FXPEPLIMEI\JT 26 the rusting of iron is reasonably slow, whereas the decomposition of TNT is noticed that reactiotis occur at varying speeds. There is an entire spectrum of speeds of reactions, ranging from very slow to extremely fast. For example, Rates of Reactions On the basis of the experiments you ve performed, you probably have already Factors Affecting DISCUSSION WORK IN PAIRS BUT EVALUATE YOUR DATA INDIVIDUALLY. buret clamp ring stand 500-mL Florence flask 250-mL Erlenmeyer flasks (4) 400-mL beaker test tube CHEMICALS clock or watch with second hand 125-mL Florence flask APPARATUS AND burets (2) l-ml pipets (2) pipet bulb thermometer prepared) (prepared from fresh solid) 0.2 M (NH 4)2S208 (200 ml) 0.1 M solution ofna 2EDTA 0.2 M KNO 3-(300 ml) 1 percent starch solution, boiled 0.2 M KI (200 ml) 0.4 M Na 2S2O3 (100 ml) (freshly 50-mL pipet 25-mL pipet with respect to the reactant concentrations; and to obtain the rate law for the chemical reaction. peroxydisulfate ion with iodine ion; to determine the order of the reaction OBJECTiVE To measure the effect of concentration upon the rate of the reaction of A OLOCK REACTION REACTIONS I: RATES OF CHEMICAL k- LM
of time. Mathematically, the rate of reaction may be expressed as follows: the greater the probability of their colliding. Hence, increasing the concentra Catalyst. Catalysts, in some cases, are believed to increase reaction rates by observing an increase in the intensity of the color of the solution as a function fore the rate of reaction. bringing partides into dose juxtaposition in the correct geometrical arrange reactant partides. If less energy is required for a successful collision, a larger tration with time ofeither A, B, C, or D. Which species you choose to observe is the number ofcollisions between them in agiven period oftime. Thus, the rate of the products C and D. In practice, then, one measures the change of concen The rate of this reaction may be measured by observing the rate of disappear A±B C+D [1] of particles per unit volume. The more particles present in a given volume, Concentration. Changing the concentration of a solution alters the number Defined Consider the hypothetical reaction of thumb, for each 100 increase in temperature, the rate of reaction doubles. Order of Reaction Let s examine now precisely what is meant by the expression rate of reaction. an increase in temperature increases the kinetic energy of the partides. An ment for reaction to occur. In other instances, catalysts offer an alternative route to the reaction, one that requires less energetic collisions between at the end of the reaction, the catalyst can be recovered chemically unchanged. percentage of the collisions will have the requisite energy, and the reaction will proportion of the collisions having the required energy for reaction. As a rule Temperature. Since temperature is a measure of the average kinetic energy, how the various factors influence the rates of reactions. (ions or molecules) must collide with sufficient energy to result in a reaction; if collisions of the reacting particles. However, even then, the reacting particles Before a reaction can occur, the reactants must come into direct contact via they do not, their collisions are ineffective and analogous to collisions of tion of a solution increases the number of collisions per unit time and there reaction increases. Also, an increase in kinetic energy results in a greater 4 Catalysis. increase in kinetic energy increases the velocity of the particles and therefore 3 The temperature; and 2 The concentration of the reactants; 1 The nature of the reactants: reactions is called chemical kinetics Experiments show that rates of homoge neous reactions in solution depend upon: billiard balls. With these considerations in mind, we can qualitatively explain ance of either of the reactants A and B, or the rate of appearance of either of a matter of convenience. For example, if A, B, and D are colorless and C is extremely fast The branch of chemistry that is concerned with the rates of occur faster. Actually, the catalyst may take an active part in the reaction, but colored, you could conveniently measure the rate of appearance of C by time required for change At Rate of appearance of C =. time required for change At RateofdissappearanceofA change in concentration of A A[AJ change in concentration of C A[C]
