Usng Spectrophotometrc Methods to Determne an Equlbrum Constant Prelab 1. What s the purpose of ths experment? 2. In step 11 of the procedure, t s stated that the absorbance of the soluton ncreases as Fe soluton s added? Explan usng uaton 3 and LeChateler's Prncple. 3. In steps 3, 5, 10 and 11, 10.00 ml of 2.00 x 10 3 M SCN s dluted to 100.0 ml and placed n a beaker. 0.100 M Fe s added to ths soluton n 10 1.00 ml ncrements. Calculate [SCN ] and [Fe ] n the 10 solutons. Do ths n an attached spreadsheet (see page 5, you wll use these values n Data Treatment and Dscusson). Include a calculaton of the 5 th soluton n your notebook. 1
Introducton Usng Spectrophotometrc Methods to Determne an Equlbrum Constant Chemcal reactons do not typcally go to completon. Reactons always reach ulbrum. When a system s at ulbrum, the rate at whch products form from reactants s the same as the rate at whch products are decomposng to produce reactants. For a general reacton, a + bb cc + dd (1) the ulbrum expresson s c d [C] [D] K = [] [B] (2) a b where K s the ulbrum constant for the reacton at a gven temperature. In ths experment, you wll determne the ulbrum constant of the reacton Fe (aq) + SCN (aq) FeSCN (aq) (3) Iron(III) on reacts wth thocyanate on (SCN ) to produce the complex on, thocyanatoron(iii), whch s blood-red. The ulbrum expresson s - K = [Fe ] [SCN ] (4) To determne the value of K for ths reacton, the ulbrum concentratons of the three ons n soluton must be determned. Beer s Law wll be used to measure the ulbrum concentraton of thocyanatoron(iii) on, (from a known value of ε at 447 nm). = (5) To determne [Fe ] and [SCN ], t can be assumed that the ron n the system must be present as ether Fe or FeSCN. Lettng [Fe ] represent the concentraton of ron(iii) ntally (before reacton), then t can be wrtten that [Fe ] = [Fe ] [FeSCN ] (6) Substtutng the relatonshp from uaton 5 for [FeSCN ], gves [Fe ] = [Fe ] (7) 2
Smlar statements may be made about the thocyanate on. If [SCN ] represents the thocyanate ntally added to the system, then t can be concluded - [SCN ] = [SCN ] Substtutng uatons 5, 7, and 8 nto uaton 4 gves (8) K = [Fe ] [SCN ] (9) In ths experment, b s 1.00 cm and of FeSCN at 447 nm s 5203 L/mole cm. By measurng the absorbance of mxtures of varous [Fe ] and [SCN ], K can be calculated. NOTE: Be very careful not to spll or dscard any soluton untl the experment s completely fnshed. ll soluton transfers must be done wthout losng any soluton. Part I Measurng K 1. Start LoggerPro and plug n the SpectroVs unt nto USB port on left. Wat for the netbook to recognze the nterface. 2. Fll a cuvette ¾ full wth 0.5 M HNO3 to make a blank. Dry the cuvette wth a Kmwpe. On the menu bar, select Experment, Calbrate, Spectrometer:1. Wat for lamp to warm up. Place the blank n the SpectroVs. Lne up the cuvette wth the reference lne. Clck Fnsh Calbraton. When t goes gray, clck OK. 3. Ppet 10.00 ml of 2.00 x 10 3 M KSCN soluton (record the exact concentraton) nto a clean 250-mL beaker. Ppet 25.00 ml of 2.0 M HNO3 nto the same beaker. Ppet 65.00 ml of dstlled water nto the beaker (2 x 25 + 15). 4. Half-fll a clean, dry 50-mL beaker wth 0.100 M Fe(NO3)3 concentraton). (record the exact 5. Usng a 1.00-mL ppet, add 1.00 ml of Fe(NO3)3 soluton to the KSCN soluton n the 250-mL beaker. Mx gently wth a dsposable dropper. Keep the dropper n the beaker for the remander of the experment. Fll a cuvette ¾ full wth ths soluton. Place the cuvette n the spectrometer. 6. Clck to Confgure Spectrometer. Under Collecton Mode, select bsorbance vs Concentraton. Check 447.0 nm as λmax. Clck OK. 3
