PRACTICAL NUMBER 1 AN INVESTIGATION OF THE EFFECT OF CHANGING CONCEN- TRATION ON THE RATE OF OXIDATION OF IODIDE BY PER- OXODISULFATE INTRODUCTION In this experiment you will determine the initial rate of reaction for various different starting concentrations of the two reactants. Some groups will study the effect on the initial rate of changing the peroxodisulfate concentration, and some the effect of changing the iodide concentration. The different groups will share results so that each group will see the effect of changing the concentration of both reactants. You will also determine the order of the reaction with respect to each reactant. Theoretical background The rate of most chemical reactions varies with the concentration of each reactant. The way in which the rate varies with the concentration of a particular reactant can be determined by varying the concentration of that reactant. At the same time all other conditions (such as concentration of other reactants, and temperature) must be kept constant. In this experiment you will determine the effect of varying the concentration of iodide ions (I - ) and peroxodisulfate ions (S 2 O 8 2- ) in turn on the rate of the following redox reaction: 2I - + S 2 O 8 2-2SO 4 2- + I 2 (1.1) The rate at which this reaction proceeds at a given temperature is given mathematically by: Rate of reaction = k [I - m [S 2 O 8 2- n (1.2) where k is known as the rate constant for the reaction, and represents the rate of the reaction when the concentration of the each reactant is 1 mol dm -3. The square brackets are to be read as "the concentration of" whatever is inside the brackets. For example, [I - should be read as "the concentration of iodide ion". m and n are the powers to which the concentration terms must be raised to make the relationship correct, and can only be determined experimentally. m is the order of the reaction with respect to iodide concentration and n is the order with respect to the peroxodisulfate concentration. m+n is the overall order of the reaction. The rate of reaction may be measured as the increase in the concentration of iodine per unit time, or as the decrease in the concentration of peroxodisulfate per unit time: 2 [ I S O 2 [ 2 8 Rate = = (1.3) t t The negative sign is necessary to give a positive rate, since the change in concentration of peroxodisulfate is negative. As the reaction proceeds it is clear that the reactants will be used up. This reduces their concentrations. As a result the rate of the reaction will also decrease. (See equation (1.2).) Thus the iodine molecules (a product) will be produced less and less rapidly, though the total amount of iodine will continue to increase. If the concentrations of the reactants (iodide and peroxodisulfate) are held constant, the rate too remains constant, making the rate easier to measure at specific concentrations. The initial rate of the reaction is easy to measure since, if the rate does not vary, the average rate and the initial rate are the same. The concentrations are held (approximately) constant in two ways: PAGE 7 OF 66
i) By reducing the iodine back to iodide with thiosulfate according to the equation: I 2 + 2S 2 O 2-3 S 4 O 2-6 + 2I - (1.4) iodine iodide The thiosulfate reacts very fast with iodine (but not with iodide, nor peroxodisulfate ion), so that virtually no free iodine builds up in the solution, and the concentration of iodide remains constant at its initial value, until the thiosulfate is completely used up. As soon as this occurs the concentration of free iodine increases. The solution turns a deep blue, almost black, since a little starch indicator is included. The interval between mixing the reagents and this colour change is the darkening time. Note that reaction (1.4) has no effect on the oxidation of the iodide other than maintaining the concentration of iodine essentially zero during the darkening time. The darkening time depends on how fast reaction (1.1) proceeds (and how much thiosulfate is present to start with). This experiment consists of a number of experimental runs, each run being a separate reaction between iodide and peroxodisulfate ions. The amount of thiosulfate present will be a very small, measured amount, exactly the same for each experimental run. ii) The amount of thiosulfate added determines the amount of peroxodisulfate used up by the time the solution darkens. In fact from equations (1.1) and (1.4) it follows that 1 mol of peroxodisulfate is used up for every 2 mol of thiosulfate consumed. The amount of thiosulfate is very small compared to the amount of peroxodisulfate in the solution and so the concentration of the peroxodisulfate only changes by a small percentage during this time. In other words the concentration of the peroxodisulfate remains approximately constant during the darkening time. The initial rate of reaction is calculated from the darkening time, since it is approximately equal to the average rate over the period whilst darkening occurs. The solution darkens when the thiosulfate has been used up by reacting with the iodine produced in reaction (1.1). The amount of iodine produced is equal to the amount of peroxodisulfate used up in this reaction. Since 1 mol of iodine is used up by 2 mol of