Kinetics & Equilibrium is a core part of IB Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
RateChange in concentration per unit time
Activation energy EaMin energy for reaction
Collision theoryReactions need successful collisions
Maxwell-BoltzmannDistribution of molecular energies
CatalystLowers Ea; not consumed
Surface areaMore area = more collisions
Rate equationrate = k[A]^m[B]^n
Order mPower to which [A] is raised
Rate constant kConstant at fixed T (units depend on order)
Half-lifeTime for [A] to halve (constant for 1st order)
Rate-determining stepSlowest step in mechanism
MolecularityNumber of species in elementary step
Arrhenius equationk = Ae^(-Ea/RT)
Pre-exponential AFrequency + geometry factor
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
How does the rate of an SN2 reaction depend on concentrations?✗ SN2 reactions are first order overall because only the substrate matters. ✓ Rate is first order in both substrate AND nucleophile (overall second order) because both are involved in the rate-determining step.
What does an NMR chemical shift (δ, in ppm) actually measure?✗ Chemical shift is the absolute resonance frequency of the nucleus in Hz. ✓ The position of a peak RELATIVE to TMS (tetramethylsilane, δ = 0), normalised by the spectrometer frequency.
What distinguishes a heterogeneous from a homogeneous catalyst?✗ Heterogeneous catalysts are solid and homogeneous catalysts are liquid — full stop. ✓ A heterogeneous catalyst is in a different phase from the reactants; a homogeneous catalyst is in the same phase.
What are the units of Kc for a reaction?✗ Kc always has units of mol dm^-3. ✓ They depend on Δn (the difference between moles of gaseous/aqueous products and reactants) — Kc can be dimensionless or have units like mol dm^-3.
When a stress shifts an equilibrium, does the system fully undo the change?✗ The system shifts to completely cancel the change and return concentrations to their original values. ✓ No — Le Chatelier's principle says the system PARTIALLY opposes the change; equilibrium shifts to a new position that minimises but doesn't eliminate the disturbance.
Practise Kinetics & Equilibrium — free games
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