7. Carefully fll a clean, dry cuvette about ¾ full wth the reddsh soluton usng the dsposable dropper. Keep the dropper n the beaker. Place the cuvette n the SpectroVs. Lne up the cuvette wth the reference lne. 8. Clck,, wat 30 second and then clck. Enter 1 (the experment #) n the dalog box askng you to enter the concentraton, then clck OK. Make a table n your notebook of Experment # and bsorbance. Record the absorbance of experment #1. 9. Wthout spllng, use the dsposable dropper to return the contents of the cuvette back nto the beaker. Do not rnse. 10. Repeat step 5. Draw soluton nto and out of the dsposable ppet a few tmes after mxng the soluton. Rnse the cuvette wth some of the soluton and return t to the beaker. Do not dscard or spll soluton. Mx, then fll the cuvette about ½ full wth the soluton usng the dsposable dropper. Put the cuvette n the SpectroVs. Lne up the cuvette wth the reference lne. Contnue to steps 8 and 9. For concentraton, enter 2 (as the experment number). The absorbance should ncrease. 11. Repeat step 10 eght more tmes (for a total of 10 data ponts). Enter 3, 4, 5, etc. as the concentraton each tme. The absorbance should ncrease each tme, but not lnearly. If the absorbance value does not ncrease, notfy the nstructor. You wll have 10 sets of data. Part II 1. To test the effect of temperature on the value of the ulbrum constant, hold the cuvette contanng soluton 10 n the beaker of hot water on the hot plate located n a specfc area of the laboratory. Keep the cuvette n the hot water for 1 mnute. Wpe off the cuvette and record the absorbance of the soluton ( ). Part III Determnng Intal Condtons from Known K and Equlbrum Concentratons 1. Ppet 10.00 ml of 2.00 x 10 3 M KSCN soluton nto a clean 250-mL beaker. 2. Ppet 25.00 ml of 2.0 M HNO3 nto ths same beaker. Ppet 10.00 ml of the unknown Fe(NO3)3 soluton. Ppet 55.00 ml of dstlled water: 2 x 25 + 5. 3. Fll a clean, dry cuvette ¾ full of the soluton and record the absorbance ( ). 4. Clck. Export all the data to your flashdrve as a.csv fle. 4
Data Treatment Usng Excel, set up 10 clearly labeled columns: Column Experment # Column B bsorbance of Exp # Column C [Fe ] (=*0.100/(100+)) Column D [SCN ] (=0.0200/(100+)) Column E [FeSCN ] (= / = B/5203) Column F [Fe ] (=C E) Column G [SCN ] (=D E) Column H K = [Fe ] [SCN ] (=E/(F*G)) Column I x bsorbance Column J y [Fe ] [SCN ] bsorbance([fe ] + [SCN ] ) [Fe ] [SCN ] (=(B(C+D))/(C*D)) (=B/(C*D)) Gve sample calculatons of columns C-J (usng the exact molartes of the ron(iii) and SCN solutons). Calculate the average of the 10 K values at the bottom of column H. Include the calculaton of K for the hot soluton (soluton 11) on the spreadsheet below the average K. Plot column J versus column I as an alternate way of determnng K that does not rely on knowng the molar absorptvty. lnear trendlne (not through 0,0) ft to the 10 data ponts has a slope of K. Compare ths K to the average K (bottom of column H). For Part II, use the average value of K (bottom of column H), the ntal concentraton of 100 ml soluton of SCN and the concentraton of FeSCN (from Beer s law and the measured absorbance) to determne the orgnal concentraton of Fe n the unknown soluton. Concluson Gve the room temperature ulbrum constants (determned by the two methods), the hgher temperature ulbrum constant and the concentraton of the unknown Fe soluton. lso: What does the magntude of the average K (bottom of column H) suggest (n general terms) about the ulbrum poston of the reacton (more toward reactants or toward products). Compare the K value from soluton 10 and the K of the warm soluton. Usng LeChateler's Prncple, determne whether the reacton of Fe wth SCN to form FeSCN s exothermc or endothermc. Explan. 5