III Chemical Kinetics MIT OpenCourseWare

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2SO4 2 I 2,persulfate iodide sulfate iodine, Rates of reaction are measured by either following the appearance of a product or the. disappearance of a reactant In this experiment the rate of consumption of the iodine will be. measured to determine the rate of the reaction As reaction 1 runs the amount of iodine I2. produced from it will be followed using reaction 2. 2S2 O3 2 I2 S4 O6 2 2I,thiosulfate iodine tetrathionate iodide. The iodine produced from the persulfate iodide reaction 1 is immediately reduced back to. iodide by thiosulfate ions 2 A known amount of thiosulfate ions will be added to the reaction. vessel which will in turn consume iodine as it is produced This continues until all the. thiosulfate has been converted to tetrathionate whereupon free iodine will start to form in the. solution via reaction 1 Because we know the amount of thiosulfate we added we can. determine the amount of iodine produced from reaction 1 stoichiometrically When all the. thiosulfate is consumed free iodine starts to form in solution By measuring the time taken for. the known amount of thiosulfate to be consumed the rate of production of iodine during that. time can be calculated, The color of the iodine formed might be intense enough that it can act as its own. indicator however for better results you will add starch which produces a deep blue starch. iodine complex,I2 C6 H1 0O5 n H2 O,blue complex 3,iodine starch. In summary iodide I and persulfate ions S2O82 react to produce iodine I2 and sulfate. SO42 in reaction 1 This iodine is immediately consumed by the thiosulfate ions S2O32 in a. pathway described by reaction 2 As soon as all of the S2O32 ions are consumed the excess. iodine produced in 1 is free to react with starch turning the solution blue 3 The amount of. thiosulfate ions added tells us how much iodine had been produced in the time taken for the. reaction to turn blue,Rate equation, The rate of the reaction at constant temperature and ionic strength can be expressed as.
the change in concentration of a reagent or product over the change in time and can be equated to. the rate law expression,S2 O82 I 2 m n,rate k S2 O82 I. The variation in concentration of persulfate a minus sign denotes consumption and the. variation in concentration of iodine production are given by. S2 O82 S2 O82 final S2 O82 initial 0 S2 O82 initial S2 O82 added. I2 I2 final I 2 initial but at the beginning of the reaction I 2 initial 0 so. I2 I2 final,I2 moles I2,t volume solution L time sec. The number of moles of iodine produced is given by the amount of thiosulfate added to the. reaction vessel, moles S2O32 volume of S2O32 added L concentration of S2 O32. The stoichiometry of reaction 2 gives,I2 moles S2 O32. t 2 volume solution L time sec,Thus we can calculate the rate by.
vol S2O32 added L conc S2O32 moles L,2 volume solution L time sec. This reaction rate is a measure of how much iodine was produced in the time it took for the. reaction to turn blue i e time taken to react with all of the thiosulfate present. Reaction Orders, In this experiment we use the initial rate method to find the order of the reaction with. respect to persulfate m and the order of the reaction with respect to iodide n The method is. based on the measurement of the rate of the reaction over a period of time This time period is. short enough for the reaction not to have proceeded significantly but long enough to be. unaffected by the time which the solutions take to mix at the start of the reaction. The rate law equation can be written as,rate k S2 O82 I n. By taking the natural log of both sides the equation becomes. ln rate ln k m ln S2O82 n ln I, For runs with different concentrations of persulfate and a constant concentration of. iodide at a constant temperature,ln rate m ln S2O82 constant.
The constant term in this equation is lnk n ln I The slope of the best fit line of a plot of ln. rate versus ln S2O82 will be equal to m the order of reaction with respect to persulfate. Similarly for runs where persulfate concentration and temperature are kept constant and. the amount of iodide is varied,ln rate n ln I constant. The constant term is lnk m ln S2O82 The slope of the best fit line of a plot of ln rate versus. ln I will be equal to n the order of reaction with respect to iodide. Activation energy Ea,Recall the Arrhenius equation. Taking natural logarithm of both sides of this equation we obtain. ln k a ln A, A plot of ln k versus 1 T yields a straight line whose slope is Ea R and whose y intercept is ln A. the natural logarithm of the Arrhenius constant, Effect of Persulfate and Iodide Concentrations on Rate. You will be provided with the following solutions, i Standardized Na2S2O3 solution about 0 1 M BE SURE TO RECORD EXACT VALUE.
