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Chemical Kinetics revision notes

Every idea in the Chemical Kinetics chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 23 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Rate of a reaction

Average and instantaneous rate
2× asked, last 2026
rav=−Δ[R]Δt=Δ[P]Δt,rinst=−d[R]dt  (slope of tangent)r_{\text{av}} = -\frac{\Delta[\mathrm R]}{\Delta t} = \frac{\Delta[\mathrm P]}{\Delta t},\qquad r_{\text{inst}} = -\frac{d[\mathrm R]}{dt}\ \ (\text{slope of tangent})
Unit mol L⁻¹ s⁻¹ (gases: atm s⁻¹). Rate depends on concentration, not amount: more of the same solution, same rate.
tt[R]t1t_1ttt2t_2secant: average ratetangent: instantaneous
Rate =−=-slope: of the chord between t1t_1 and t2t_2 (average) or of the tangent at tt (instantaneous)
Rate in terms of each species
5× asked, last 2026
aA+bB→cC:r=−1ad[A]dt=−1bd[B]dt=1cd[C]dta\mathrm A + b\mathrm B \to c\mathrm C:\quad r = -\frac{1}{a}\frac{d[\mathrm A]}{dt} = -\frac{1}{b}\frac{d[\mathrm B]}{dt} = \frac{1}{c}\frac{d[\mathrm C]}{dt}
Divide each species' rate of change by its coefficient; minus sign for reactants. Every species then gives the same rr.
ttconc.12NX2OX5\ce{N2O5}NOX2\ce{NO2}OX2\ce{O2}
NOX2\ce{NO2} forms twice as fast as NX2OX5\ce{N2O5} is used; dividing each by its coefficient gives one rate
  • Not one species' rate as the reaction rate. Divide by its coefficient. In 2 NX2OX5→4 NOX2+OX2\ce{2N2O5 -> 4NO2 + O2}, NOX2\ce{NO2} forms twice as fast as NX2OX5\ce{N2O5} is used; unknown coefficients are in the ratio of the species' rates.
Factors that change the rate
3× asked, last 2026
V→V/f:  every gas concentration×f,r→f n r  (n=overall order)V \to V/f:\ \ \text{every gas concentration}\times f,\qquad r \to f^{\,n}\,r\ \ (n = \text{overall order})
Rate rises with concentration (pressure; not zero order), temperature, catalyst, surface area. Only TT and a catalyst change kk.
  • Not 'smaller volume, slower reaction'. The same molecules in less volume are more concentrated. Elementary A(g)+B(g)→C(g)+D(g)\ce{A(g) + B(g) -> C(g) + D(g)} (order 2) at one third the volume runs 32=93^2=9 times faster.
Also remember
  • Iodide–HX2OX2\ce{H2O2} clock: a little thiosulphate (less than the KI) removes the IX2\ce{I2} formed; once it is used up, IX2\ce{I2} turns fresh starch blue. Time the first blue.

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Practise Chemical Kinetics

20 more concepts in 5 topics

Rate law, order and molecularity; Finding the order; Zero- and first-order reactions; Temperature and the Arrhenius equation; and Energy profiles, collisions and catalysts. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Rate law, order and molecularity

  • Rate law and order6×
  • Units of the rate constant5×
  • Molecularity and elementary steps2×
  • Mechanism and rate-determining step3×

Finding the order

  • Initial-rate (ratio) method9×
  • Half-life method10×
  • Graphs that identify the order15×

Zero- and first-order reactions

  • Zero-order reactions9×
  • First-order integrated rate law12×
  • First-order half-life and multiples26×
  • Two first-order decays together4×
  • Gas-phase first order by pressure10×
  • Pseudo-first-order reactions3×

Temperature and the Arrhenius equation

  • Arrhenius equation11×
  • Two-temperature form14×
  • Plot of ln k against 1/T9×
  • Lowering Ea: rate-constant ratio6×
  • Also remember1

Energy profiles, collisions and catalysts

  • Activation energy and energy profile7×
  • Collision theory3×
  • Catalysts4×

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