Usng Spectrophotometrc Methods to Determne an Equlbrum Constant Prelab 1. What s the purpose of ths experment? 2. Wll the absorbance of the ulbrum mxture (at 447 nm) ncrease or decrease as Fe soluton s added? Explan usng LeChateler's prncple on uaton 3. 3. In steps 1 and 2 of the procedure, 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. Consder dong ths n a spreadsheet (you wll use these values n Data Treatment and Dscusson), but nclude one sample calculaton n the notebook. 19
Introducton Usng Spectrophotometrc Methods to Determne an Equlbrum Constant Not all chemcal reactons go to completon. Usually chemcal systems approach what s called 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, the ulbrum expresson s wrtten as a + bb cc + dd (1) 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 study the ulbrum of the reacton Fe (aq) + SCN (aq) FeSCN (aq) (3) The ron(iii) on reacts wth the thocyanate on to produce the complex on thocyanatoron(iii) whch s blood-red. Other complexes between ron(iii) and the thocyanate on exst but the complex shown above wll predomnate snce the ron(iii) concentraton wll be hgh compared to the concentraton of the thocyanate on. The ulbrum expresson s - K = [SCN ] (4) In order to determne the ulbrum constant 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) complex on, where ε s known. = (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 ron ntally (before reacton), then t can be wrtten that [Fe ] = [Fe ] [FeSCN ] (6) Substtutng the relatonshp from uaton 5 for [FeSCN ], gves 20
= (7) 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 = [SCN ] (9) In ths experment, the pathlength of the cuvette 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 ], you wll be able to calculate K for each experment and an average K. NOTE: You must be very careful not to spll or dscard any of your solutons untl the experment s completely fnshed. ll soluton transfers must be done wthout losng any soluton. Part I Measurng K Procedure 1. Ppet 10.00 ml of 2.00 x 10 3 M KSCN soluton (record the exact concentraton) nto a clean 100.0 ml volumetrc flask. Ppet 25.00 ml of 2.0 M HNO 3 nto the same volumetrc flask. dd dstlled water to the mark on the flask. Mx the contents thoroughly and transfer all of ths soluton to a clean, dry 250-mL beaker. 2. Obtan about 25 ml of 0.100 M Fe(NO 3 ) 3 n a clean, dry 50-mL beaker (record the exact concentraton). Usng a 1.00-mL ppet, add 1.00 ml of the Fe(NO 3 ) 3 soluton to the 250-mL beaker contanng the KSCN soluton. Mx thoroughly by swrlng the beaker. Carefully pour some of ths soluton nto the 100-mL volumetrc flask used n step 1, swrl, and carefully (wthout spllng) return the soluton to the 250-mL beaker. 3. Set the Spectronc 20 to 447 nm. Set to 100% T (calbrate) wth 0.5 M HNO 3 soluton as a blank. Usng a dsposable ppet, fll a cuvette wth the soluton from step 2 and measure the absorbance. 21
4. Wthout spllng, pour the contents of the cuvette back nto the beaker. Use the same dsposable ppet (above) to remove any drops of soluton. Do not rnse. 5. Repeat steps 2 through 4 nne more tmes (for a total of 10 data ponts). Draw soluton nto and out of the dsposable ppet a few tmes after swrlng the beaker n step 2. Rnse the cuvette wth some of the soluton and return t to the beaker. Do not dscard. Swrl agan and contnue to step 3. The absorbance should ncrease each tme, but not lnearly. 6. To test the effect of temperature on the value of the ulbrum constant, place the cuvette contanng soluton 10 n a 50-mL beaker of hot tapwater. Let t stand for 2 mnutes. Wpe off the cuvette and record the absorbance of the soluton. Part II Determnng Intal Condtons from Known K and Equlbrum Concentratons Procedure 1. Ppet 10.00 ml of 2.00 x 10 3 M KSCN soluton nto a clean 100.0 ml volumetrc flask. 2. Ppet 25.00 ml of 2.0 M HNO 3 nto ths same volumetrc flask. Usng a 10.00-mL ppet, add 10.00 ml of the unknown Fe(NO 3 ) 3 soluton. dd dstlled water to the mark on the flask. 3. Measure the absorbance at 447 nm after calbraton. Data Treatment Gve a sample calculaton of [Fe ] (usng the exact molarty of ron(iii)) after addton to the SCN soluton, [SCN ] (usng the exact molarty of the SCN ) after the addton of the ron(iii) soluton, [FeSCN ] from uaton 5, where b = 1.00 cm and = 5203 L/mole cm, [Fe ] from uaton 6, [SCN ] from uaton 7 and K, from uaton 8, of soluton 1. Usng a spreadsheet, set up 7 clearly labeled columns wth the followng. Column 1 bsorbance,. Column 2 [Fe ] Column 3 [SCN ] Column 4 [FeSCN ] (= / = /(5203 L/mole cm)(1 cm)) Column 5 [Fe ] Column 6 [SCN ] Column 7 K = [Fe ] [SCN ] Calculate, at the bottom of column 7, the average of the 10 K values. Include the calculaton of K for the warm soluton on the spreadsheet below the average K. 22
Usng the average value of K, the ntal concentraton of SCN and the concentraton of FeSCN (from Beer s law), determne the orgnal concentraton of Fe n the unknown. n alternate way of determnng K that does not rely on knowng the molar absorptvty nvolves plottng [SCN ] (as y) versus ( + [SCN ] ) [SCN ] (as x) lnear trendlne (that does not go through 0,0) ft to the 10 data ponts has a slope of K. Compare ths K to the average K of your prevous calculatons. 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 address: What does the magntude of the average K suggest (n general terms) about the ulbrum poston of the reacton (more toward reactants or toward products). Usng LeChateler's Prncple, the average K, and the K of the warm soluton, s the reacton of Fe wth SCN to form FeSCN exothermc or endothermc. Explan. 23