IB›IB Chemistry HL›Mind mapsR2.2 How fast? The rate of chemical changeIB Chemistry HL: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsCollision theoryReact only if E ≥ Ea and correct orientationRate rises with concentration, pressure, surface areaTemperature raises the proportion with E ≥ EaA catalyst lowers Ea via a new pathwayCatalyst leaves ΔH unchangedMechanismsSeries of elementary stepsSlowest step is the rate-determining stepMolecularity: uni-, bi-, termolecular (rare)Transition state: energy maximumIntermediate: energy minimum, finite lifetimeRate equationrate=k[A]m[B]n\text{rate} = k[A]^m[B]^nrate=k[A]m[B]nOrders found only by experimentOverall order is m+nm + nm+n×2 concentration gives ×1, ×2, ×4: order 0, 1, 2Rate of changekineticskkkEaE_aEat1/2t_{1/2}t1/2Order graphsZero order: concentration falls linearlyFirst order: constant half-lifeSecond order: half-life increasesk units: first order s⁻¹k units: second order dm3 mol−1 s−1\text{dm}^3\text{ mol}^{-1}\text{ s}^{-1}dm3 mol−1 s−1Arrheniusk=Ae−Ea/RTk = Ae^{-E_a/RT}k=Ae−Ea/RTk rises exponentially with temperatureln k against 1/T: gradient −Ea/RIntercept is ln AA reflects correctly oriented collisionsExam tipsNever read orders from the balanced equationk units: divide by concentration to the overall orderEa from gradient is in J: divide by 1000Maxwell–Boltzmann: curve flatter, peak shifts right when hotter