/n Laboratory Worksheet 2.2 The 'iodine clock' reaction International Foundation Year Science, Engineering and Computing Pathway Tutor: Dr Kartheek Sooda The Iodine Clock lab will be held in the Leslie Silver building Remember to bring your lab books with you. Before the lab, you need to read the relevant parts of your textbook: Chapter 22 pp. 324 - 340 Also, answer the following pre-lab questions: 1. What is the role of H2O2 in the reaction? 2. Write down the steps for deducing Order of Reaction from raw data (with known initial concentrations). 3. Construct a table to enable you to record the reaction time for each reaction mixture (1-7) The Partial Report should be submitted via Turnitin This submission needs to include: Introduction ○ Cover page ○ O Results (graphs can be produced using Excel, or inserted as photographs) Introduction The equation for the reaction being studied is as follows: H2O2(aq) + 2H(aq) + 21¯(aq) → 2H2O(l) + I2(aq) The aim of this practical is to determine the order of reaction with respect to the concentrations of: • hydrogen peroxide • H+ ions • iodide ions. The reaction takes place in a solution of sodium thiosulfate and starch. As soon as the iodine is produced it reacts with thiosulfate ions to produce iodide ions: 2S2O3²¯(aq) + I2(aq) → S406²˜¯(aq) + 2ï(aq) This means that the starch solution cannot react with the iodine and does not turn blue-black. However, when all the sodium thiosulfate solution has reacted, the iodine formed reacts immediately with the starch and the solution turns blue-black. Therefore, the rate of reaction is inversely proportional to the time taken for the blue-black colour to disappear (i.e. rate time The reaction is started by the addition of the required amount of potassium iodide solution each time, followed by rapid mixing. Safety Wear eye protection at all times. Sulfuric acid is an irritant. Apparatus (per group) 25.0 cm³ of 0.1 mol dm³ potassium iodide solution • 25.0 cm³ of 0.5 mol dm³ sulfuric acid • 25.0 cm³ of 0.1 mol dm³ hydrogen peroxide solution • 10 cm³ of 0.01 mol dm³ sodium thiosulfate solution • 10 cm³ of 0.2% starch solution • • 10 cm³ measuring cylinders or burettes containing the above solutions seven test tubes • stopwatch • distilled water • eye protection Procedure 1 The reaction mixtures are given in Table 1 below. Construct a results table for recording the time taken for each reaction mixture. Test Volume/cm³ tube KI H2SO4 H2O2 Na2S2O3 Starch Distilled Total solution water 1 3.0 3.0 3.0 1.0 1.0 4.0 15.0 2 2.0 3.0 3.0 1.0 1.0 5.0 15.0 3 1.0 3.0 3.0 1.0 1.0 6.0 15.0 4 3.0 2.0 3.0 1.0 1.0 5.0 15.0 5 3.0 1.0 3.0 1.0 1.0 6.0 15.0 6 3.0 3.0 2.0 1.0 1.0 5.0 15.0 7 3.0 3.0 1.0 1.0 1.0 6.0 15.0 Table 2 Reaction mixtures. 2 Make up the reaction mixtures as shown in the appropriate table above, but without the potassium iodide and the sulfuric acid (IRRITANT). Use a separate Comboplate well or labelled test tube for each mixture. 3 Add the potassium iodide and acid, as shown in the table, to the first mixture. Start the stopwatch. 4 When the blue-black colour appears, stop the watch and record the time. 5 Carry out the other six experiments and measure the time taken for each reaction. 1 6 Calculate the value of for each reaction mixture. time Questions 1 1 Why do we calculate the value of for each reaction? time 2 Why is water added to the reaction mixture? 3 State which experiments are used to determine each of the following: 4 a the effect of [I] on the rate of reaction b the effect of [H*] on the rate of reaction с the effect of [H2O2] on the rate of reaction. Determine the order of reaction with respect to the concentration of each of the three reagents. Explain your answer. 5 Write the rate expression for the reaction. 6 a Write an equation for the rate-determining step. b Why is this different from the stoichiometric equation?/nCalibri Regular (Be B àÅ× | • A× | Ev providing a quantitative basis for kinetic analysis: A FI > ITT Test Volume (cm3) tube Time (s) 1/time (S-1) H202 Na2S2O3 Starch Distilled Total Solution Water =2 1 3 1 1 4 15 15 0.066667 33332 3456 1 1 5 15 22 0.045455 1 1 6 15 27 0.037037 1 1 5 15 39 0.025641 1 1 6 15 52 0.019231 1 1 5 15 60 0.016667 7 1 9 115 59 0.016949

Fig: 1