Formal Lab Report Exp. 34 An Equilibrium Constant
When chemical substances react, the reaction typically does not go to completion. Rather, the system goes to some intermediate state in which both the reactants and products have concentrations that do not change with time. Such a system is said to be in chemical equilibrium . When in equilibrium at a particular temperature, a reaction mixture obeys the Law o f Mass Action (or the Law o f Chemical Equilibrium ), which imposes a specific condition on the concentrations of reactants and products that is expressed by the equilibrium constant for the reaction. In this experiment, the reaction between iron(III) ion (Fe3+), and thiocyanate ion (SCN- ), will be studied. When solutions containing Fe3+ and SCN- are mixed, a reaction occurs forming the FeSCN2+ complex ion that has a deep red color. The reaction may be written: Fe3+ (aq) + SCN- (aq) ¨ FeSCN2+ (aq) . (1) The reaction between iron(III) ion and thiocyanate ion does not go to completion. Instead, equilibrium is established at some intermediate state where both the reactants and products have concentrations that do not change with time. As a result of the reaction, the amounts of Fe3+ and SCN- at equilibrium will be less than the amount of each present before the reaction occurred. From the balanced chemical equation, we can see for every mole of FeSCN2+ that is formed, one mole of Fe3+ and one mole of SCN- will be consumed. According to the Law of Mass Action, the equilibrium constant (Kc), for Equation (1) may be formulated as shown in the following expression: Kc = FeSCN2+ Fe3+ SCN- , (2) where the molar concentrations of FeSCN2+, Fe3+, and SCN- are measured at equilibrium . The value of Kc in Equation (2) is constant at a given temperature. As a result, mixtures containing Fe3+ and SCN- will react until Equation (2) is satisfied. Additionally, the same value of Kc will be obtained no matter what initial amounts of Fe3+ and SCN- were used. The purpose of this experiment is to measure Kc for several mixtures of Fe3+ and SCN- made up in different ways. The mixtures will be prepared by mixing solutions containing known concentrations of iron(III) nitrate [Fe(NO3)3], and potassium thiocyanate (KSCN). The reaction between iron(III) ion and thiocyanate ion is a particularly good one to study because the value of Kc is of a convenient magnitude and the intense color of the FeSCN2+ complex ion makes the determination of its equilibrium concentration quite simple. Knowing the initial composition of a mixture and the equilibrium concentration of FeSCN2+, the equilibrium concentrations of the rest of the pertinent species may be calculated and then used to determine Kc.
2. Prepare the blank solution. After the spectrophotometer has been turned on for 10 minutes and the wavelength scale has been set at 447 nm, rinse a cuvet with several portions of the blank solution. Dry the outside of the cuvet with a clean Kimwipe, removing water and fingerprints.2 Handle the lip of the cuvet thereafter. If a cuvet has two clear and two cloudy sides, be sure light passes through the clear sides and handle the cuvet on the cloudy sides.
3. Calibrate the spectrophotometer. Place the cuvet, three-fourths filled with the blank solution, into the sample compartment, align the mark on the cuvet with that on the sample holder, and close the cover. Set the meter on the spectrophotometer to read zero absorbance (or 100%T).3 Remove the cuvet. Consult with your instructor for any further calibration procedures. Once the instrument is set, do not perform any additional adjustments for the remainder of the experiment. If you accidentally do, merely repeat the calibration procedure.
4. Record the absorbance of the standard solutions. Empty the cuvet and rinse it thoroughly with several small portions of Solution…