thiosulfate in equation (1.4) we have 2 [ S2 O3 = (1.5) 2 t where [S 2 O 2-3 represents the change in concentration of the thiosulfate ion until darkening occurs. Since all the thiosulfate is used up, it is numerically the same as the initial concentration [S 2 O 2-3 i. t is the darkening time, t d. This gives [ S O 2t 2 = 2 3 i (1.6) d Several experimental runs will be carried out. In each case we start with a different concentration of iodide ion, but all other concentrations (including that of the thiosulfate, S 2 O 3 2- ) and the temperature are the same. The darkening time varies. Equation (1.2) now becomes: m = const. [ I (1.7) where k[s 2 O 8 2- has been replaced by const. Taking logarithms of both sides of this equation gives ln( InitialRat e) = ln( const.) + mln[ I (1.8) so that a plot of ln () against ln [I - gives a straight line of slope m. This can be used to find m. Similar runs will be carried out in which the concentration of peroxodisulfate ion is varied. A second graph of ln () against ln [S 2 O 8 2- allows the calculation of n. You will use a spread sheet to PAGE 8 OF 66
accomplish the calculations. Reaction (4.1) proceeds at a convenient rate under the conditions we use. You will determine the way in which each of the reactant concentrations alters the rate of reaction. Your lecturer will split the groups into two sets. The first set will study the effect of altering the concentration of the peroxodisulfate ion on the rate of the reaction and the second set will do the same thing for the iodide ion. Before leaving the laboratory, each group must collect the results from another group of students so each student can determine both m and n. METHOD You will probably be supplied with solutions of sodium peroxodisulfate (0.04M), potassium iodide (0.05M), sodium thiosulfate (0.001M) and soluble starch indicator. In the results section note down the actual concentrations of these solutions as stated on their containers. You will make up 10 solutions containing peroxodisulfate ions and 10 others containing iodide ions. Pairs of solutions will be mixed to start the reaction and the darkening time measured. Each such reaction is termed an experimental run Some groups of students will study the effect on the initial rate of reaction of altering the concentration of peroxodisulfate, and some the effect of altering the concentration of iodide. You will be told which reactant you are altering at the beginning of the practical. I. STUDYING EFFECT OF ALTERING CONCENTRATION OF PEROXODISULFATE 1) Make up solutions A and B as described below. 2) Solutions A. Into 10 clean test tubes measure 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 cm 3 respectively of the peroxodisulfate solution using a burette. To the first nine test tubes add water by burette to bring the total volume up to 10 cm 3. 3) Solutions B. Into each one of a further 10 test tubes, pipette 10 cm 3 of the iodide solution. To each of these add carefully 1 cm 3 of the thiosulfate solution from a graduated pipette, and 1 cm 3 of starch solution, using a different pipette if possible. 4) Quickly add the contents of the first test tube containing solution A to the first test tube containing solution B whilst starting a stop watch. Make sure that the contents of this test tube have been mixed thoroughly by pouring them back and forth between the two test tubes four times. Avoid warming the tubes with your hands. Place the test tube in a rack and watch the solution carefully. As soon as it begins to darken, stop the stop watch. Record the darkening time (t d seconds) in results table 1. 5) Proceed, as in step 4, with the remaining test tubes. II. STUDYING EFFECT OF ALTERING CONCENTRATION OF IODIDE 1) Make up solutions C and D as described below. 2) Solutions C. Into each of 10 test tubes, pipette 10 cm of the peroxodisulfate solution. 3 3) Solutions D. Into 10 test tubes measure 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 cm 3 respectively of the iodide solution using a burette. To the first nine test tubes add water from a burette to bring the total volume up to 10 cm 3. To all ten test tubes carefully add 1 cm 3 of the thiosulfate solution from a graduated pipette, and 1 cm 3 of the starch solution. 4) Quickly add the contents of the first test tube containing solution C to the first test tube containing solution D whilst starting a stop watch. Make sure that the contents of this test tube have been mixed thoroughly by pouring them back and forth between the two test tubes four times. Avoid PAGE 9 OF 66
