Chemistry › Kinetics
Kinetics
Unit C05 · 3 lessons.
A 10 °C rise makes molecules collide about 2% more often and can double the rate of a reaction, or more. Collision theory says a reaction needs collisions with at least the activation energy, and the Maxwell–Boltzmann distribution shows how many molecules have that much energy and how temperature and a catalyst change the fraction that do. The practical methods measure a rate from gas volume, mass loss or a colour change.
The answer that loses marks says a higher temperature works because particles collide more often. It is true, but it is the smaller effect. The mark is for the larger one: many more molecules now have energy equal to or greater than the activation energy, so a greater proportion of collisions are successful. A catalyst leaves the distribution alone and provides a route with a lower activation energy.
AQA Papers 2 and 3, OCR A Papers 1 and 3, Edexcel Papers 2 and 3 and Cambridge Papers 1 and 2 examine it, and each paper's page under past papers lists what else it covers.
Specification sections: AQA 3.1.5.1, 3.1.5.3, 3.1.5.4, 3.1.5.2, 3.1.5.5 · OCR A 3.2.2 · Edexcel 9.1 to 9.9, 16.1 to 16.3 · CAIE 8.1, 8.2, 8.3
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Collision theory, activation energy and the rate of reaction
Rate of reaction as the change in concentration per unit time and its units, the two conditions for a successful collision, activation energy and the transition state on an enthalpy profile, the effect of concentration, gas pressure and surface area on collision frequency, why the rate falls as reactants are used up, finding a rate from the gradient of a tangent to a volume–time or concentration–time graph, a mean rate from mass loss, and Cambridge's effective collisions and the boards' names for the enthalpy profile.
- The Maxwell–Boltzmann distribution, temperature and catalysts
The Maxwell–Boltzmann distribution of molecular energies and how to sketch it mark by mark, the most probable and mean energies, what the area under the curve means, the effect of temperature on the curve and on the number of molecules with at least the activation energy, why a small temperature rise gives a large rise in rate, catalysts as an alternative route of lower activation energy on an enthalpy profile and on the distribution, temperature and a catalyst compared, heterogeneous and homogeneous catalysts, why industry uses catalysts, and catalysts and atom economy in making ethanoic acid.
- Measuring the rate of a reaction
Following a reaction by gas volume in a gas syringe, by mass loss on a balance, by colour in a colorimeter and by a precipitate hiding a cross; choosing a method for a given reaction; the reaction of sodium thiosulfate with hydrochloric acid and its sulfur dioxide hazard; colorimetry and the calibration curve; the resolution of a balance and the uncertainty in a mass loss; varying the thiosulfate concentration by dilution; why 1/t measures the rate; the iodine clock and a rate in mol dm⁻³ s⁻¹ from a fixed amount of product; and required practical 3, the effect of temperature on rate, with its method, results, uncertainties and improvements.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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