// CHEMISTRY L5 — KINETICS
How fast, and what controls it.
The rate equation is a reaction's fingerprint — found only by experiment, never from the equation. Change the order and watch how doubling a concentration reshapes the rate.
// FIG. 01 — RATE vs CONCENTRATION
ORDER w.r.t. [A]:
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// THE RATE EQUATION
rate = k [A]ᵐ [B]ⁿ
The powers m and n are the orders of reaction with respect to each reactant, and their sum is the overall order. Crucially, they are not the numbers in the balanced equation — you can only find them by experiment, by changing one concentration at a time and measuring what happens to the rate. k is the rate constant, fixed at a given temperature.
// READING THE ORDER
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// THE RATE-DETERMINING STEP
Most reactions happen in several steps (a mechanism). The whole thing can only go as fast as its slowest step — the rate-determining step — like a checkout queue moving at the pace of its slowest till. Only the reactants involved up to and including that slow step appear in the rate equation. That's the powerful link between kinetics and mechanism: the rate equation is direct evidence for how a reaction actually proceeds.
// THE ARRHENIUS EQUATION — TEMPERATURE & k
Raising the temperature speeds reactions up because the rate constant k itself grows. The Arrhenius equation, k = Ae^(−Eₐ/RT), captures it: a higher activation energy Eₐ means a smaller k (slower reaction), and a higher temperature T means a larger k. A rough rule of thumb: a 10 °C rise roughly doubles the rate — a huge effect from a small change, because it lifts far more molecules over the activation-energy barrier.
// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).