Experimental methods for rate dataEdexcel A-Level Chemistry: Revision notes

The rate is found by following the concentration of a reactant or product (or a property that depends on it) over time. The best technique depends on the reaction, and in the exam you must justify your choice by linking it to something that changes during the reaction.

• A coloured reactant or product: colorimetry.
• A gas produced: gas volume (gas syringe) or mass change if the gas is dense enough.
• A reactant or product that can be removed and measured chemically: titration of samples.

Other suitable techniques, such as measuring pH or electrical conductivity, are acceptable when a suitable change occurs.

In a titrimetric method, samples of equal volume are removed from the reaction mixture at timed intervals and analysed. Because the reaction continues while you titrate, each sample must be quenched, which means stopping the reaction, for example by cooling, diluting or removing a catalyst or reagent.

In the acid-catalysed reaction of propanone with iodine, CH₃COCH₃ + I₂ → CH₃COCH₂I + HI, excess sodium hydrogencarbonate neutralises the acid catalyst and stops the reaction. The remaining iodine is titrated with sodium thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻, with starch added near the end-point (blue-black to colourless).

A colorimeter measures the absorbance of light by a coloured solution. Absorbance is proportional to the concentration of the coloured species.

1. Choose a filter of the colour that is absorbed most strongly, which is the complementary colour of the solution; for an orange-brown bromine solution a blue filter is used.
2. Make solutions of known concentration and measure their absorbance to plot a calibration curve.
3. Zero the colorimeter with the solvent.
4. Follow the reaction by taking readings at timed intervals, and convert absorbance to concentration with the calibration curve.

This gives continuous monitoring without removing samples, and works only when a reactant or product is coloured.

If a gas is evolved, the reaction can be followed by:

• Gas volume: collect the gas in a gas syringe (or upturned measuring cylinder) and record the volume at timed intervals. It suits any gas that is not very soluble, such as hydrogen from magnesium and acid. Check there are no leaks.
• Mass change: place the flask on a balance and record the loss in mass as the gas escapes, with cotton wool in the neck to prevent spray loss. It works best for a dense gas such as carbon dioxide, since hydrogen is so light that the change in mass is too small to measure accurately.

The rate at any time is the gradient of the tangent to the volume–time or mass–time graph.

In the initial-rate method a series of separate experiments is carried out in which the initial concentration of one reagent is varied and everything else is held constant. The initial rate in each is found from the gradient of a tangent at t = 0 on a concentration–time or volume–time graph, and is compared with the concentration to find the order.

A clock reaction is an acceptable approximation. A fixed amount of a second reagent is added, so a sudden colour change occurs after time t. Since the same amount of reaction has occurred at that point, the rate is proportional to 1/t. It is only an approximation because it gives an average rate over the first part of the reaction, but it is reliable when only a small fraction of the reactants has been used.

The Harcourt–Esson iodine clock uses the reaction H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O. A small fixed amount of sodium thiosulfate and starch is included. The thiosulfate reacts with iodine as soon as it forms (I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻), so there is no colour until the thiosulfate has been used up, when iodine appears and the starch turns blue-black.

The time is recorded from mixing to the colour change. The concentration of one reagent (for example iodide) is varied, with water making up the same total volume, and 1/t is plotted against concentration. A straight line through the origin means first order. The same approach with a different reagent gives its order.

In continuous monitoring a single experiment is followed over time, giving a concentration–time or volume–time graph. Tangents give rates at different concentrations, and half-lives or rate–concentration graphs give the order.

In the iodine–propanone practical, the iodine concentration is followed by quenched titrations. Because propanone and acid are in excess, the concentration of iodine falls linearly with time: a straight-line graph shows the reaction is zero order with respect to iodine, and the gradient is the rate.

Worked example. A 10.0 cm³ sample needs 20.0 cm³ of 0.0050 mol dm⁻³ thiosulfate. n(S₂O₃²⁻) = 1.0 × 10⁻⁴ mol, n(I₂) = 5.0 × 10⁻⁵ mol, so [I₂] = 5.0 × 10⁻³ mol dm⁻³.