16.3-16.4 - Measuring Reaction Rates

Start With The Measurable Change

A student says, "This reaction is fast because it fizzes a lot." That is an observation, but it is not yet rate data. Rate data need a measurable quantity that changes with time and can be linked to the amount or concentration of a reactant or product.

For any rate experiment, ask three questions before naming apparatus:

1. Which species is changing in a measurable way?
2. Does the measurement track that species directly, or is it only a proxy?
3. Can the measurement be repeated often enough without disturbing the reaction?

A good rate technique gives a signal that changes smoothly as the reaction proceeds. The signal might be concentration, absorbance, mass, gas volume, pressure, pH, conductivity or the time taken to reach a fixed visible change.

The word "suitable" is doing real work. A method is suitable only if the measurement is connected to the reaction progress, the apparatus has adequate resolution, the reaction can be followed safely, and the data can be processed into a rate or a graph.

Common-error contrast. Do not justify a method only by saying "it is easy to see". A visible change is not automatically quantitative. For example, "the solution turns pale" is weaker than "a colorimeter can record absorbance at regular time intervals, and absorbance is linked to the concentration of the coloured species."

The Named Techniques

You need to select and justify techniques, not just list them. The same reaction may have more than one possible method, but some methods are cleaner than others.

Titration method logic. Titration is useful when concentration cannot be read continuously from the reaction flask. For example, a reaction mixture can be sampled every few minutes, each aliquot can be quenched by cooling, dilution or a stopping reagent, and the remaining reactant can be titrated. The quench step matters because the aliquot must represent the composition at the sampling time, not the composition several minutes later during analysis.

For iodine titration, use $\ce{I2 + 2S2O3^2- -> 2I- + S4O6^2-}$: $[\ce{I2}]=c_{\mathrm{thio}}V_{\mathrm{titre}}/(2V_{\mathrm{sample}})$. Quenching must leave the analysed iodine unchanged. Cooling or dilution may only slow a reaction; verify that continued reaction during analysis is negligible, and account for any dilution when converting the titre to the original sample concentration.

Colorimetry method logic. A colorimeter can provide many data points without removing samples. It is especially powerful when the coloured species is the reactant being used up or the product being formed, because absorbance can be followed against time. If a question asks for a justification, say what is coloured and why the absorbance change tracks the reaction.

Choose a wavelength strongly absorbed by the coloured species and zero the instrument with a solvent/reagent blank that excludes that species. Use clean cuvettes in the same orientation and path length. A calibration series of known concentrations tests the usable linear range; absorbance is proportional to concentration only within that range. Bubbles or turbidity scatter light and can create an unrelated signal.

Gas methods logic. Gas volume and mass change are often alternatives. Gas volume is more direct if the gas can be collected reliably; mass loss is simpler if the gas escapes but harder to use when the mass change is small or the gas is very soluble. Either way, the initial apparatus must be sealed appropriately: a gas syringe setup should be leak-free, while a mass-loss setup must allow gas to leave.

Initial-Rate Experiments

An initial-rate experiment measures how fast the reaction starts under a chosen set of starting concentrations. Separate experiments are carried out, and in each one the initial concentration of one reagent is changed while the other conditions are kept constant.

The method structure is:

1. Prepare a mixture with known initial concentrations.
2. Start timing at the same mixing point.
3. Measure a small early change.
4. Repeat with one initial concentration changed.
5. Keep total volume, temperature and other reagent concentrations controlled.

For example, $15.0\,\mathrm{cm}^{3}$ of $0.200\,\mathrm{mol\,dm}^{-3}$ solution made up to $50.0\,\mathrm{cm}^{3}$ gives $0.0600\,\mathrm{mol\,dm}^{-3}$.

The key idea is that the reaction is judged near the beginning, before concentrations have changed very much. This is why the data can later be used to connect initial concentration to initial rate.

Clock reactions as an approximation. In a clock reaction, the measured value is usually the time taken for a fixed visible change to occur, such as a sudden colour change or the appearance of enough precipitate to obscure a mark. If the visible change always happens after the same small amount of product has formed, then:

That approximation is valid only when the endpoint represents the same small extent of reaction each time. It is not valid if the endpoint amount changes between runs or if the reaction has already consumed a large fraction of a reactant before the clock change appears.

Worked example. A reaction forms a cloudy precipitate. The endpoint is defined as the time when a printed cross under the flask can no longer be seen. A student changes the initial concentration of reactant A, keeps the total volume constant by adding water, keeps temperature constant, and measures the time to the same visual endpoint.