Investigation 6: Temperature Dependence of the Equilibrium Constant ...
2014-15 Chemistry 122 or Chem112/Temperature Dependence/Procedure
Investigation 6: Temperature Dependence of the Equilibrium Constant for a Nickel Complex Question:
How does the value of the equilibrium constant, K, depend on temperature?
Pre-lab required reading Atkins & Jones (6th ed.): Sections 11.8 – 11.11 Primers:
Spectrophotometry SpectraVis Plus spectrophotometer Volumetric glassware use – General Volumetric glassware use – volumetric flask Volumetric glassware use – volumetric pipet Coordination chemistry Safety and Waste Disposal
Eye protection and gloves should be worn at all times. Nickel salts and their solutions will irritate the eyes upon contact. Be careful handling triethylenetetramine solutions. Triethylenetetramine is corrosive to the skin, eyes, and respiratory system. Use caution with the sodium perchlorate solution, it is a strong oxidizer. Do not mix it with organic materials or acid. Dispose of all solutions in the appropriate waste container.
Background In this lab, observation will be made on the reaction of aqueous nickel (II) coordination complexes. The color of the solution is caused by the complexation of the Ni2+ ion in solution and depends on how many water molecules are coordinated to the metal. The nickel (II) ion is surrounded by water in the aqueous NiCl2 solution, Ni(H2O)62+. When a solution of triethylenetetramine is added, the water molecules are replaced as shown in the following schematic:
One can monitor the amount of blue or yellow in the equilibrium mixture to determine the concentration of either the octahedral or square planar complex ion in solution, respectively. Absorbance spectra (plots of absorbance versus wavelength) for the yellow and blue complexes are shown in Figure 1. The square planar yellow nickel complex has an absorbance maximum near 450 nm whereas the octahedral blue nickel complex has absorbance maxima at 330, 540, and 910 nm. In this experiment, we will monitor absorbance of the yellow complex at 450 nm.
2014-15 Chemistry 122 or Chem112/Temperature Dependence/Procedure
Ni-triethylenetetramine complex absorption spectra
Absorbance
1 0.8 Mostly blue complex
0.6 0.4
Mostly yellow complex
0.2 0 200
400
600
800
1000
Wavelength (nm) Figure 1: Absorbance spectra for the two forms of the nickel triethylenetetramine complex. The equilibrium expression, Kc, for this equilibrium (Eq. 1) is: [
] [
(1)
]
The value of Kc should be independent of all factors except a change in temperature. At high ionic strength, the concentration of the blue complex is very small and the concentration of the yellow complex is large. We will use this relationship to make yellow standard solutions by dissolving the complex in a 3.0M NaClO 4 solution (has a high ionic strength). In the equilibrium part of the experiment, you will be working with a solution that is green in color, so that neither [blue] nor [yellow] is much larger than the other. One can use the Beer’s law expression (A =∙b∙c) to determine the relationship between the concentration of the yellow nickel complex and absorbance: [
For the yellow complex:
]
(2)
If we make a calibration curve or Beer’s law plot at the wavelength of interest using solutions of known concentration, we can then determine the concentrations of both species at equilibrium. The equilibrium constant and the absolute temperature (T) at which the equilibrium constant was determined can be used to determine the G of the reaction using equation 1, G
=
=
H
RTlnK
(3)
where R is the gas constant. Since G
-
TS
(4)
where H is the enthalpy change and S is the entropy change for the reaction, the two expressions for G found in equations 3 and 4 can be set equal (equation 5). RTlnK = H TS (5) Rearranging equation 5 gives (6) In this experiment, values of K will be determined experimentally for various temperatures (T). By plotting lnK versus 1/T, H and S for the reaction can be determined (equation 6).
2014-15 Chemistry 122 or Chem112/Temperature Dependence/Procedure
Procedure Part 1: Making Beer’s Law Plots 1. Obtain enough stock solution (0.1M) of nickel triethylenetetramine complex to make one set of solutions with concentrations between 0.01 M and 0.1 M nickel triethylenetetramine complex. Make a set of four solutions in 10 mL volumetric flasks in the correct concentration range using 3.0 M NaClO4 to dilute to the mark. 2. Measure the absorbance of each solution at 450 nm and create a Beer’s Law plot (absorbance vs. concentration). Fit the data with a linear least squares fit, if any data does not fit the trend re-measure absorbance and remake solutions as needed. The slopes of the best-fit linear equations will be used to obtain a value for the concentration of the yellow form of the nickel complex in the equilibrium solutions below. Part 2: Equilibrium measurements 1. Obtain around 3 mL of 0.1 M NiCl2. Note the color, remove some solution, transfer to a cuvette and obtain an absorbance spectrum. Record the max. 2. Combine 5.0 mL of 0.1 M NiCl2 and 5.0 mL of 0.1 M triethylenetetramine in a small testtube. Note the color, remove some solution, transfer to a cuvette and obtain a spectrum. Record the max. 3. Place the test tube in a hot water bath and gently heat the mixture to 5060C. Note any color changes as the solution warms. Remove some solution, transfer to a cuvette and obtain a spectrum. Record the max. 4. Set up the spectrometer to collect the absorbance as a function of time and temperature. Remove some solution from the test tube, transfer to a cuvette and collect the absorbance as a function of temperature as the solution cools to room temperature. Be sure to export the data as a cvs file. 5. Use the absorbance and the Beer’s law plot generated above to determine the equilibrium concentration of the yellow nickel complex at each temperature. 6. Determine the value of K at each temperature and prepare a plot of lnK vs 1/T.
References Atkins, P.; Jones, L. “Chemical Principles: The Quest for Insight”, 6th ed.; Freeman: New York. 2013. Ben-Dor, L., Marcus, Y. J. Chem. Ed., 75, 1998, 1458-1459.
