Aaron Azose Partner: David Creed Chem 317 AB 6/7/2013 Linkage Isomers of Nitro-Pentaammine-Cobalt(III) Introduction Linkage isomerism is a phenomenon in coordination chemistry by which a ligand with two or more possible basic sites can interact with the metal ion through any of its basic atoms. Ligands that show linkage isomerism include NO2-, NCS-, CN-, NCO-, NO-, and CO2-. In the late 1800s, Jorgensen was the first to report the discovery of linkage isomers.1 His study of nitro (Co-NO2) and nitrito (Co-ONO) pentammine cobalt (III) complexes examines the same linkage isomerization that will be discussed in the following report. Further studies of the differences in IR spectra between nitrito and nitro have been performed.2 The following study seeks to identify the mechanism of conversion from [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2, or from the nitrito derivative to the nitro derivative. Previous studies have suggested the isomerization occurs via a first order mechanism.3 Two proposed reaction mechanisms are seen below. In Mechanism 1, a unimolecular conversion between nitrito and nitro proceeds through a transition state in which the NO2 ligand is coordinated to Co through both N and O. In Mechanism 2, one molecule of [Co(NH3)5ONO]Cl2 loses its NO2 group and proceeds to regain a NO2 group (bound through the O) through a transition state that has the NO2 ligand bound to one Co atom through nitrogen and to the other Co atom through oxygen.
2+
2+
‡
2+
Mechanism 1.
2+
2+
2+
‡
2+
2+
2+ 2+
Mechanism 2. To determine the mechanism of the above reaction, first the order of the reaction must be determined. Three possible rate determining steps are labeled within Mechanism 1 and Mechanism 2. Of these, we would expect RDS. 1 and RDS 2 to give reactions that are first order in [Co(NH3)5ONO]Cl2, as they consist of unimolecular steps. RDS 3 would give a reaction that is second order in [Co(NH3)5ONO]Cl2, as it is a bimolecular step requiring the collision of two molecules of [Co(NH3)5ONO]Cl2.
To determine the order of the above reaction, the conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 will be performed at various temperatures. During the course of the reaction, samples of the reaction mixture will be removed to cuvettes and the UV-Vis spectra will be taken. By choosing the wavelength at which the most change is expected on conversion from nitrito to nitro and making plots of the absorbance at that wavelength versus time, the order of the reaction will become apparent (as zero order, first order, and second order reactions each give a linear plot of a different function of absorbance). Additional confirmation of the reaction order will be obtained by performing the above reaction at a single temperature and varying initial concentrations of [Co(NH3)5ONO]Cl2 and examining the half-life of the reaction. A first order reaction gives a half-life of ln(2) / k that is independent of the concentration of [Co(NH3)5ONO]Cl2, whereas a second order reaction gives a half-life that depends on the concentration of [Co(NH3)5ONO]Cl2. If the above plots identify the reaction as first order, an Eyring plot will allow calculation of ∆H‡ and ∆S‡. As RDS 1 describes a step in which a less ordered starting material becomes slightly more ordered, ∆S‡RDS1 will be negative and small in magnitude. As RDS 2 describes a dissociative step, ∆S‡RDS2 will be positive and larger in magnitude. The calculation of ∆H‡ and ∆S‡ will allow us to differentiate between Mechanism 1 and Mechanism 2 from the data.
Experimental Synthesis of [Co(NH3)5Cl]Cl2: Ammonium chloride (10.036 g, 0.188 mol) was dissolved in ammonium hydroxide (60 mL). CoCl2•6H2O (20.015 g, 0.084 mol) was added slowly to the solution, creating a slurry that darkened in color to orange and then brown. Concentrated hydrogen peroxide (16 mL, 30%)
was added dropwise to the solution over 20 minutes, accompanied by evolution of heat and color change to dark purple. Concentrated HCl (60 mL) was added dropwise to the solution over 35 minutes accompanied by heat, evolution of white gas, and color change to red. The solution was heated over 10 minutes to 85ºC and held at 85º for 14 minutes. On heating, the solution turned purple. The solution was removed from the heat and cooled to room temperature. The wet purple solid was washed on a fritted filter twice with 20 mL cold 6M HCl. The wet purple solid was covered and placed in an oven for two days. On removal from the oven, a white solid was seen on top of the purple solid. Yield of [Co(NH3)5Cl]Cl2 was 24.00 g (0.096 mol, 113.9%)\ Synthesis of [Co(NH3)5ONO]Cl2: [Co(NH3)5Cl]Cl2 (10.057 g, 0.040 mol) was added to a solution of 80ºC water (160 mL) and concentrated ammonium hydroxide (15.0 mL). A deep reddish purple color was observed. Stirring and heat was applied for 45 minutes to dissolve all solid. The solution was removed from heat and placed in a beaker of cold water. Hydrochloric acid (appx. 10 mL, 6M) was added to neutralize the pH. Sodium nitrite (10.030 g, 0.145 mol) was added to the solution and dissolved. Hydrochloric acid (10 mL, 6 M) was added, resulting in a dark orange color and evolution of gas from solution. The orange solution was vacuum filtered on a frit and washed with cold water (25 mL twice), cold ethanol (25 mL twice), and cold diethyl ether (25 mL). Yield of the orange solid [Co(NH3)5ONO]Cl2 was 6.81 g (0.026 mol, 65.0%). Synthesis of [Co(NH3)5NO2]Cl2: [Co(NH3)5ONO]Cl2 (0.869 g, .00333 mol) was placed in an 80ºC oven for 57 minutes, yielding a lighter yellow-orange solid [Co(NH3)5NO2]Cl2.
