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2206-008 Chemistry HL Candidate Name Candidate Number Date of Practical : Eileen Cham Yee Lin : 2206-008 : 4th May 2009 Practical Assessment 22 – Iodine clock reaction Research Question How does the concentration of a reactant (manipulated by using different concentrations of potassium iodide solution) affect its rate of reaction with another reactant (hydrogen peroxide solution, of a fixed concentration and volume), which is calculated as the inverse of the time measured for the triiodide ion formation from the reaction to completely obscure the “X” mark at the bottom of a flask? Introduction There are a few factors that affect the rate of reaction: temperature, reactant concentration, reactant surface area, pressure, the presence of catalyst and the physical states of reactants. This experiment will investigate the effect of reactant concentration on the rate of reaction in the reaction between potassium iodide and hydrogen peroxide. The hydrogen peroxide concentration is fixed while the concentration of potassium iodide is manipulated; the formation of the product – triiodide ions can be detected with the blue-black starch complex. Because both of these reactants react quickly with each other, sodium thiosulphate solution is used as a delaying mechanism to delay the formation triiodide ions by reacting with the triiodide ions to reform more iodide ions. When the sodium thiosulphate is exhausted, remaining triiodide ions form a blue-black starch complex with the starch present in the solution. The chemical equations involved in this reaction are: H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) → I3- (aq) + 2 H2O (l) I3- (aq) + 2 S2O32- (aq) → 3 I- (aq) + S4O62- (aq) A stopwatch is used to measure the time taken for the triiodide ions to completely obscure a mark “X” at the bottom of the flask; the times recorded are then inversed and the rate of reactions for individual runs can be found. This experiment is also known as the Iodine Clock Experiment. 1

Rate of reaction for iodine clock reaction

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