Questions
5 questions per paper
Difficulty
Medium-Hard
Importance
High yield for JEE Advanced and NEET rank-deciding section.
Overview
Chemical Kinetics is a foundational physical chemistry chapter dealing with the speed of reactions and the molecular pathways they follow. It is a high-yield topic in JEE and NEET, requiring mastery over calculus-based derivations and graph interpretation. Aspirants must focus on translating reaction mechanisms into rate laws and understanding the temperature dependence of rate constants.
Rate Laws and Order of Reaction
The rate law expresses the relationship between reaction rate and reactant concentrations, defined by experiment rather than stoichiometry. Understanding the difference between elementary reactions and complex mechanisms is vital for determining the overall order.
- Rate = k[A]^x[B]^y
- Order is the sum of exponents (x + y)
- Molecularity applies only to elementary steps
- Units of k: (mol L^-1)^(1-n) s^-1
- Pseudo-first order reactions (e.g., acid-catalyzed hydrolysis of esters)
Integrated Rate Equations
These equations provide the concentration of reactants at any given time 't' based on initial concentrations. Memorizing the linear plot forms is crucial for solving graphical problems in competitive exams.
- Zero order: [A] = [A]0 - kt; half-life t1/2 = [A]0/2k
- First order: ln[A] = ln[A]0 - kt; t1/2 = 0.693/k
- Second order: 1/[A] = 1/[A]0 + kt
- First order slope: -k (for ln[A] vs t plot)
- Radioactive decay follows first-order kinetics
Arrhenius Equation & Activation Energy
The Arrhenius equation describes how temperature influences the rate constant through activation energy (Ea) and frequency factor (A). High-level exam questions often involve two-temperature comparison problems.
- k = A * e^(-Ea/RT)
- log(k2/k1) = (Ea/2.303R) * [(T2-T1)/(T1*T2)]
- Ea represents the minimum energy barrier for effective collisions
- Catalysts provide an alternative path with lower Ea
- Effect of temperature: Rate constant roughly doubles for a 10-degree rise
Reaction Mechanisms
Complex reactions occur through a series of elementary steps where the slowest step is the rate-determining step (RDS). Aspirants must learn to derive the rate law from mechanisms involving fast equilibrium intermediates.
- Rate of reaction is dictated by the slowest step
- Steady State Approximation (SSA) used for reactive intermediates
- Pre-equilibrium method for reactions with rapid initial steps
- Mechanism must be consistent with the experimental rate law
Formula Sheet
Rate = -d[R]/dt = d[P]/dt
k = A * exp(-Ea/RT)
ln[A]t = ln[A]0 - kt
t1/2 (first order) = 0.693/k
log(k2/k1) = Ea/2.303R * (T2-T1)/(T1*T2)
Exam Tip
Always plot the graph provided in the question; if it is linear, determine the order immediately from the slope and intercept before attempting complex calculations.
Common Mistakes
- Confusing molecularity (a theoretical value) with order of reaction (an experimental value).
- Forgetting to convert activation energy units (kJ/mol vs J/mol) when using the gas constant R.
- Assuming the rate law exponents match stoichiometric coefficients for all reactions.
More Revision Notes
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