Spectroscopic Characterization of [Co(NH3)5Cl]Cl2: IR (KBr): 3282.9 cm-1 (s, νNH), 1308.6 cm-1 (m, δNH3), 846.6 cm-1 (m, ρNH3). UV/vis: λ = 252.01 nm. Absorbance = 0.974. ε = 1108.71 M-1cm-1. Spectroscopic Characterization of [Co(NH3)5ONO]Cl2: IR (KBr): 3276.5 cm-1 (s, νNH), 1319.0 cm-1 (m, δNH3), 851.8 cm-1 (m, ρNH3), 1066.0 cm-1 (m, νNO). UV/vis: λ = 269.5 nm. Absorbance = 1.246. ε = 1250.75 M-1cm-1. Spectroscopic Characterization of [Co(NH3)5NO2]Cl2: IR (KBr): 3273.3 cm-1 (s, νNH), 1313.0 cm-1 (m, δNH3), 842.0 (and 822.2) cm-1 (m, ρNH3), 1050.2 cm-1 (m, νNO). UV/vis: λ = 324.82 nm. Absorbance = 1.300. ε = 1304.96 M-1cm-1. Collection of Kinetics Data: A stock solution (100 mL, 9.96 x 10-3 M) was prepared with 0.260 g (9.96 x 10-4 mol) [Co(NH3)5ONO]Cl2. A buffer solution (500 mL) consisting of 2.672 g NH4Cl and 3.380 mL concentrated NH3OH was prepared. 90 mL of the buffer solution was placed in each of four different temperature baths – 25.0ºC, 34.0ºC, 38.5ºC, and 44.0ºC and was allowed to equilibrate with the temperature in the bath. With a stopwatch running, 10 mL stock solution was added to each buffer solution and the time of addition was noted. At recorded time points during the reaction, samples of the reaction mixture were removed to cuvettes and UV-Vis spectra were taken. Collection of Kinetics Data at Varying Concentrations: Three 100mL stock solutions of [Co(NH3)5ONO]Cl2 were prepared. Solution 1 contained 0.132 g (5.06 x 10-4 mol, 5.06 x 10-3 M) [Co(NH3)5ONO]Cl2, solution 2 contained 0.260 g (9.96 x 10-4 mol, 9.96 x 10-3 M) [Co(NH3)5ONO]Cl2, and solution 3 contained 0.392 g (1.50 x 10-3 mol, 1.50 x 10-2 M) [Co(NH3)5ONO]Cl2. Three flasks, each with 90 mL buffer
solution (as above) were heated to 45.0ºC. As above, 10 mL of a solution was added to a flask and UV-Vis spectra were taken at measured time points during the reaction.
Results Data characterizing the IR and UV-Vis spectra of [Co(NH3)5Cl]Cl2, [Co(NH3)5ONO]Cl2, and [Co(NH3)5NO2]Cl2 are seen below in Tables 1 and 2.
Characterization of IR Spectra of Cobalt Pentaammine compounds Peak Characterization N-H stretching
Frequency in [Co(NH3)5Cl]Cl2 3282.9 cm-1
Frequency in [Co(NH3)5ONO]Cl2 3276.5 cm-1
Frequency in [Co(NH3)5NO2]Cl2 3273.3 cm-1
δNH3
1308.6 cm-1
1319.0 cm-1
1313.0 cm-1
ρNH3
846.6 cm-1
851.8 cm-1
842.0 cm-1
N-O stretching
N/A
1066.0 cm-1
1050.2 cm-1 Table 1.