Rate = k[a[bf [2] second time, another 2 X io4 mol ofs2082 has reacted, making a total of constant, k. thiosulfate, Na You will add to the solution a small amount of another reagent (sodium 2S.,O blue-black color will disappear. When the blue-black color reappears the add another portion of Na 3), which will cause a change in the color of the solution. 2S2O3 after the appearance of the color, and the turn blue-black when 2 X lo- mol of S2O82 has reacted. You will quickly S2082_ has reacted. For reasons to be explained shortly, the solution will The amount is such that the color change will occur when 2 x io- mol of Your goal will be to determine the values of x andy as well as the specific rate S O2 2 = or Rate of disappearance ofs2o82 =k[s 2Os 2ix[IjY [51 determined is of the form disulfate,s2082, that reacts as a function of time. The rate law to be Reaction of In this experiment you will measure the rate of the reaction Peroxydisulfate and you will determine the rate law by measuring the amount of peroxy Ion with Iodine S2082 + 2F I + 2SO on 2 [4] of k can be calculated. value that is independent of the concentration. It is characteristic of a given of the reaction by noting the effects of changing reagent concentrations It should be emphasized that k, the specific rate constant, has a definite 2 + 1 = 3, or a third-order reaction. It is possible to determine the order order in B. The overall order of the reaction is the sum of the exponents, reaction. In this case, the reaction is said to be second order in A and first to which the concentrations in the rate law are raised are termed the orderthe same) would cause the rate to increase bya factor of 4, because the rate of the double. On the other hand, doubling the concentration of A (keeping [B] the reaction is proportional to the square of the concentration of A. The powers reaction and depends only on temperature. Once the rate is known, the value concentration of B (keeping [A] the same) would cause the reaction rate to on the rate of the reaction. would be the rate law. It should be evident from Equation [3] that doublingthe Rate = k[a1 2[B} [3] is to determirie the rate law. Stated slightly differently, one goal of measuring rate, and k is the specific rate constant. One of the objectives of chemical kinetics the rate of the reaction is to determine the numerical values of x and y. powers to which the respective concentrations must be raised to describe the where [A] and [B] are the molar concentrations of A and B, x and y are the Suppose that we found x = 2 and y = 1 for this reaction. Then the reactants. Thus, the rate of our hypothetical reaction may be expressed as In general, the rate of the reaction will depend upon the concentration of
x 17.5 s 7.8 x io tmi (22.0 (9.8 s2o82-2 per of a starch-iodine complex that is formed from iodine, 12, and starch in the Consequently, until the same amount of S9032 that is added is dl con 12 + 2SO2_ 21 S4062 12 is formed according to Equation [4]. The thiosulfate that is added to the The blue-black color that will appear in the reaction is due to the presence S9082 and 1. concentrations of S2082 and 1 and observing the effects upon the rate of sumed, there will not be a ufficient amount of 12 in the solution to yield the equal portions are termed aliquots), and from the stoichiometry of Equations consumed according to Equation [4]. it is rapidly regenerated according to Equation [6] and therefore its concentration does not change during a given Figure 26.1. The best straight line passing through the origin is drawn, and The more rapidly the 2 X I mol ofs2082 is consumed, the faster is the experiment. 2 x io mol of S2O8 has reacted. Note also that although iodine. 1, is [4] and [61 you can verify that when this quantity of S2032 has reacted, you will he able to determine the rate of the reaction. By changing the initial several times, keeping carejid note of the time for the appearance :f the solution. The color therefore will not appear until a detectable amount of solution reacts exrremel raprdl c with the iodine, as follows: reaction. To determine the rate of the reaction. a plot of moles ofs2o that the slope is determined. The slope,.s2os2/.t. corresponds to the moles of Rate blue-black colors. By graphing the amount of S2O5 consumed versus time. the reaction. VU wil determine the order of the reaction with respect to blue-black color. You will add 4 X 1O- mol of S2032 each time ithese have reacted versus the time required for the reaction is made, as shown in Determination of Graphical [6] ) 2(2 x lo ) mol of SO82 that has reacted. You will repeat this proedure FIGURE 26.1 Graphical determination of rate. Time in ond = 4.5 x io nt s 1O mol 2.0) 4.5) 4.5 5 IC 15 20 22 25 30 35 s Since the rate corresponds to the change in the concentration of S20. S2082 that have been consumed per second and is proportional to the rate.
