Unformatted text preview:

VIDEO EXPERIMENT 5A KTNETIC STT'DY USING SPECTROPHOTOMETRY'Flri c avnarimanl studies the well--known sol-ution reaction between bromate andbromide ions in the presence of varying concentrations of hydroni-um ions:BrO3- + 5 Br- + 6 H3O* * 3 Brz + 9H2OYou wilf be able to use the resul-ts from the kinetic runs to find the order ofthe reaction with respect to hydronium ions and decide whether this agrees witha proposed mechanism or reaction pathway.There are two main probl-ems in most kinetic st.udies. The first is the need tomonitor the progress of the reaction by measuring the decrease in concentrationof a reactant or the increase in concentration of a product. The second problemlies in the treatment of the resul-ts obtai_ned.For the reason described below, we use a spectrophotometer to measure veryIow concentrati-ons of bromine as it is produced. Although sophisticated indetaiJ-, the principles of the instrument are straiqhtforward.. The opticalcomponents produce two identical beams ofradiation which are monochromatic, i.e., theyconsist of one wavelength only. One of thesepasses through a sample of the reactingsolution which is kept at a steady 20aC in j_tssilica cell-. The other beam passes throughdistill-ed water contained j-n an opticallyi-denticalreference cell. The intenslty of the referencebeam (Io) is then el-ectronically compared withthat from the sample (It) by means of tworadiation detectors. these express the ratio ofthe intensities in the formf ogro ( Ia/ It)known as the absorbance of the solution. Theimnart:nnc nf J-his r:rr:nfil-rz i-n rrq iq fh:t- ni76rurrrr vqqirL4Lf Lv qv rJ LrrqL, vv9rthe ranoe ttsecJ- it i c di ractl \/ nr^n^rf i an=l t-^svvs, ylvvvt Lrvrrqr uvthe concentration of the radi-ation-absorbinqspecies. Thus the ver:ica] position of the pen on the chart recorder is a directmeasure of the bromine concentration. Note that zero absorbance for this machineis at the top of the chart paper. It will be seen that the absorbance of thesol-ution is not the same at atl- wavelengths. In fact, the values for bromineshow a maxinum val-ue or peak at 267 nm, which l_ies in therrlrr:trinlar ranign. It is convenient to use this peak as a reference point atwhich to measure the bromine absorbance for each specErum.The rate of a given reaction normall_y changescontinually; thus concentrations of product andreer:tani- s v.ar\/ non-l i nc:rl rr wi J- h f imo and iharrsrru Llrsyapproach gj-ven val-ues asymptotically. This causes adifficul-ty when we wish to compare one kinetj_c runwith another. At what instant should we choose tomeasure a rate? A cornmon sol-ution is to use amathematicaf treatment, but in this experiment thecjesicrn al lnws rrq f ^ i^i+iai raf oq hrr <trrr{rrinavsrryrr q!!vw- ur uv UvrlryqrE L!l!LLaL !J rLuuyfftg Ithe reaction until- is it only about 5? complete. This Irestricts measurements to the early, almost Linear ,j ,n:ri- of fhe -cA.tj-on Curve and reqUires a mOnitOrinq- Lw2linstrument of great sensitivity. Hence the choice ofa spectrophotorneter.The reaction mixtures for each run are made in aninitially identical fashion by adding exactly 5 mL of0.7 M potassium bromide and 10 mL of 0.01 M potassiumbromate solutions to each 50 mL graduated fl_ask. Thisis then made up to between 40 and 45 mL withdistiLl-ed water and a measured volume of 0.5 M..rU i'llauAF^^li 1^-; ^ -^ilryqruurrterrc acrd is added quickly from a microburette whj-ch j-s sliqhtLvpressurized to speed up the addition. (Why is rapid addition rr""u""u.y, and is a,loss of accuracy invorved?) After topping off the flask, the spectrum isrepeatecily scanned in the ultravioLet region at fixed time intlrvals. For eachof four different hydronium ion concentrationsr 1zou wil-l- obtain a series of peakabsorbance val-ues elated to bromi-ne concentration whi-ch, when plotted againstihai r nnrrocnani'---* --r-.^Jl.n9times, wil-l- yield an initial_ reactionThe rate of the forward process canconcentrations of the react.ants in theRate : k IBro:-] t Ier-]. IH3o*] nwhere k is the rate constant and l, m and n are the ordersrespect to the individual reactants. The values of l- and munity--that is, the reacti-on is first order i-n both bromatesince we keep the concentrations of these constant for eachexpression simplifies toRate : k' [H3O+]nand by taking logs,Ioc{r:l-cI : nl^dIH-n+l + ]nn tzr4v: \rssv/ rr+vY Lrrjv J I rvY ^Thus a plot of 1og(rate) against 1og[H3Ot] should give another straj-qht linewith n as the slope.With this estimated val-ue for the order of the reaction in hydronium ion, youwill- be in a position to test the feasibil-ity of a proposed mechanj-sm for thebromate-bromide reaction, which can be represented as fol_.]_ows:rate as the slope of the straight l_lne.be expressed as a function of thefollowing way:of the reaction withare each known to beand bromide ions.rttn - f hc rrf a€^ ^!sl-ow€^ ^!H3o' + Br-H^n+ + Elrn^-rlJv . e! vJFlRr + tlRrO^HOBr + HBr+ HBr + H2O+ HBrO3 + H2O+ HOBr + HOOBr+ H2O + Br2HOOBr+HBr + 2HOBrThe slow third step would be rate-determininq.ADDITIONAL NOTESi- When the spectrophotometer is demonstrated, notj-ce the two collimatingsl-its which select the narrow beam of monochromatic radiat.ion to be passed+-hrarralr +ha crnDfe and reference. In order to make them and the beam clearl-vt'icil'r'la +.r-ra ori.ts have been manuatly opened to a gap of about 2 mm. To avoid, lv1yLv toptical errors which would resu]t from using such a wide slit width for anaCtUal_ meaSUreme-+ f 1.ra,, 1-.---^ L^:rlE7 rney nave been reduced to their normar working value ofabout 0.025 mm during the kinetic runs.i i trvnari-^nts to determine reaction orders in accurate kinetic studies arelAyv4 4r!rvrrtr<rrr'l lrr ranorJ-^'l a nurnber of ti-mes and the results averaged to el-iminate randomerrors. The answers you obtain may therefore be subject to some errori but sincereaction orders are normally smalt and integral, it should be obvious how toround of f the n val-ue -THE STUDENT'S PART IN THE EXPERIMENTYou will need to note the volume of 0.5 M hydrochloric acid added to eachflas k.During each kj-netic run, you should. write down the times in minutes at which+-ha <n] rrr-inn cnacLrtxn is scanned, taking the first as zero mi-nutes. you need notworry that the tracing of the


View Full Document

CABRILLO CHEM 1B - A KTNETIC STT'DY

Download A KTNETIC STT'DY
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view A KTNETIC STT'DY and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view A KTNETIC STT'DY 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?