Equilibrium Constant I.Today’s Experiment: Fe 3+ (aq) + HSCN(aq) FeSCN 2+ (aq) + H + (aq) orange colorless dark red colorless 1.Determine [FeSCN 2+ ] using Spec20 and Beer’s Law 2.Determine the other concentrations from an ICE Table 3.Calculate K at three different Temperatures 4.Use the Temperature data to determine  H,  S, and  G for the reaction II.Take a look at the Pre-Lab Problems III.Beer’s Law and Making a Calibration Curve 1.Colored compounds absorb light that is shined through them 2.A =  l CAbsorbance = (Extinction Coefficient)(length)(Concentration)

3. We will use test tubes #1-5 to make a calibration curve using Beer’s Law 4. Excess Fe 3+ (0.200 M) pushes the reaction to the right: [HSCN] o = [FeSCN 2+ ] 5. We must use Fe 3+ solution as a blank to cancel out Fe 3+ absorbance 6. Record %T and calculate A for the five different [HSCN] concentrations 7. Plot A vs. [FeSCN 2+ ] to give a straight line. Calibration Curve 8. Once you find the Absorbance of any other [FeSCN 2+ ] solution, you can find its concentration from the calibration curve. A [FeSCN 2+ ] Slope =  l

IV.Procedure for finding K (Tubes #6-9) 1.Make 4 different solutions of HSCN, Fe 3+ 2.Find A with the Spec20 3.Find [FeSCN 2+ ] from the Calibration Curve 4.Use an ICE Table to find all the other concentrations 5.All solutions are made with M HNO 3, so [H + ] = Other initial concentrations are found using the Dilution Equation Solution 6 Room Temperature Species Fe 3+ HSCNFeSCN 2+ H+H+ Initial Molar Concentration Change in Molar Concentration -x +x Equilibrium Molar Concentration x xx x Calculated Values0.500

V. Procedure for finding  H,  S, and  G (Tubes #6-9 at different T’s) 1.You will use the same tubes (#6-9) at an ice bath (around 5 o C) 2.Hot tap water bath temperature (around 45 o C) [Not too hot! Boils off HSCN!) 3.You already have the room temperature data from these tubes (around 25 o C) 4.Plot lnK vs. 1/T for your three different temperatures 5.Use the following equations to calculate  H,  S, and  G for the reaction 6.Notes: a.Use parafilm to cover the test tubes as you mix the solutions thoroughly b.Spec20: 0% with nothing in it; 100% with Iron Solution only as Blank c.Fill cuvet with most dilute first, rinse with next most dilute, and so on 1/T lnK Slope = -  H/R Intercept =  S/R