Kinetics
Collision theory, Maxwell–Boltzmann, rate equations, Arrhenius
Maxwell–Boltzmann distribution
The Maxwell–Boltzmann distribution shows the spread of molecular energies. The area under the curve equals the total number of molecules, and it stays constant.
At higher temperature the peak shifts right and sits lower, so many more molecules exceed Ea — that is why rate rises so steeply. A catalyst provides an alternative route with a lower Ea: the curve itself doesn't change, but the Ea line moves left.
Rate equations (A-level year 2)
rate = k[A]m[B]nThe orders m and n come only from experiment — never from the coefficients of the chemical equation. The rate-determining step contains the species that appear in the rate equation.
Arrhenius equation
k = A e−Ea/RT · ln k = ln A − Ea/(RT)A plot of ln k against 1/T is a straight line with gradient −Ea/R — how Ea is measured (required practical 3 uses this idea).
Try the Maxwell–Boltzmann simulation with the temperature slider.
3.1.9.1Finding orders from initial-rate data
- Comparing experiments 1 and 2: [A] doubles while [B] stays constant, and the rate doubles — so the reaction is first order in A.
- Comparing experiments 2 and 3: [B] doubles while [A] stays constant, and the rate goes up ×4 — second order in B.
- So rate = k[A][B]², giving an overall order of 3. The units of k are rate ÷ (mol dm⁻³)³ = dm⁶ mol⁻² s⁻¹.
- Find k by substituting any one experiment's numbers into the rate equation.
3.1.9.1Concentration–time vs rate–concentration graphs
On a [X]–time graph, zero order gives a straight line down and first order gives a curve with a constant half-life. The rate at any time t is the gradient of the tangent at that point.
On a rate–[X] graph, zero order is a horizontal line, first order is a straight line through the origin, and second order is an upward curve.
The full Kinetics chapter — every remaining section, the Mastery Vault, chapter quizzes and AI-marked exams — is in ChemLab.
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