Rate and Activation Energy of the Iodination of Acetone

Transcription

1 nd Activtion Energ of the Iodintion of Acetone rl N. eer Dte of Eperiment: //00 Florence F. Ls (prtner) Abstrct: The rte, rte lw nd ctivtion energ of the iodintion of cetone re detered b observing the disppernce of the chrcteristic color of iodine in wter. B sstemticll vring the concentrtion of rectnts, the rte lw is detered to be: 0 = [cetone] 0 [] 0. Once the rte lw epression is found, the ctivtion energ of the rection is found b mesuring the rte of the rection t multiple tempertures. The clculted ctivtion energ of 9 ± / mol is sufficientl lrge to me the rection slow enough to observe, but not so lrge s to me the rection eceedingl slow. Introduction: The ctivtion energ of chemicl rection is the energ required to initite collisions energetic enough to llow the trnsformtion of rectnts to products nd is described b the Arrhenius eqution. Ae where: = rte lw constnt A = frequenc fctor = universl gs constnt, 8.34 / mol K E = ctivtion energ of the rection Detertion of the rte lw constnt requires the rte of the rection to be eplored s the concentrtions of rectnts re sstemticll vried. This llows the orders of the rection with respect to ech regent to be detered, which is the foundtion of the rte lw epression. The iodintion of cetone is convenient rection to stud for number of resons: the color of the iodine rectnt is chrcteristic nd redil observble, so the chnge in [I ] cn be esil monitored; the rte of the rection is fst enough to llow multiple runs, but not so fst to be difficult to mesure; the rection is ero-order with respect to iodine so the chnge in concentrtion is liner with time. The fct tht the rection is ero order in cetone is especill importnt becuse it llows the rte of the rection to be clculted s n verge rte of the chnge in iodine concentrtion over (reltivel) long time periods. O O (q) + I (q) C + H + (q) (q) + I - (q) + H + (q) H C 3 C CH 3 H 3 C CH I Eperimentl Procedure: Detertion of the Lw Epression: A number of smples were prepred to eplore the dependence of the rte of rection upon the concentrtion of rectnts, summried in Tble. For ech smple, the listed volumes of 4.00 cetone solution, hdrochloric cid solution, nd wter were dded to one test tube (Tube A ), nd the listed volume of 5.00 iodine solution ws dded to second test tube (Tube B ). The tubes were plced in room temperture wter bth (0.8 C) nd the temperture ws llowed to equilibrte for t lest 5 utes. While the temperture of the smples ws stbiliing, the spectrophotometer ws clibrted nd SpectroPro ws set up to collect bsorbnce vs. time dt for 0 utes, smpling ever second. Once the spectrometer ws prepred nd the smples t temperture, the two tubes were removed from the wter bth nd dt collection ws strted in SpectroPro s the smples were combined. The miture ws poured bc nd forth between the two tubes number of times to mi thoroughl nd poured into smll test tube for use in the spectrophotometer. To imie temperture increses from inside the spectrophotometer, the tube ws not plced in the smple comprtment until most of the iodine color fded from the rection solution. The time ws recorded when the bsorbnce reding leveled off t or ner ero. Additionl eperiments were performed t room temperture, sstemticll doubling the concentrtion of rectnts while eeping the totl smple volume t. For ech condition, repet runs were performed until consistent rection times were observed. Detertion of the Activtion Energ: Clcultion of the ctivtion energ of this rection requires rection rte dt t dditionl tempertures. For this eperiment, two dditionl temperture conditions were used. The first ws cool bth. A 800mL beer ws filled E

2 with wter nd ice ws dded to bring the temperture of the bth down to pproimtel 0 C. The bth ws mgneticll stirred nd temperture ws monitored using LoggerPro. Smples were prepred s listed in Tble nd the tubes were plced in the cool bth to llow the temperture to equilibrte for 5-0 utes. During equilibrtion nd rection, the bth temperture ws mintined t 9.8±0. C. The smples were mied nd immeditel returned to the cool bth. This rection ws observed visull, recording the time required for ll visible color to fde from the solution. A third temperture condition, the cold bth, ws lso used. For the cold bth, beer ws filled with ice nd enough wter ws dded to fcilitte het trnsfer. Agin, tubes were prepred s described in Tble nd equilibrted to temperture for 0 utes before miing nd immeditel returned to the cold bth. The temperture of the cold bth ws checed periodicll nd mintined t 0 C b periodic removl of ecess wter nd ddition of ice. These smples were lso observed visull. Tble : Volumes of regents used nd rection times for ech eperiment. Tube A Tube B Ep t # Temp. ml Totl Solution n Time ml (q) ml H ( C) cetone(q) O(l) ml I (q) Volume, ml () Cool Cool Cold Cold Notes: All volumes were mesured with volumetric pipettes. esurements were ten t 0.7 C. The double-i run ws repeted fourth time becuse the first three times vried. esults nd Discussion: Initil concentrtions of ll regent were clculted bsed upon the stoc concentrtions nd the volumes listed in Tble. ection times nd concentrtions re summried in Tble. The rte of the rection is clculted s the chnge in concentrtion of iodine over rection time, lso summried in Tble. Tble : Initil concentrtions nd rections rtes for ll rections. Ep t # n Time ection [cetone] () 0 [] 0 [I ] 0 ( / ) Cool Cool Cold Cold