Investigation 6: Temperature Dependence of the Equilibrium Constant for a Nickel Complex Question:
How does the value of the equilibrium constant, K, depend on temperature?
Pre-lab required reading Atkins & Jones (6th ed.): Sections 11.8 – 11.11 Primers:
Spectrophotometry SpectraVis Plus spectrophotometer Volumetric glassware use – General Volumetric glassware use – volumetric flask Volumetric glassware use – volumetric pipet Coordination chemistry Safety and Waste Disposal
Eye protection and gloves should be worn at all times. Nickel salts and their solutions will irritate the eyes upon contact. Be careful handling triethylenetetramine solutions. Triethylenetetramine is corrosive to the skin, eyes, and respiratory system. Use caution with the sodium perchlorate solution, it is a strong oxidizer. Do not mix it with organic materials or acid. Dispose of all solutions in the appropriate waste container.
Background In this lab, observation will be made on the reaction of aqueous nickel (II) coordination complexes. The color of the solution is caused by the complexation of the Ni2+ ion in solution and depends on how many water molecules are coordinated to the metal. The nickel (II) ion is surrounded by water in the aqueous NiCl2 solution, Ni(H2O)62+. When a solution of triethylenetetramine is added, the water molecules are replaced as shown in the following schematic:
One can monitor the amount of blue or yellow in the equilibrium mixture to determine the concentration of either the octahedral or square planar complex ion in solution, respectively. Absorbance spectra (plots of absorbance versus wavelength) for the yellow and blue complexes are shown in Figure 1. The square planar yellow nickel complex has an absorbance maximum near 450 nm whereas the octahedral blue nickel complex has absorbance maxima at 330, 540, and 910 nm. In this experiment, we will monitor absorbance of the yellow complex at 450 nm.
2014-15 Chemistry 122 or Chem112/Temperature Dependence/Procedure
Ni-triethylenetetramine complex absorption spectra
Absorbance
1 0.8 Mostly blue complex
0.6 0.4
Mostly yellow complex
0.2 0 200
400
600
800
1000
Wavelength (nm) Figure 1: Absorbance spectra for the two forms of the nickel triethylenetetramine complex. The equilibrium expression, Kc, for this equilibrium (Eq. 1) is: [
] [
(1)
]
The value of Kc should be independent of all factors except a change in temperature. At high ionic strength, the concentration of the blue complex is very small and the concentration of the yellow complex is large. We will use this relationship to make yellow standard solutions by dissolving the complex in a 3.0M NaClO 4 solution (has a high ionic strength). In the equilibrium part of the experiment, you will be working with a solution that is green in color, so that neither [blue] nor [yellow] is much larger than the other. One can use the Beer’s law expression (A =∙b∙c) to determine the relationship between the concentration of the yellow nickel complex and absorbance: [
For the yellow complex:
]
(2)
If we make a calibration curve or Beer’s law plot at the wavelength of interest using solutions of known concentration, we can then determine the concentrations of both species at equilibrium. The equilibrium constant and the absolute temperature (T) at which the equilibrium constant was determined can be used to determine the G of the reaction using equation 1, G
=
=
H
RTlnK
(3)
where R is the gas constant. Since G
-
TS
(4)
where H is the enthalpy change and S is the entropy change for the reaction, the two expressions for G found in equations 3 and 4 can be set equal (equation 5). RTlnK = H TS (5) Rearranging equation 5 gives (6) In this experiment, values of K will be determined experimentally for various temperatures (T). By plotting lnK versus 1/T, H and S for the reaction can be determined (equation 6).
2014-15 Chemistry 122 or Chem112/Temperature Dependence/Procedure
Procedure Part 1: Making Beer’s Law Plots 1. Obtain enough stock solution (0.1M) of nickel triethylenetetramine complex to make one set of solutions with concentrations between 0.01 M and 0.1 M nickel triethylenetetramine complex. Make a set of four solutions in 10 mL volumetric flasks in the correct concentration range using 3.0 M NaClO4 to dilute to the mark. 2. Measure the absorbance of each solution at 450 nm and create a Beer’s Law plot (absorbance vs. concentration). Fit the data with a linear least squares fit, if any data does not fit the trend re-measure absorbance and remake solutions as needed. The slopes of the best-fit linear equations will be used to obtain a value for the concentration of the yellow form of the nickel complex in the equilibrium solutions below. Part 2: Equilibrium measurements 1. Obtain around 3 mL of 0.1 M NiCl2. Note the color, remove some solution, transfer to a cuvette and obtain an absorbance spectrum. Record the max. 2. Combine 5.0 mL of 0.1 M NiCl2 and 5.0 mL of 0.1 M triethylenetetramine in a small testtube. Note the color, remove some solution, transfer to a cuvette and obtain a spectrum. Record the max. 3. Place the test tube in a hot water bath and gently heat the mixture to 5060C. Note any color changes as the solution warms. Remove some solution, transfer to a cuvette and obtain a spectrum. Record the max. 4. Set up the spectrometer to collect the absorbance as a function of time and temperature. Remove some solution from the test tube, transfer to a cuvette and collect the absorbance as a function of temperature as the solution cools to room temperature. Be sure to export the data as a cvs file. 5. Use the absorbance and the Beer’s law plot generated above to determine the equilibrium concentration of the yellow nickel complex at each temperature. 6. Determine the value of K at each temperature and prepare a plot of lnK vs 1/T.
References Atkins, P.; Jones, L. “Chemical Principles: The Quest for Insight”, 6th ed.; Freeman: New York. 2013. Ben-Dor, L., Marcus, Y. J. Chem. Ed., 75, 1998, 1458-1459.