6.0 X rrit,-3 4.5 X io molls Experiment B. Kinetics Experiments A. Preliminary PROCEDURE Helpful. Comments Total volume = 75.0 ml Total volume = 50.0 ml 1 drop EDTA solution 23.0 ml KNO Solution 1: 25.0 ml KI solution Solution 2: 25.0 ml KI solution 3 solution 48.0 ml KNO solution 1.0 mlna 1.0 ml starch solution 1.0 ml Na 2S2O3 2S2O3 Total volume = 73.0 ml Total volume = 50.0 me 1 drop EDTA solution 1 drop EDTA solution 1 drop EDTA solution 3 solution solution 23.0 ml KNO Solution 4: 12.5 me K! solution 35.5 me KNO 3 solution 1.0 ml starch solution Solution 3: 50.0 ml K! solution solution solution 1.0 mlna 1.0 mena 2S2O3 1.0 ml starch solution 3 solution 2S2O3 1.0 ml starch solution time): Solution preparation. Prepare four addition of Na reaction solutions as follows (one at a 2S2O3 has on the color. 2 Repeat the procedure in (1), but when the solution changes color add 4 drops of 0.4 M Na drops of starch solution and mix thoroughly, and then add 5 ml of 0.2 M (NH 4)2S208 solution. Mix. Wait a while and observe color changes. 2S2O3, mix the solution, and note the effect that the 1 Dilute 5 me of 0.2 M KI solution with 10 ml of water in a test tube, add 3 4 You will perform a few preliminary experiments to become acquainted purpose of the drop of the EDTA solution is to minimize the effects of 3 The reaction studied in this experiment is catalyzed by metal ions. The medium the same in each run in terms of the concentration of ions; it does not enter into the reaction in any way. blue-black when exactly 2 X io mól of S2082 has reacted. 1 According to the procedure of this experiment, the solution will turn above analysis, determine the order of the reaction with respect to both of with the observations in this experiment so that you will know what to By varying the initial concentrations of S2032 and F, you can, via the doubles the rate of the reaction and the reaction is first order in S2082. these species. [F] = 2.0 M, we then know. that doubling the concentration of S2082 [F] = 2.0 M, and a rate of 3.0 X i0 mol/l-s when[s2o] = 1.0 M and If we obtain a rate of 6.0 X 10 moljl-s when [S20829 = 2.0 M and 2 The purpose of the KNO 5 The initial concentrations of the reactants have been provided for you on the report sheet. 3 solution in this reaction is to keep the reaction trace quantities of metal ion impurities that would cause spurious effects on the reaction. expect in the reactions. U.015 L disappearance of S2O8, that is,[s2o8]t. If the total volume of the second, dividing the slope by the volume of the solution yields the rate of solution in this example were 75 ml, the rate would be as follows:
Equipment setup. Set up two burets held by a clamp on a ring stand as shown in Figure 26.2. Use these burets to measure accurately the volumes of the K! and KNO 3 solutions. Use two separate l-ml pipets for measuring the volumes of the Na 2S2O3 and starch solutions and use 25-mL and 50-mL pipet to measure the volumes of the (NH 4)2S208 solutions. Each solution mustbefrech1vpreparedte& y1 -r P-, 3, and 4 one at a time as you make your measurenents. Rate measurements. Prepare solution 1 in a 250-mL beaker that has been scrupulously cleaned and dried. Pipet 25.0 ml of(nh 4)2S208 solution into a clean, dry l00-ml beaker. Be ready to begin timing the reaction when the solutions are mixed (READ AHEAD). The reaction starts the moment the solu tions are mixed! BE PREPARED! ZERO TIME! Quickly pour the 25.0 ml of (NH 4)2S208 solution into solution 1 and stir vigorously with a clean stirring rod; note the time you begin mixing to the nearest second. At the instant when the blue-black color appears, 2X io- mol of S2082 has reacted. ) S IMMEDIATELY (be prepared!) add a 1-mE aliquot of Na 2S2O3 solution from the pipet and stir the solution; the color will disappear. Record the time for the reappearance of the blue-black color. Add another 1-mL aliquot ofna 2S2O3 solution and note the time for the reappearance of the color. The time interval being measured is that between the appearance of the blue-black color. For good results, these aliquots of Na 2S2O3 must be measured as quickly, accurately, and reproducibly as possible. Continue this procedure until you have added seven (7) aliquots to solution 1. You are finished with solution 1 when you have reached all your times on the report sheet. (The time intervals are cumulative.) Solutions 2, 3, and 4 should be treated in exactly the same manner except L C that 50.0-mE portions of(nh 4)2S208 solutions should be added to solutions 1 2 and 4 and 25 ml of(nh 4)2S208 solution should be added to solution 3. (CAUTION: Be on guard solution 3 will react much more rapidly than solution 1.> In each of these reactions the final total solution volume is 100 ml. KNO 3 SOlUtIOn IU solution flge 262
