Chemistry 112: Determination of an Equilibrium Constant INTRODUCTION When two or more reactants are mixed together and p

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Chemistry 112 Determination Of An Equilibrium Constant Introduction When Two Or More Reactants Are Mixed Together And P 1 (78.96 KiB) Viewed 40 times

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Chemistry 112: Determination of an Equilibrium Constant INTRODUCTION When two or more reactants are mixed together and products are formed, the reaction rarely goes to completion. Usually some of the reactants form products and some remain unchanged. When the concentration of both products and reactants remain unchanged with time, the system is in chemical equilibrium The equilibrium state, at a given temperature, can be quantitatively described by a constant, Ke, which gives the relationship between the molar concentrations of all species present. For the reaction Eq. 1 KE aA+bB = C +dD ICHID [AL'" where IAL (B). (C), and (D) represent the molar equilibrium concentrations of the species. This expression describes the necessary conditions for an equilibrium situation. If any amounts of A, B, C and Dare med and the above expression for K. is not satisfied (that is, the concentration quotient term does not equal the equilibrium constant) the system is not at equilibrium. A reaction will occur and the concentration of each species will change until the equilibrium concentration of each species is formed. In Chemistry 109, we studied the reaction of Fe with SCN The balanced chemical equation for the reaction was: Eq2 FeSCN = FeSCN + In this experiment the equilibrium constant for this reaction is measured by mixing various concentrations of Fe and SON allowing the solutions to come to equilibrium and determining the molar concentration of each species at equilibrium. This sequence of studies, that is, (1) the determination of the stoichiometry of a reaction and (2) the determination of the equilibrium constant for the reaction is a typical procedure to be followed in studying a new reaction. From the stoichiometry of the reaction, the equilibrium constant expression is E9.3 K- FeSCN1 [Fe "HSCN1 To determine K, the molar equilibrium concentration of each species in the expression must be known. Since Fe" and SON are not colored species and FeSONis a colored species, the molar equilibrium concentration of Fe(SCN) - can be determined spectrophotometrically. If known amounts of Fe and SCN are mixed together, the equilibrium concentrations of these species may be determined from the amount of Fe(CN) and the balanced chemical equation (2) Consider the following example.Letsay that somt of 160 x 10'M Feand SoomL of 6.00 x 10M SON are mixed. The concentration of the Fe(SCNformed is determined by measuring the absorbance of the solution Using Beer's Law. (A c), and the known value of el the concentration of Fe(CN)** is found to be 2.00 x 10 M. Then, since one liter of TOTAL solution has been formed, and moles - MxV, there are 2.00 x 10 moles of Fe(CN}present at equilibrium Before the reaction there were (1.6 x 10 M (0.500L) 80.0 x 10moles of Feand (6.00x 10M (500L) = 3.00 x 10 moles of SCN Therefore, from the balanced chemical equation, 200 x 10* moles of Fe must have reacted with 200 x 10 moles of SCN to form 200 x 10 moles of Fe(CN)* This leaves 78.0 x 10* moles of Fe" and 1.00 x 10 moles of SCN at equilibrium These results are summarized in the following table:
Initial Change during reaction Equilibrium Sample Calculation for the Determination of K moles Felt moles SCN 80.0 x 10+ 3.00 x 10- -2.00 x 104 - 2.00 x 10 78.0 x 10" 1.00 x 104 moles Fe(SCN)2 0 +2.00 x 104 2.00 x 104 The number of moles at equilibrium must be converted back to molar concentrations before substituting into the equilibrium expression and solving for K. This is trivial in this example, since we have 1 L of solution. Then we selve for Ke K = [Fe(SCN)?1 12,0 x 104) = 256 [Fe][SCN1 (78.0 x 10^)(1.00 x 104 Remember, when using the equilibrium expression, molar concentrations must be used; when considering the amount of reactants used according to the stoichiometry of the reaction, moles of substance must be considered PROCEDURE 1. Obtain a 5ml pipet, a suction bulb, and two burets from the stockroom. 2. Pipet SmL of 2.00 x 10' M Fe(NO) in to each of five large test tubes labelled 1 - 5. 3. Fill one buret with about 20mL of 2.00 x 10 M KSCN. Fill the second buret with about 30mL of 0.500M HCIO 4. In test tube 1. place 1.00mL of 200 X 10 M KSCN and 4.00 ml of O.SOOM HOUSE the readings on the buret to measure these volumes. Mix thoroughly. The acid prevents the Fes from forming the red precipitate Fe(OH) 5. In test tube 2. place 2.00 mL of 2.00x 10 M KSCN and 3.00 ml. of O.SOOM HCIO. Wash and dry the stirring rod well and mix thoroughly 6. In test tube 3, place 3.00 mL of 2.00 x 10 M KSCN and 2.00 mL of O.SOOM HClO.. Wash and dry the stirring rod well and mix thoroughly 7. In test tube 4, place 4.00 ml. of 2.00x 10 M KSCN and 1.00ml of O.SOOM HCIO Wash and dry the stirring rod well and mix thoroughly 8. In test tube 5. place 5.00ml of 2.00 x 10 M KSCN. 9. In a sixth test tube, prepare a standard by mixing SmL of 0.2M Fe(NO), and 1.00ml of 2.00 x 10 M KSCN. Dilute to 20ml with O.SOOM HCIO.. Mix thoroughly. Since the concentration of Fe" is very high compared to the concentration of SCN assume, according to Le Chatelier's principle, that the reaction is shifted far to the right and that all of the SCN is converted to Fe(SCN) This means the concentration of Fe(SCN) - is 1.00 x 10-'M. 10. Set the wavelength on the spectrometer to 447 nm. Standardize the instrument at zero absorbance using distilled water as the blank solution. Check the standardization of the instrument before each measurement. 11. Measure and record the absorbance of the standard solution using the spectrometer. Calculate el for Fe(SCN) 12. Measure and record the absorbance of each of the solutions prepared in steps 4-8 13. Calculate the equilibrium constant for the reaction to the correct number of significant figures.
DATA: DETERMINATION OF AN EQUILIBRIUM CONSTANT Name Data Test Tube 1 2 3 Absorbance [FeSCN2") at equilibrium 0.067 0.157 0.247 0.361 0.479 Test Tube (standard, 1 x 10-M). Absorbance 0.290 4 5 el Results Initial Moles Fe SCN- Moles at Equilibrium Fe(SCN) SCN 1 2 elm 3 5 Equilibrium Molar Concentrations Fe SCN FeSCN2 K K 2 3 4 5 Average value of K →
Show calculations demonstrating how you determined: (5) Initial number of moles Fe (2) Equilibrium concentration (MOLAR or mol/L) of FeSCN2. (3) Moles of Fe and Fe(SCN)** at equilibrium (4) Equilibrium concentration (MOLAR or mol/L) of Fe” (5) The equilibrium constant