MODEL RELEASED. Reaction rate experiment. Student using a stopwatch to time a reaction. The progress of the reaction is monitored with a colorimeter (lower centre). This measures the amount of light passing through the reaction vessel (cuvette), and is used when one of the chemicals is coloured. The cuvette is the small square tube inserted into the colorimeter. It contains propanone reacting with iodine, causing the colour of the iodine to fade. The rate of the reaction can be used to determine the reaction kinetics and values such as the rate constant. Rate constants at different temperatures are used to investigate reaction energetics.