Chemistry 112 Determination Of An Equilibrium Constant Introduction When Two Or More Reactants Are Mixed Together And P 1 (75.5 KiB) Viewed 12 times
Chemistry 112 Determination Of An Equilibrium Constant Introduction When Two Or More Reactants Are Mixed Together And P 2 (61.17 KiB) Viewed 12 times
Chemistry 112 Determination Of An Equilibrium Constant Introduction When Two Or More Reactants Are Mixed Together And P 3 (14.02 KiB) Viewed 12 times
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 aA + bB cC + dD, [C][D] [A] where [A], , [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 D are mixed and the above expression for Ke, 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: Fe³+ + SCN FeSCN²+ K₂ = Kc = Eq. 1 Eq.2 In this experiment the equilibrium constant for this reaction is measured by mixing various concentrations of Fe" and SCN", 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 [FeSCN²] [Fe][SCN] Eq. 3 To determine K, the molar equilibrium concentration of each species in the expression must be known. Since Fe** and SCN are not colored species and Fe(SCN) is a colored species, the molar equilibrium concentration of Fe(SCN)²* 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(SCN) and the balanced chemical equation (Eq. 2)
PROCEDURE 1. Obtain a 5mL pipet, a suction bulb, and two burets from the stockroom. 2. Pipet 5mL of 2.00 x 10³ M Fe(NO3)3 in to each of five large test tubes labelled 1-5. 3. Fill one buret with about 20mL of 2.00 x 10 ³M KSCN. Fill the second buret with about 30mL of 0.500M HCO. 4. In test tube 1, place 1.00mL of 2.00 x 10 ³M KSCN and 4.00 mL of 0.500M HCIO4. USE the readings on the buret to measure these volumes. Mix thoroughly. The acid prevents the Fe³ from forming the red precipitate Fe(OH)3. 5. In test tube 2, place 2.00 mL of 2.00 x 10 ³M KSCN and 3.00 mL of 0.500M HCIOA. Wash and dry the stirring rod well and mix thoroughly. 6. In test tube 3, place 3.00 mL of 2.00 x 10 ³M KSCN and 2.00 mL of 0.500M HCIO. Wash and dry the stirring rod well and mix thoroughly. 7. In test tube 4, place 4.00 mL of 2.00 x 10 M KSCN and 1.00mL of 0.500M HCIO. Wash and dry the stirring rod well and mix thoroughly. 8. In test tube 5, place 5.00mL of 2.00 x 10 ³M KSCN. 9. In a sixth test tube, prepare a standard by mixing 5mL of 0.2M Fe(NO)); and 1.00mL of 2.00 x 10 ³M KSCN. Dilute to 20mL with 0.500M HCIO. Mix thoroughly. Since the concentration of Fe" is very high compared to the concentration of SCN", assume, according to Le Châtelier'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)2 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)²+ 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 Test Tube 1 2 3 4 5 Absorbance 0.386 1.727 2.465 2.472 2.931 [FeSCN2+] at equilibrium Test Tube 6(standard, 1 x 10M). Absorbance 3.094 32.
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