PAG10: Clock Reactions, Orders and Rate Constants

An iodine clock produces iodine which is initially removed by a known small amount of thiosulfate; a persistent starch colour appears after that scavenger is exhausted. With a fixed threshold and total volume, a shorter clock time indicates a faster average early production rate. Use the reaction stoichiometry to convert the threshold reagent to iodine amount when an absolute rate is required.

Keep the threshold amount, temperature, total volume, mixing and other reactant concentrations constant when changing one initial concentration. Replace a smaller reactant-solution volume with solvent. The concentration in the reaction mixture is stock concentration multiplied by delivered volume divided by total volume. A threshold consuming an appreciable fraction of a reactant gives a poorer approximation to its initial rate.

For an illustrative reaction rate = k[A]ᵐ[B]ⁿ, compare experiments with B fixed to find m, then experiments with A fixed to find n. Orders come from measured evidence rather than balanced-equation coefficients. If doubling A doubles rate, m = 1; if doubling B quadruples rate, n = 2. A clock time itself varies inversely with the rate proxy, so doubling time means halving that proxy.

The table below is an original generic clock dataset, not a claim that every iodine-clock formulation has these orders. Repeat conditions and assess scatter before using exact integer orders. A spreadsheet can calculate mixed concentrations and reciprocal times, but label whether its column is a relative rate, an iodine-production rate or a rate constant.

If the specified threshold is Δ[I₂], estimate the early iodine-production rate as Δ[I₂]/t. A rate convention for another reactant can differ by a stoichiometric factor, so state which rate the equation uses. Determine k by dividing this rate by the concentration powers. For total order three, k has units dm⁶ mol⁻² s⁻¹ when rate is in mol dm⁻³ s⁻¹.

Manual mixing and colour judgment add uncertainty, especially for fast clocks. Use the approved timing arrangement or an objective colour sensor where appropriate. Smaller fixed thresholds can improve the initial-rate approximation, but must still give a clear, reproducible endpoint. Use the teacher-approved dilute reagents, warming and waste instructions; do not alter the formulation simply to make the colour change dramatic.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.