R. J. C. De Guzman / Chemistry 26.1 (2017) Page | 1 Chemical Kinetics: The Iodine Clock Reaction Raphael Joshua C. De Guzman a a Institute of Biology, University of the Philippines Diliman Performed 10 February 2017; Submitted 17 February 2017 A B S T R A C T Chemical Kinetics is described as how fast a reaction occurs depends on the reaction mechanism the step-by- step molecular pathway leading from reactants to products. The Iodine Clock Reaction was performed by the class to be able to demonstrate Chemical Kinetics at work by manipulating factors such as concentration of reactants, temperature, and catalyst. In the Iodine Clock Reaction, persulfate (S 2 O 8 2- ) and iodide (I - ) react in two stages, producing sulfate (SO 4 2- ) and iodine (I 2 ), then tetrathionate (S 4 O 6 2- ) and iodide (I - ). It was concluded that the resulting rate of reaction is directly proportional to these three factors. The data was plotted in graphs using a scatter graph and the value of r 2 was obtained as well as the equation of the lines. These were used to get the Activation energy. The experiment can yield even better results if human and mechanical errors were mitigated. __________________________________________________________________________________________ Introduction A dose of medicine will cause biochemical reactions inside the body and it is crucial to know the rate of its reaction. Any miscalculations may even endanger someone’s life, and here’s where Chemical Kinetics comes in. Chemical Kinetics is an area of Chemistry concerned with the study of rates of chemical reactions, rate, laws of chemical reactions, and reaction mechanisms. Petrucci (2001) described Chemical Kinetics as how fast a reaction occurs depends on the reaction mechanism the step-by-step molecular pathway leading from reactants to products. This information is obtained by studying the chemical kinetics of a reaction, which depend on various factors: reactant concentrations, temperature, physical states and surface areas of reactants, and solvent and catalyst properties if either are present . The Iodine Clock Reaction was performed by the class to be able to demonstrate Chemical Kinetics at work. This experiment explores the reaction between persulfate (S 2 O 8 2- ) ions, and iodide (I - ) ions. In this experiment, the rate of formation and consumption of I 2 is studied in a two-step reaction. In the first reaction, persulfate ions react with iodide ions to form sulfate (SO 4 ) and iodine(I 2 ). Iodine then react with thiosulfate (S 2 O 3 2- ) ions to form tetrathionate (S 4 O 6 2- ) ions and iodide ions. The time it takes for thiosulfate ions to be used up allows the rate of the formation of iodine in that moment to be calculated (Analytical Chemistry Academic Group, 2017). Methodology The Iodine Clock Reaction experiment utilized the following chemicals namely: potassium iodide (KI), which was the source of iodide ions; potassium persulfate (K 2 S 2 O 8 ), which was the source of persulfate ions; starch, which was the indicator of the consumption of thiosulfate; potassium chloride (KCl); sodium thiosulfate pentahydrate (Na 2 S 2 O 3◦5H 2 O), which was the source of thiosulfate ions; potassium sulfate (K 2 SO 4 ) functioned as the catalyst; and lastly, copper (II) sulfate pentahydrate (CuSO 4◦5H 2 O). The glassware used were 500-mL volumetric flasks for solution preparation; 10-mL measuring pipette, which was used for transferring the solutions into beakers; and the beakers, which were used to hold, mix, and observe the solution and the effects of combining the solutions. Equipment used were hot plate to prepare the 1% fresh starch solution and the top loading balance, which was used in the preparation of solutions. The experiment consisted of four parts: solution preparation, Persulfate and Iodide concentrations on reaction rate, effect of temperature on the reaction rate, and effect of the catalyst in the reaction rate. In the first part, solutions were prepared. These include 50.0mL of 0.01M CuSO 4 and 500.0mL each of 0.2M KI, 0.2M KCl, 0.1M K 2 S 2 O 8 , 0.1M K 2 SO 4 , and 4.0M Na 2 S 2 O 3 . The weighted powdered forms of the said substances were measured on the top loading balance. These were subsequently dissolved in distilled water. 20.0mL of 1% fresh starch solution was also prepared via the moistening of 0.20 grams of soluble starch with sufficient H 2 O until a paste formed. The paste was then boiled. The contents of Beaker A were poured into B. The timer ran from the moment the contents were combined until the solution turned blue. This was all conducted at room temperature. The following part of the Iodine Clock Procedure called for the observation on the effect of temperature on the reaction rate.