without loss of time. can add these aliquots to their reactions at the appearance of the blue color each of seven clean, dry test tubes with I ml. ofna 25,O; solution. thrv rj the burets from bottles of stock solutions also kept there. If the students fill L 3A+2B 2C+D 9 Assuming that a chemical reaction doubles in rate for each 100 tempera 10 A reaction between the substances A and B has been found to give the following data: what is the rate of consumption of S2082? possible practical value does this type of information have? were increased 40 C? ture increase, by what factor would the rate increase if the temperature 1 Whatfactors influence the rate of a chemical reaction 4 Write the chemical equations involved in this experiment and show that How would the particle size of the zinc affect the rate of its dissolution? either A or B quadruples the rate of the reaction. Write the rate law for obeys the rate law rate = k[a] this reaction. What is the order of reaction with respect to A and B for a reaction that 3 2[Bj appearance of the blue color of the starch-iodine complex. the rate of disappearance of[s2o82] is proportional to the rate of 3 5 It is found for the reaction A + *C that doubling the concentration of 6 If 2 X 10 mol ofs2082 in 50 ml of solution is consumed in 188 s, 7 Why are chemists concerned with the rates of chemical reactions? What 8 Suppose you were dissolving a metal such as zinc with hydrochloric acid. What is the general form of a rate law? c cc c ccc; cc. 2 answer the following questions: Before beginning this experiment in the laboratory, you should be able to REVIEW QUESTIONS sheet. scissa, horizontal axis), using the data in columns 3 and 4. Calculate the slope For each solution, plot on the graph paper provided the moles ofs2082 2 The cumulative time from the start of the reaction to each appearance of 3 The corresponding numbers of moles S2082 consumed (column 4). interval fràm the preceding color appearance for each succeeding aliquot consumed (as the ordinate, vertical axis) versus time in seconds (as the ab (column 2); color (column 3); the appearance of color for the first aliquot ofs2032 and the time of each plot, and from these calculations answer the questions on your report 1 The time interval from the start of the reaction (addition of S2082) to four solutions: Calculations Tabulate on the data sheet for each aliquot of Na 2S2O3 added to each of the solutions. The students may obtain the solution from these burets and refill Time may be saved in this experiment by setting up several burets on side tables in the laboratory filled with (NH 4)2S208, KNO 3, KI, and Na Timesaving Hint 2S2O3 to Instructors
and B and the rate law and calculate the specific rate constant. Using the above data, determine the order of the reaction with respect to A 2.0 x 10 3.0 x 10_2 2.0 x (moifl) (mol/l) (mol/h) [A] [B] Rate of appearance of C 1.0 x 10 2 10_2 3.0 8.1 x 10-6 3.0 3.24 x 3.0 7.30 x 1.0 1.20 x i0_6!,o x ft3 x 1O_6 (moll) (mol/l) (mollh) [Al [BI RateofappearanceofC
12 REPORT SHEET FOR EKPERLMENT 26 Date Name 2 7 12 x io 14 x 5 10 x io- 4 4 3 U) 2 x i0 8 x io 6 x io Aliquot no. appearances of color Cumulative time(s) Time(s) between Total moles of SO82 consumed 4 x io- Solution 2. Initial[S20829 0.10 M; initial [F] 0.05 M. Time experiment started 7 14 x U 5 I.;,.3 6 x io- 12 x io lox 4.; 4 x i04 2 x io4 8 x io Time(s) between Aliquot no. appearances of color Cumulative time(s) consumed Total moles of S2O8 Solution 1. Initial[S2082] 0.05 M; initial [F] 0.05 M. Time experiment started Experiment B. kinetics 2 The color of the starch * complex is 1 What are the colors of the following ions: K ; I Experiment A. Preliminary REACTIONS I: A CLOCK REACTION RATES OF CHEMICAL r
3 your graph), calculate k from your data. Rate =k[s I (.; 5 Aliquot no. appearances of color Cumulative time(s) Time(s) between 2O82_j[I_y 7 (0 5 4 c Aliquot no. Solution4. Initial[S2082i = O.1OM; initial [I 1 = 0.025M.Time experiment started 14 x 10 12 x 10 4 x 2 x io 8 x io consumed 6 x io4 Total moles ofs2o 10 x 8 x io 12 x i0 2 What effect does doubling the concentration of 1 have on the rate of this reaction? 5 From your knowledge of x and y in the equation (as well as the rate in a given experiment from 4 Write the rate law for this reaction that is consistent with your data. 3 What effect does changing the[s2082] have on the reaction? Solution 1 Solution 2 Calculations 4 4 x i0 6 x i04 2 x i0 consumed Total moles of S2082 10 x 14 x io4 Solution 3. Initial[S2O82i = 0.05 M; initial [1j 0.10 M. Time experiment started 3 ij Solution 3 Solution 4 1 Rate of reaction, z[s 2O829/zt, as calculated from graphs (that is, from slopes of lines): appearances of color Cumulative time(s) re(s) between