ii 0 1M potassium persulfate K2S2O8 iii 0 2M potassium iodide KI iv 0 2M potassium. chloride KCl v 0 1M potassium sulfate K2SO4, The rate coefficient k of ionic reactions depends on the ionic strength or salinity of the solution. Potassium chloride KCl and potassium sulfate K2SO4 are used to maintain the ionic strength. of the solutions, 1 Prepare a 4 0x10 3 M solution of sodium thiosulfate as follows Rinse a clean 250mL. volumetric flask with distilled water Pipette an aliquot of 10 mL of the standardized. thiosulfate solution into the volumetric flask and add distilled water to the mark on the neck. of the flask Stopper and invert the flask a few times to mix its contents Transfer the diluted. thiosulfate solution into a clean labeled plastic bottle This diluted solution will be used along. the experiment, 2 Label a 50mL Erlenmeyer flask A and a 50mL beaker R the reaction beaker For each. run of the reaction make up glassware as shown in the chart below Between runs rinse the. flasks THOROUGHLY with distilled water,Erlenmeyer A Reaction Beaker R. 2 drops fresh starch solution magnetic bar, Runs 0 2M KI 0 2M KCl 0 1M K2S2O8 0 1M K2SO4 4 0x10 3 M.
mL mL mL mL S2 O 3 2,1 10 0 5 5 5,2 5 5 5 5 5,3 2 5 7 5 5 5 5. 4 5 5 7 5 2 5 5,5 5 5 10 0 5, 3 For each run start stirring the reaction beaker Then dump the contents of flask A into it. and immediately begin timing Record the Blue Time the time in seconds needed for the. solution to turn blue for each run Deposit all waste in the liquid waste container Do. not add the S2O32 solution until you are ready to mix mixtures A and R together. Discussion and Calculations,Prepare the folowing graphs. 1 ln rate versus ln S2O82 for runs where I is constant runs 2 4 and 5. 2 ln rate versus ln I for runs where S2O82 is constant runs 1 2 and 3. 3 ln k versus 1 T for runs at constant concentrations but variable temperature. For these graphs draw a best fit line The slopes of graph 1 and graph 2 will give you m. and n respectively round them to their nearest integer values The slope of graph 3 will give. you Ea R and the intercept lnA, In your calculations you should keep in mind that the starting concentration in the. reaction vessel for each reagent is not simply what was printed on the bottle For instance for. the first run in Part One you used 20 mL of the 0 2M KI solution however when the reaction is. run the actual concentration of iodide at start is not 0 2M Find the concentration of the two. reagents iodide and persulfate used in each run Hint what dilutions have occurred. The rate for every run in this experiment can be calculated by. S O 2 5 4 0 10 4,rate 2 3 diluted Ms 1,2 25 time time.
Since the total volume in every reaction is 25 mL and 5 mL of the dilute thiosulfate solution is. used in every reaction the only quantity in this equation that will change is the time Be sure to. account for all dilutions in the sodium thiosulfate solution concentration. Once you have determined m and n the rate constant k is calculated from. S2O8 m I n, Your final k value at room temperature should be the average of the k values obtained for runs. 1 through 5 in Part One Make sure to give the units for k. 1 Determine the experimental rate law, 2 Propose a mechanism consistent with the experimental rate law Do not worry if it is the. correct mechanism only that the experimental rate law can be derived from it. 3 Determine the activation energy and Arrhenius constant for the reaction. Data Sheet, Effect of Reagent Concentration Temperature of solution. KI KCl K2 S 2 O8 K2SO4 S2 O 3 2 Time,mL mL mL mL mL sec. 1 10 0 5 5 5,2 5 5 5 5 5,3 2 5 7 5 5 5 5,4 5 5 7 5 2 5 5.
5 5 5 10 0 5, Effect of Temperature note pick a run from above to use as room temperature run. KI KCl K2 S 2 O8 K2SO4 S2 O 3 2 Temperature Time,mL mL mL mL mL K sec. III Chemical Kinetics based on the kinetics of the reaction The orders of the reaction are defined by the mechanism of

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