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Clock Reaction Revisited: Catalyzed Redox Substrate-Depletive Reactions

Quantitative demonstration of chemical kinetics often requires sophisticated equipment and materials. By using inexpensive and commonly available chemicals, we propose here two clock reactions, oxygen–safranin–benzoin clock and cysteine–iodine–hydrogen peroxide clock. Students simply measure and add stock solutions to start clock reactions. Sudden discoloration or coloration of the reaction mixture marks the end of the reaction, and this can be conveniently timed. Students then complete calculations and graphing in a spreadsheet file by following common protocols. Rates of reaction are calculated from the ratio of [limiting reactant]0 and clock time. Orders of reactions and rate constants are determined by the method of initial rate. Additional discussions on topics such as catalysis, reaction mechanism, redox reaction, and pseudo-zeroth-order reaction are also possible.