3 Smple Clcultions for Tble vlues: Initil concentrtion of cetone, run : cetone stocvolume of cetone stoc mL cetone.00 volume of solution totl Initil concentrtion of hdrochloric cid, run : stoc volume of stoc mL 0.48 volume of solution totl Initil concentrtion of iodine, run : I volume of I stoc stoc mL I.60 volume of solution totl of rection, run : 3 ΔI I I finl of rection Δt t t 3.7 finl For nlsis of the orders of rection in the rte lw epression, it is convenient to use verge rection rtes for ech set of conditions. The conditions re summried in Tble 3 s verges. Tble 3: Averge rection rtes for ech set of conditions. [cetone] 0 [] 0 [I ] 0 ection Error on the verge ( / ) rte Stndrd un ± (.5%) Double cetone ± (4.%) Double cid ± (.0%) Double iodine ± (4.5%) Cool uns ± (4.3%) Cold uns ± (0.6%) The consistentl sub-5% errors on these verge rection rtes indicte tht the dt should be quite relible for detertion of the rte lw epression nd the ctivtion energ of the rection. Compring the stndrd run to the double cetone run, the rte of the rection pproimtel doubled when the concentrtion of cetone doubled; therefore, the rection is first order with respect to cetone. Compring the stndrd run to the double cid run, the rte of the rection pproimtel doubled when the concentrtion of cid doubled; therefore, the rection is first order with respect to cid. Compring the stndrd run to the double iodine run, the rte of the rection is unchnged when the concentrtion of iodine doubled; therefore, the rection is ero order with respect to iodine. Therefore, the rte lw epression for this rection is: 0 = [cetone] 0 [] 0 [I ] 0 0 = [cetone] 0 [] 0 The ect mthemticl vlue of the rte lw orders cn lso be found to indicte the error in these mesurements. Compring the stndrd run to the double cetone run: Compring the stndrd run to the double cid run: = =.07 cetone I cetone I cetone I cetone I

4 Compring the stndrd run to the double iodine run: cetone I cetone I = These clculted orders re ll ver close to the qulittivel detered orders, so these eperimentl results do not hve significnt error, s reflected in the verge rection rtes in Tble 3. The vlue of the rte lw constnt is found b plugging the concentrtion nd rte dt bc into the rte lw epression nd solving for. Tble 4: lw constnts Ep t # [cetone] 0 [] 0 [I ] 0 ection ( / ) ( - - ) Cool Cool Cold Cold For Ep t : 0 = [cetone] 0 [] / = (.00) (0.48) = The vlue of should be constnt for rection t ech temperture, so the vlues re verged for ech temperture, results re shown in Tble 5. Tble 5: Averge rte lw constnts t three tempertures Temperture Averge, (K) - - Error on verge ±0.00 (7.0%) ± (5.%) ± (0.5%) The ctivtion energ of this process cn be detered b using the Arrhenius eqution. Given the dt in Tble 5, the comprtive form of the Arrhenius eqution cn be used three times to detere the ctivtion energ of this process. E T T Compring the room temperture nd cool dt: molk 83.0K E = / mol = 03 / mol 94.0K

5 Compring the room temperture nd cold dt: molk 73.0K 94.0K E = / mol = 9.9 / mol Compring the cool nd cold dt: molk 73.0K 83.0K E = / mol = 80.8 / mol The verge ctivtion energ is 9 ± / mol bsed upon pirwise dt. This % error seems high, but this is lrgel result of using reltivel smll temperture rnge for the eperiments. As cn be seen in the dt, the clculted ctivtion energ for the lrgest temperture chnge (room temperture to cold) is most consistent with the verge, while the smller room-cool nd cool-cold vlues vr significntl. The ctivtion energ cn lso be clculted grphicll using the liner form of the Arrhenius eqution. E T A Arrhenius Plot for Iodintion of Acetone = ² = E E E E E E E E-03 /Temperture E slope 67K 8.34 molk E E = / mol = 9.8 / mol The vlue of the ctivtion energ detered from the liner form of the Arrhenius eqution is in ver close greement with tht found from the pir-wise tretment of the dt, with less thn % difference between the two methods. A more relible nlsis of the ctivtion energ could be done with lrger temperture rnge nd/or more tempertures. Conclusions: Detertion of the rte lw nd ctivtion energ of chemicl rection requires number of steps. B vring the concentrtions of rectnts in the iodintion of cetone it ws detered tht the rection is first order with respect to both cetone nd hdrochloric cid concentrtion nd ero order with respect to iodine concentrtion. esuring the rection rte t multiple tempertures llows clcultion of the ctivtion energ of the process, in this cse the ctivtion energ of the rection is found to be 9 / mol. The error in ll prts of this eperiment is reltivel smll. This rection is fst enough to be performed in resonble mount of time, but not so fst tht it is difficult to mesure. This mens tht the clculted ctivtion energ, 9 / mol, is moderte brrier to rection.