The rates at which chemical reactions occur, it is very important as it enables the exploration of reaction mechanism. Even the simplest chemical may consist of a complex sequence of events. Thus, it is customary practice to simplify matters by devising experiment in which, effectively, the concentration of one species only is changing. In this experiment, reactants of different volumes are given, iodine stored in KI(aq) solution is first mixed with given volume of sulphuric acid, acetone and distilled water into a flask.
Students are to be worked in pairs. Our group carried out experiment 2 (20cm3 of Acetone) and only experiment 1, 2 and 3 were underwent in our class. Group(Experiment): 1 The flask of mixture is placed in a thermostat. At a fix time interval, portions of mixture are added to a flask containing NaHCO3 solution.
Since iodine reacted with acetone to give I- ions, the solution is then titrated against sodium thiosulphate solution, the amount of iodine molecules remained after reaction with acetone are reacted with sodium thiosulphate to give I- ions. At a fixed time interval, the procedure is repeated, thus the reaction rate can be determined by monitoring the volume of titre in each titration. In the experiment, the NaHCO3 is used as a quenching material to quench the reaction of iodine with acetone. CH3COCH3 + I2 > CH3COCH2I + H+ +I- I2 (aq) + 2S2O32- (aq) > 2I- (aq) + S4O62- (aq).
Requirements:0. 02M I2 dissolved in aqueous KI solution 1. 0M acetone dissolved in water 1. 0M sulphuric acid 0. 5M sodium bicarbonate solution Very dilute sodium thiosulphate 25 cm3 pipette 10 cm3 pipette At least 5 clean, dry conical flasks Water trough Burette Measuring cylinder 100cm3 beakers Procedures: ( for pair 2) 25. 0cm3 of sulphuric acid and 20. 0 cm3 of acetone solution were mixed together with 5. 0 cm3 of distilled water into a clean by using 25cm3 pipette and 10 cm3 pipette, dry conical flask, labeled flask B.
The flask was stoppered and placed in a thermostat at 25? (water bath). 50.0 cm3 of iodine in KI solution was added in another clean, dry conical flask by using a 25cm3 pipette, labeled flask B. The burette was filled with sodium thiosulphate solution. 10 cm3 portions of the NaHCO3 solution was added into each of three conical flask. The contents of flasks A and B were mixed thoroughly and a stopclock was started at once. The flask containing the reaction mixture was reclamped in the thermostat. The temperature of the mixture was checked carefully, and after about 5 minutes, 10 cm3 of the reaction mixture was pipetted into one of the flask containing the NaHCO3 solution, time at which the act was done was noted.
The contents of the flask were mixed thoroughly and the mixture was then titrated against sodium thiosulphate solution. When the mixture was nearly colourless, two or three drops of a fresh starch solution were added and then the titration was continues until one drop of the sodium thiosulphate solution discharged the blue starch – iodine complex colour. After about 10, 15, 20 and 30 minutes, further 10 cm3 portions of the reaction mixture were withdrew from the thermostat and the above procedure was carried out each time. All times and titration data were recorded. Data and Analysis:
Graph2 ( please refer to the graph paper attached) The table below shows the volume of sodium thiosulphate solution used against time. ( data for graph 1) Time started/ min 6:00 12:30 18:00 21:00 24:00 27:00 Final reading/ cm3 39. 55 29. 50 43. 90 28. 95 42. 65 32. 60 Initial reading/ cm3 23. 15 14. 20 29. 50 14. 75 28. 95 19. 70 Titre/cm3 16. 40 15. 30 14. 40 14. 20 13. 70 12. 90 Temperature readings remain constant during the experiment: 22. 5?. Analysis: From the table, it was noticed that the amount of sodium thiosulphate solution used is directly proportional to the concentration of the remaining iodine.
The slope of graph 1 equals to the negative value of the rate of reaction, it implies that iodine concentration drops at a uniform rate. Therefore, rate of change of the iodine concentration is independent of iodine concentration. The reaction of iodine with acetone is zero order with respect to I2. Graph2 ( please refer to the graph paper attached) By combining the results of Group 1, 2 and 3 class results are obtained. The data is used to plot Graph II: rate of reaction against the volume of acetone added.
Volume of acetone/ cm3 25. 0 20. 0 15. 0 Analysis: Data marked with ( ) are abandoned because of its great deviation form the estimated value. With the rate of reaction increases with increasing volume of acetone together with a straight line passing through origin, the reaction of iodine with acetone is in first order with respect to acetone. Volume of mixture left = 47 cm3 Observations: Iodine solution was reddish brown in color while the other reactants are colourless.
Gaseous bubbles were evolved when the reaction mixture is mixed with NaHCO3.due to formation of carbon dioxide gas. 2NaHCO3(aq) + H2SO4(aq) > Na2SO4(aq) + 2CO2(g) + 2H2O(l) During the titration process, the colour of the mixture containing iodine changed from reddish brown to straw yellow gradually. Once the mixture reached the straw yellow colour, 2-3 drops of starch solution were added into the mixture. The colour turned to dark blue, the solution became colourless after several more drops of sodium thiosulphate were added. Precautions: For mixing the content of flasks A and B, the solution should be transferred alternately between two flasks for 2 – 3 times.
The solution is to swirled gently to allow the content inside to mix throughly. Conclusion: For a straight line passing through origin obtained in Graph II and a linear decreasing graph is in Graph I, the reaction of iodine with acetone was to be found in first order with respect to acetone and zero order with respect to I2. Discussion: The experiment cannot be conducted with more than one variable amounts reactions. For reaction more than one reactant, only the concentration of the reactant under investigated (acetone) changes, and the other reactants remain unchanged.