RP 7: Rates: initial rate & continuous monitoring

Hazards

Method

1. Plan the iodine-clock mixturesH₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O. Mix KI, starch, a fixed small amount of sodium thiosulfate and the acid in one flask; start the clock as you add the H₂O₂. The thiosulfate silently consumes the first portion of iodine — the instant it runs out, free I₂ hits the starch and the mixture snaps blue-black.
2. Vary one concentration onlyChange the concentration of a single reagent between runs, topping up with water so the total volume (and therefore every other concentration) stays constant. Time to the colour flip: initial rate ∝ 1/t, because the same fixed amount of iodine is made in every run.
3. Find the ordersPlot log(1/t) against log(concentration): the gradient is the order with respect to that reagent (0, 1 or 2). Repeat for each reagent in turn to assemble the full rate equation.
4. Continuous monitoring: gas collectionFor a gas-producing reaction (e.g. Mg + HCl), collect the gas in a gas syringe and record the volume every 10 s. Plot volume against time: the curve is steepest at the start and flattens as the limiting reagent runs out.
5. Tangents, not chordsRate at any moment = gradient of the tangent to the curve at that instant. Initial rate = the tangent at t = 0. Draw it with a ruler touching the curve at one point only, and quote the coordinates you used for the gradient.

Mistakes examiners see every year

What to expect

The classic H₂O₂/I⁻/H⁺ clock comes out first order in both H₂O₂ and I⁻ (indicative): log–log plots with gradients ≈ 1. Gas-collection curves flatten as the limiting reagent is consumed; doubling the metal's surface area steepens the initial tangent but leaves the final volume unchanged.