Monitoring the progress of a reaction

There are many ways to monitor the progress of a reaction in the attempt to determine the rate of the reaction. We can broadly categorise them as:

    1. Real-time monitoring methods; and
    2. Quenching methods

Real-time monitoring involves measuring a physical property of the system while the reaction is in progress. For example, a reaction that produces a gas can be monitored by measuring the volume of gas produced at different times with a syringe or by noting the pressure of the gas at various time intervals with a pressure gauge (A). Another way to observe the progress of reaction in real-time is to record the change in mass of a reaction mixture over time (B).

The reaction between hydrochloric acid and calcium carbonate to give carbon dioxide is an example that can be monitored by either method A or B.

2H_3O^+(aq)+CaCO_3(s)\rightarrow Ca^{2+}(aq)+CO_2(g)+3H_2O(l)

Colorimetry, an analytical method to determine the concentration of dissolved coloured compounds, is also commonly used to observe the progress of a reaction (C). It begins with radiating the reaction mixture with a specific wavelength of light, which is selected by passing a continuum light source through a monochromatic filter. The monochromatic light is partly absorbed by the coloured compound before it exits the mixture. A detector then captures the exiting light and conveys the data to a computer for analysis. Since the amount of monochromatic light absorbed by the coloured species is proportional to the concentration of the species, the progress of the reaction can be monitored in real-time. The reaction between iodide and hypochlorite to form hypoiodite, which absorbs near 400 nm, is an example of a reaction that can be monitored using colorimetry:

ClO^-(aq)+I^-(aq)\rightarrow IO^-(aq)+Cl^-(aq)

Quenching methods, on the other hand, involves extracting a sample of the reaction mixture called an aliquot, quenching or stopping the reaction in the aliquot, and analysing the quenched mixture by other analytical techniques like titration (D), spectrometry, chromatography, etc. Some of the ways to quench a reaction mixture include:

    1. diluting a reaction mixture with water, e.g. for the decomposition of oxalic acid in concentration sulphuric acid
    2. cooling a reaction suddenly, e.g. by spraying an aqueous reaction mixture with cold isopentane
    3. adding a reagent that combines with one of the reactants to stop the reaction, e.g. adding acid to quench the hydrolysis of ethyl acetate in a basic solution
    4. inactivating or removing a catalyst in a reaction mixture, e.g. adding a base in the acid-catalysed iodination of acetone to remove the catalyst:

(CH_3)_2CO(l)+I_2(aq)\; \: \begin{matrix} H^+\\\rightarrow \end{matrix}\: CH_3COCH_2I(l)+HI(aq)

Aliquots of the reaction mixture are extracted over several time intervals and added to excess aqueous sodium hydrogen carbonate to neutralise the acid. The concentration of the remaining iodine is then determined by titration with sodium thiosulphate.


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