Unit 11
Reaction rates
Thermodynamics says whether a reaction can happen, not how long it takes. In this unit you time an iodine clock on the virtual bench to find reaction orders, read half-lives from integrated rate laws, extract activation energies from Arrhenius plots, integrate elementary steps to test reaction mechanisms, and finally see how enzymes and metal surfaces speed reactions up by factors of billions.
- 1 Reaction rates and rate laws Mix two colourless solutions and twenty seconds later they snap to blue-black. Time this chemical clock, vary the concentrations and find the reaction orders yourself — they are not the coefficients of the equation. 25 min
- 2 Integrated rate laws and half-lives How does concentration fall with time? Plot the data three ways — [A], ln[A] and 1/[A] against t. Only one is straight, and it gives you the order, the rate constant and the half-life. 25 min
- 3 Collision theory and activation energy Warming by 10 °C adds under 2 % more collisions, yet many reactions go twice as fast. Build an Arrhenius plot from 1921 measurements on N₂O₅ and extract the activation energy. 25 min
- 4 Reaction mechanisms An overall equation tells you only the start and the finish; the mechanism is the step-by-step story in between. Integrate every elementary step in the simulator and see how the rate-determining step, the steady state and pre-equilibrium turn a mechanism into a testable rate law. 30 min
- 5 Fast reactions: relaxation and the diffusion limit Neutralisation is over before mixing is — so how is it measured? Eigen nudged systems already at equilibrium and watched them return: pure water relaxes in about 36 μs. Then compute diffusion coefficients from ionic conductivities and see why H⁺ and OH⁻ combine at 1.4 × 10¹¹ L mol⁻¹ s⁻¹. 30 min
- 6 Catalysis Hydrogen peroxide keeps for months in the bottle but fizzes on a cut. Catalysts lower the barrier without moving the equilibrium — and enzymes and metal surfaces share one more trait: saturation. 25 min