warming the tubes with your hands. Place the test tube in a rack and watch the solution carefully. As soon as it begins to darken, stop the stop watch. Record the darkening time (t d seconds) in results table 2. ALL GROUPS 1) Be prepared to repeat any or all of your experimental determinations. 2) Calculate the total volume of each solution after mixing and enter this in the appropriate results table. 3) Calculate the initial concentrations of the peroxodisulfate, iodide, and thiosulfate immediately after mixing the solutions using the following relationship: Volume before mixing Concentrat ion after mixing = Concentration before mixing (1.9) Volume after mixing 4) The volume before mixing is the volume run out of the burette or pipette, and the volume after mixing is the volume produced after mixing solutions A and B, or C and D, but before any reaction occurs. Enter these concentrations in the results table. 5) Calculate the initial rate according to equation (1.6) and enter your values in the results table. 6) Use Excel (or similar) to plot a graph of ln(initial rate) against ln[s 2 O 2-8 or ln[i - (whichever you varied in your experiment). The type of plot to use is a scatter plot. Add the trend line and a title, and label your axes appropriately. The slope of this graph is the order of the reaction with respect to S 2 O 2-8 or I - (as appropriate). Whilst the slope may not be exactly a whole number, the order must be. Note that logarithms of quantities have no units, but otherwise correct units must be shown. You can get help on using Excel 2003 on Dr. Banks s website (http://fbanks.info/chem225/practicals/practical_1_help.htm). There is a sample spread sheet with graphs that you can download and practice on. 7) Use Excel or similar to plot a second graph of initial rate against a function of concentration (of either iodide or peroxodisulfate as appropriate) as dictated by the order. For example, if the order is 1, you plot initial rate against concentration. If the order is 2, you plot initial rate against square of concentration. The aim is to obtain a straight line. Note that theory dictates that it must pass through the origin. The slope of the straight line will be k [ I if [S 2 O 2-2 8 is varied and k [ S2O8 if [I - is varied. Use this to calculate k. 8) Share your results with those of another group so that you have the order of the reaction with respect to both iodide and peroxodisulfate. When presenting your work you must attribute other groups results to that group. Clearly indicate which results are your own and whose any others are. Using others results without attributing them is a serious offence and will result in a mark of zero. RESULTS Original molarity of peroxodisulfate (S 2 O 8 2- ) solution = mol dm -3 Original molarity of iodide (I - ) solution = mol dm -3 Original molarity of thiosulfate (S 2 O 3 2- ) solution = mol dm_3 PAGE 10 OF 66
Total vol. of reacting solutions after mixing = cm 3 âinitial concentration of thiosulfate ion in reaction mixture = mol dm -3 âchange in conc. of peroxodisulfate ion, [S 2 O 8 2- until darkening = mol dm -3 Table 1.1 Solutions A and B mixed, i.e. peroxodisulfate concentration varied. Origin of results System number cm 3 S 2 O 8 2- Solution A cm 3 I - Solution B Darkening time t d /s [S 2 O 8 2- after mixing/m [I - after mixing/m 2 [S 2O3 = 2td /Ms -1 i 1 1 9 10 0 2 2 8 10 0 3 3 7 10 0 4 4 6 10 0 5 5 5 10 0 6 6 4 10 0 7 7 3 10 0 8 8 2 10 0 9 9 1 10 0 10 10 0 10 0 PAGE 11 OF 66
Table 1.2 Solutions C and D mixed, i.e. iodide concentration varied. Origin of results System number cm 3 S 2 O 8 2- Solution C cm 3 I - Solution D Darkening time t d /s [S 2 O 8 2- after mixing/m [I - after mixing/m 2 [S 2O3 = 2td /Ms -1 i 1 10 0 1 9 2 10 0 2 8 3 10 0 3 7 4 10 0 4 6 5 10 0 5 5 6 10 0 6 4 7 10 0 7 3 8 10 0 8 2 9 10 0 9 1 10 10 0 10 0 PROBLEMS Answer the following problems. You may be tested on your understanding of them. 1) By examining your graph decide on the value of m or n in equation (1.2). Obtain the other value from another group and write the rate equation for the reaction. Justify your decisions. 2) State the overall order of reaction (1) and the order with respect to each of the reactants. PAGE 12 OF 66
3) For the experiments investigating the change in concentration of peroxodisulfate, consider systems no. 1 (the worst possible case) and no. 10 (the best possible case). a) In each case calculate the change in the peroxodisulfate concentration (due to reaction) during the period between mixing and darkening. (You may already have calculated it.) Express each one as a percentage of the initial value of this concentration. b) Considering each case in turn, do you think that the concentration of peroxodisulfate remained approximately constant during the time for which the reaction was observed? Explain. c) What do you think is meant by the terms the worst possible case and the best possible case in the context of this question? 4) Why is it important to avoid warming the test tubes with your hands? Taking into account your answer to this question, you may be able to suggest an improvement to the design of the experiment. In other words, what should have been done and wasn't? 5) Why does the solution darken? Why does it not do so immediately after mixing the two solutions (A and B, or C and D)? What would you notice if the thiosulfate were accidentally omitted from the reaction mixture? 6) What would you notice (or fail to notice) if (i) the starch indicator, (ii) the peroxodisulfate, and (iii) the iodide solution were omitted from the reaction mixture? 7) What would you expect to be the effect of substituting potassium peroxodisulfate, sodium iodide, and potassium thiosulfate for the recommended reagents? 8) Why is the volume of potassium iodide solution used in the first set of experiments, and the volume of sodium peroxodisulfate solution used in the second set of experiments, kept constant? PAGE 13 OF 66