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Syllabus
🩺 NEET UG
Subject:Chemistry Syllabus

Chemical Kinetics

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

Chemical Kinetics studies how fast reactions proceed and what governs that speed — defining rate and reaction order experimentally, using integrated rate laws to predict concentration over time, the Arrhenius equation's temperature dependence, and collision theory's molecular explanation of why reactions speed up with temperature, concentration and catalysts.

Chemistry Syllabus Physical Chemistry ~25% weightage
Live Animation1 / 4
Rate = k[A]^m[B]^n
1Rate Constant (k)Fixed at a given T
2Order in A (m)Found experimentally
3Order in B (n)Found experimentally
4Overall Order (m+n)Sum of exponents

1Rate of Reaction & Order

Reaction rate measures how fast concentration changes with time; reaction order — found only by experiment — describes how the rate actually depends on concentration, which need not match the balanced equation's coefficients.

Learning Objectives

  • Define average and instantaneous rate of reaction in terms of concentration change over time.
  • Write the rate law (rate = k[A]^m[B]^n) and define reaction order as the sum of exponents m+n.
  • Explain why reaction order must be determined experimentally and can differ from stoichiometric coefficients.
  • Distinguish order of reaction from molecularity of an elementary step.

Key Terms

Rate of ReactionThe change in concentration of a reactant or product per unit time, expressed as -d[R]/dt or +d[P]/dt.
Rate LawThe experimentally determined equation rate = k[A]^m[B]^n relating rate to reactant concentrations.
Order of ReactionThe sum of the exponents (m+n) of the concentration terms in the experimental rate law; can be zero, fractional, or a whole number.
MolecularityThe number of reactant species colliding simultaneously in an elementary step; always a whole number, unlike order, and meaningful only for elementary reactions.

Rate measures the speed of a reaction by tracking how quickly a reactant disappears or a product appears, per unit time.

Rate = -(1/a)(d[A]/dt) = -(1/b)(d[B]/dt) = +(1/c)(d[C]/dt) for aA + bB → cC

Rate law and order

Rate = k[A]^m[B]^n Order = m + n
  • Order is found ONLY by experiment (the initial rate method), never assumed from the balanced equation.
  • Order can be zero (rate independent of concentration), fractional (e.g. 1.5, common in chain or surface reactions), or a whole number.
Common MistakeClassic NEET trap: for 2NO + O2 → 2NO2, the experimentally determined rate law is rate = k[NO]²[O2], giving an overall order of 3 — NOT simply readable from the equation's stoichiometric coefficients.

Order vs molecularity

  • Molecularity applies only to a single elementary step and is always a small whole number (1, 2, rarely 3).
  • Order applies to the overall, possibly multi-step reaction, and can be fractional or zero.
  • For a single-step elementary reaction, order equals molecularity; for multi-step reactions they usually differ.
Exam TipThe slowest step in a multi-step mechanism — the rate-determining step — dictates the experimental rate law for the overall reaction.