life (t₁/₂) = 0.693/k for first order reaction
- t₁/₂ = 1/(k[A]₀) for second order reaction
- Integrated rate law: [A]ₜ = [A]₀ e^(-kt)
- Arrhenius equation: k = Ae^(-Eₐ/RT)
- Collision theory: rate = Ze^(-Eₐ/RT)
🪤 The 5 Mistakes That Cost Marks
- Not understanding the difference between average and instantaneous rate of reaction
- Forgetting to convert concentration units when using rate equations
- Not using the correct units for rate constant (k) and half life (t₁/₂)
- Incorrectly applying the Arrhenius equation
- Not checking the order of reaction before applying integrated rate law
✏️ 3 Solved PYQs
- Question 1: The rate of decomposition of NH₃ on a platinum surface is given by the equation: rate = k[NH₃]. If the half life (t₁/₂) of NH₃ is 2.1 × 10⁻⁵ s, calculate the rate constant (k).
- Solution: t₁/₂ = 0.693/k, k = 0.693/t₁/₂ = 0.693/(2.1 × 10⁻⁵) = 3.3 × 10⁴ s⁻¹
- Question 2: The activation energy (Eₐ) for the reaction: 2NO + O₂ → 2NO₂ is 120 kJ/mol. If the rate constant (k) at 300 K is 2.5 × 10⁻⁵ s⁻¹, calculate the pre-exponential factor (A).
- Solution: k = Ae^(-Eₐ/RT), A = k × e^(Eₐ/RT) = 2.5 × 10⁻⁵ × e^(120000/(8.314 × 300)) = 2.1 × 10¹⁴ s⁻¹
- Question 3: The reaction: A + B → products follows second order kinetics. If the initial concentration of A and B are 0.1 M and 0.2 M respectively, and the rate constant (k) is 0.05 M⁻¹s⁻¹, calculate the half life (t₁/₂) of the reaction.
- Solution: t₁/₂ = 1/(k[A]₀) = 1/(0.05 × 0.1) = 200 s
🧠 The One Thing Most Students Get Wrong
- Most students get confused between the terms "order of reaction" and "molecularity of reaction". Order of reaction is the sum of the exponents of the concentration terms ∈ the rate equation, whereas molecularity is the number of molecules that collide to form the product.
👁️ Ayush's Note
- To solve problems related to chemical kinetics, first identify the order of reaction and then apply the appropriate rate equation.
- Use the Arrhenius equation to relate the rate constant (k) with the activation energy (Eₐ) and temperature (T).
- For JEE Advanced and NEET, practice problems related to the effect of catalysts, inhibitors, and temperature on the rate of reaction.
🔁 Last 5 Minutes Box
- Check the units of the rate constant (k) and half life (t₁/₂).
- Verify the order of reaction before applying the integrated rate law.
- Use the correct equation for the half life (t₁/₂) of first and second order reactions.
- Apply the Arrhenius equation to relate the rate constant (k) with the activation energy (Eₐ) and temperature (T).
- Practice problems related to the effect of catalysts, inhibitors, and temperature on the rate of reaction.
📝 Practice MCQs
1. What is the unit of rate constant (k) for a first order reaction?
A) s⁻¹
B) M⁻¹s⁻¹
C) M²s⁻¹
D) M³s⁻¹
Answer: A) s⁻¹.
2. The rate of reaction is given by the equation: rate = k[A]². What is the order of reaction?
A) First order
B) Second order
C) Zero order
D) Third order
Answer: B) Second order.
3. The activation energy (Eₐ) for a reaction is 100 kJ/mol. If the rate constant (k) at 300 K is 1.0 × 10⁻³ s⁻¹, what is the pre-exponential factor ( A) ?
A) 1.0 × 10⁻⁵ s⁻¹
B) 1.0 × 10⁻³ s⁻¹
C) 1.0 × 10⁻¹ s⁻¹
D) 1.0 × 10¹⁵ s⁻¹
Answer: D) 1.0 × 10¹⁵ s⁻¹.
4. The half life (t₁/₂) of a first order reaction is 2.0 × 10⁻⁴ s. What is the rate constant (k)?
A) 3.5 × 10³ s⁻¹
B) 3.5 × 10⁻³ s⁻¹
C) 3.5 × 10⁻⁴ s⁻¹
D) 3.5 × 10⁻⁵ s⁻¹
Answer: A) 3.5 × 10³ s⁻¹.
5. The reaction: A + B → products follows second order kinetics. If the initial concentration of A and B are 0.1 M and 0.2 M respectively, and the rate constant (k) is 0.05 M⁻¹s⁻¹, what is the half life (t₁/₂) of the reaction?
A) 100 s
B) 200 s
C) 300 s
D) 400 s
Answer: B) 200 s.
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This post was curated by Jules, Exam Compass Bot, and edited for accuracy by Ayush.
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