Chemical Equilibrium & Finding the Equilibrium Constant Kc Data & Specific Directions Test Tube # Abs470nm #1 0.101 Keq = [Fe(SCN)2+Jeg / ([Fe3+]eq x [SCN-Jeg) #2 0.119 #3 0.151 #4 0.260 The first table on page 141 in the lab manual is the experimental data set. The volume of iron(III) chloride solution is 5.0 mL in all test tubes. If you look at the prep scheme, you will see that for Test Tube #1, to the 5.0 mL of iron(III) chloride you add 2.0 mL of the potassium thiocyanate and 3 mL of the solvent water. This totals 10.0 mL and in fact that is the total volume for all four test tubes. (As the KSCN solution volume increases by 1.0 ml, the volume of added water decreases by 1.0 mL to maintain the total for each tube at 10.0 ml.) The entries for the tubes for the first column is 5.0 mL in each case. For the second column entry, the first volume is 2.0 mL and increases by 1.0 mL each tube. The volume of added water equals the amount to keep the total of all three additions in each tube to equal 10.0 mL. Each initial test tube concentration then must be calculated using the dilution equation. (Remember: Mstock X Vstock = MNew X VTotal) Mstock FeCl3 = 0.00185 M Mstock KSCN = 0.0020 M Using these equations get you the entries into the columns four and five for the initial concentration of iron (111) ion and the initial concentration of thiocyanate ion, respectively. The sixth column is the data above and as stated in the manual, the last column is the absorbance value for the blood red compound product divided by the value 2000. (Remember our Beer’s Law work that said the absorbance of any color compound is abs = constant x Concentration. For this compound, the constant equals 2000 so the absorbance divided by 2000 equals the concentration of the color compound; in this case it is our product of interest.) The second table is an example ICE scheme for Test Tube #1. Your initial concentrations for Test Tube #1 go in the first line spaces, your x values into the second line with the appropriate plus or minus signs, the third line is the addition of each column as usual for an ICE scheme. In none of the test tubes is there an initial concentration of the color product. Once we have values for the last column of Table 1, each entry must be the product concentration at equilibrium, x. The third table is the bottom equilibrium line from an ICE scheme for each test tube. Follow the way you did the second table for Test Tube #1 and take the other three bottom lines and write them down in Table 3. Lastly, take those equilibrium concentration values and plug them into the equilibrium expression given above so you can calculate Keg for each tube. Average the four K values to obtain an average. As stated in the manual, historically the accepted value is 125. After completing page 141, do questions 1 and 2 from page 142. Chemical Equilibrium & Finding the Constant (Kc) of Thiocyanatoiron(III) Name Partner Name(s) Test Tube Vol FeCl3 Vol KSCN (mL) (ml) Vol H2O (mL) [Fe3+] (M) (SCN) (M) A470 nm (FeSCN2+leg (M) 1 2 3 4 Using your row 1, complete this ICE table for the reaction Fe3+ (aq) + SCN- (aq) → [FeSCN2+] (aq) [Fe3+] (M) [SCN-] (M) (FeSCN2+](M) Initial 0 Change Equilibrium In most class examples, your end goal is to solve for x to find the equilibrium concentrations. Here, you have a direct measurement of [FeSCN2+leq (using Beer’s Law), which tells you the value of x. Using this value of x and the initial iron(III) and thiocyanate ion concentrations, you can determine all the equilibrium concentrations. Your job is to plug these equilibrium concentrations into the equilibrium expression to determine Keq: Test Tube [Fe3+ leq (M) (SCN)eq (M) (FeSCN2+leq (M) Keg 1 2 3 4 Average Keq Questions on Back 141 1. Believe it or not, there are chemists who specialize in measuring equilibrium constants like these. As a result, we know the expected result of this experiment to a fair degree of accuracy. Under the conditions of this experiment, the accepted value of the equilibrium constant is 125 (your answer should be within a factor of 2 of this value). What is the % error of your equilibrium constant? 2. Why did we not measure the absorbance of this compound using red light? 3. As you’ll see in a few chapters, another trick you can do with equilibrium constants is to measure how they change with temperature in order to determine AHrxn Performing this experiment on the iron(III) thiocyanate complex reaction, it was determined that this reaction is slightly exothermic (AHrxn -6700J/mol). According to Le Chatelier’s principle, will the equilibrium constant become larger or smaller if the temperature is increased? Explain. = 142
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