Questions
3–5 questions per paper
Difficulty
Medium
Importance
Core — never skip
Overview
Chemical Kinetics is the branch of physical chemistry that deals with the speed of chemical reactions and the mechanism by which they occur. It is a high-weightage topic in board exams, specifically focusing on the mathematical relationship between concentration, time, and rate constants. Mastering this topic requires a strong grasp of rate laws, integrated rate equations, and the effect of temperature on reaction rates.
Rate of Reaction
The rate of reaction is the change in concentration of a reactant or product per unit time. For board exams, you must be proficient in writing the rate expression using stoichiometric coefficients and interpreting graphs of concentration versus time.
- Rate = -d[R]/dt = +d[P]/dt
- Rate depends on concentration, temperature, and catalysts
- Average rate vs instantaneous rate
- Rate expression for aA + bB -> cC + dD: Rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt
Order and Molecularity
Order is an experimental value representing the sum of powers of concentration terms in the rate law, whereas molecularity is a theoretical concept of the number of reacting species colliding simultaneously. Distinguishing between these two is a staple 2-mark question in board examinations.
- Order can be zero, fractional, or integer
- Molecularity is always a whole number (1, 2, or 3)
- Pseudo-first order reactions (e.g., hydrolysis of esters)
- Rate law must be determined experimentally, not from balanced equations
Arrhenius Equation
The Arrhenius equation describes the quantitative relationship between temperature and the rate constant of a reaction. It is frequently tested through numerical problems involving two different temperatures to calculate activation energy.
- k = A * exp(-Ea / RT)
- ln(k2/k1) = (Ea / 2.303R) * [(T2 - T1) / (T1 * T2)]
- A is the Arrhenius factor or pre-exponential factor
- Ea represents the minimum energy barrier for a reaction
Formula Sheet
Rate = k[A]^x[B]^y
k = (2.303/t) * log([R0]/[R]) (1st order)
k = [R0] - [R] / t (0th order)
t(1/2) = 0.693/k (1st order)
k = A * e^(-Ea/RT)
Exam Tip
Always verify the units of the rate constant 'k' in the question, as they reveal the order of the reaction even if it isn't explicitly stated.
Common Mistakes
- Confusing the units of rate constant k for different orders (e.g., mol L-1 s-1 for zero order vs s-1 for first order).
- Ignoring stoichiometric coefficients when writing the rate expression for reactions where a, b, c, and d are not equal to 1.
- Applying the integrated rate equation for first-order reactions to second-order reactions by mistake.
More Revision Notes
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