[chickeychicken]

if im determining the Ea of the catalyzed and uncatalyzed iodination of propanone, can i not use a colorimeter? can i simply measure the time taken for the colour change to happen?

[Corribus]

In principle the reaction progress can be tracked by any measurement that scales to the concentration of either the reactants or the products vs time. The problem with colorimetry is that there's no easy generalized link between the color coordinates and concentration of a single mixture component. This is because the perceived color depends not only of absorption, but also scattering, reflection, fluorescence etc., across a broad spectral range, for all components in the mixture. Making absorption measurements with a spectrophotometer is a far superior way to monitor the reaction progress because you can select an absorption wavelength corresponding to only a single reaction component, and the measurement is usually not influenced by other spectral processes.