CBSE Class 12 Chemistry: Chemical Kinetics — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Define rate of reaction. Differentiate between order and molecularity.
  Ans: Rate of reaction is the change in concentration of reactant or product per unit time. Rate = -d[R]/dt = d[P]/dt. Order: Sum of powers of concentration terms in the rate law (experimentally determined). Can be 0, 1, 2, fractional. Molecularity: Number of reacting species in an elementary step (theoretical). Always a whole number (1, 2, or 3). Order applies to overall reaction; molecularity to elementary steps only.
• Q: Derive the integrated rate equation for a first-order reaction. What is its half-life?
  Ans: For first-order: Rate = k[A]. -d[A]/dt = k[A]. Integrating: ∫d[A]/[A] = -k∫dt. ln[A] = -kt + ln[A₀]. ln([A₀]/[A]) = kt. Or: k = (2.303/t)log([A₀]/[A]). Half-life t₁/₂: At t₁/₂, [A] = [A₀]/2. k = (2.303/t₁/₂)log(2) = 0.693/t₁/₂. t₁/₂ = 0.693/k. Half-life is independent of initial concentration for first-order reactions.
• Q: What is Arrhenius equation? A reaction has Ea = 50 kJ/mol. By what factor does rate increase from 300 K to 310 K?
  Ans: Arrhenius equation: k = Ae^(-Ea/RT). ln(k₂/k₁) = (Ea/R)(1/T₁ - 1/T₂). Given: Ea = 50000 J/mol, T₁ = 300 K, T₂ = 310 K. ln(k₂/k₁) = (50000/8.314)(1/300 - 1/310) = 6013.5 × (10/93000) = 0.6466. k₂/k₁ = e^0.6466 = 1.91. Rate increases by approximately 1.91 times (~2 times for 10° rise, consistent with general rule).

Exam Tips for This Chapter

• Revise all definitions and laws from Chemical Kinetics — commonly asked as 1-2 mark questions
• Practice diagrams related to Chemical Kinetics — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Chemical Kinetics carries significant marks in the board exam