UV-Vis Spectra of Cobalt Pentaammine Compounds [Co(NH3)5Cl]Cl2
[Co(NH3)5ONO]Cl2
[Co(NH3)5NO2]Cl2
λmax
252.01 nm
269.5 nm
324.82 nm
ε
1108.71 M-1cm-1
1250.75 M-1cm-1
1304.96 M-1cm-1 Table 2.
Af values were obtained from the spectra taken after heating each of the solutions. For data analysis purposes, Af was calculated separately for each of the temperature baths. Af values for various temperatures are displayed in Table 3.
Final Absorbance at 324.82 nm After Heating Solutions of Varying Temperatures Temperature (ºC)
Final Absorbance
25.0
1.215
34.0
1.320
38.5
1.141
44.0
1.235 Table 3. Figure 1 shows an overlay of UV-Vis spectra taken during a conversion of
[Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC. Data is taken from samples approximately 10 minutes apart for 70 minutes. Close-ups of the isosbestic points from Figure 1 are shown in Figures 2 and 3. Figures 4-15 show plots of various functions of absorbance at various temperatures. For each temperature, plots for zero order (At vs t), first order (ln((Af-At)/(Af-Ai) vs t), and second order ((Af-Ai)/(Af-At) are given. UV-Vis spectra for [Co(NH3)5Cl]Cl2, [Co(NH3)5ONO]Cl2, and [Co(NH3)5NO2]Cl2 are attached at the end of the report. These plots are derived from starting with Beer’s law: At = εCoONO [CoONO]t l + εCoNO2 [CoNO2]t l. From this equation, assumptions were made that the initial starting material was 100% pure and that isomerization takes place with no side reactions and complete conversion of starting materials to products. These assumptions lead to the conclusion that [CoONO]t = ((Af-At)/(Af-Ai)) [CoONO]i. Thus, graphs are made of this modified absorbance, (Af-At)/(Af-Ai). Data were processed by correcting for different baseline absorbances in the 800-900 nm region before graphs were made.
UV-VIS Spectra Overlay of Kinetic Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC 1.1
Absorbance
0.9 0.7 0.5 0.3 0.1 -0.1 200
Isosbestic point 250
300
350
400 450 Wavelength (nm)
500
550
600
Figure 1.
UV-VIS Spectra Overlay of Kinetic Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC 1.1
Absorbance
0.9 0.7 0.5 0.3 0.1 -0.1 270
275
280
285
290 295 300 Wavelength (nm)
305
310
315
320
Figure 2.
UV-VIS Spectra Overlay of Kinetic Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC 1.1
Absorbance
0.9 0.7 0.5 0.3 0.1 -0.1 350
355
360
365
370 375 380 Wavelength (nm)
385
390
395
400
Figure 3.
Zero Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 25.0ºC 0.7 0.6
Absorbance
0.5 0.4 0.3 0.2 0.1 0 0
20
40
60 80 Time (minutes)
100
120
140
Figure 4.
First Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 25.0ºC 0 0
20
40
60
80
100
120
140
ln((Af-At)/(Af-Ai))
-0.1 -0.2 y = -0.0047x - 0.0527 R² = 0.9915
-0.3 -0.4 -0.5 -0.6 -0.7
Time (minutes)
Figure 5.
Second Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 25.0ºC 2 1.8
(Af-Ai)/(Af-At)
1.6 y = 0.0065x + 1.032 R² = 0.995
1.4 1.2 1 0.8 0.6 0.4 0.2 0 0
20
40
60 80 Time (minutes)
100
120
140
Figure 6.
Zero Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 34.0ºC 1.2
Absorbance
1 0.8 0.6 0.4 0.2 0 0
20
40
60 80 Time (minutes)
100
120
140
Figure 7.
First Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 34.0ºC 0 -0.2
0
20
40
60
80
100
120
140
ln((Af-At)/(Af-Ai))
-0.4 y = -0.013x + 0.0158 R² = 0.9977
-0.6 -0.8 -1 -1.2 -1.4 -1.6 -1.8
Time (minutes)
Figure 8.
Second Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 34.0ºC 5 4.5
(Af-Ai)/(Af-At)
4 3.5
y = 0.0282x + 0.7391 R² = 0.974
3 2.5 2 1.5 1 0.5 0 0
20
40
60 80 Time (minutes)
100
120
140
Figure 9.
Zero Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 38.5ºC 1.2
Absorbance
1 0.8 0.6 0.4 0.2 0 0
10
20
30
40 50 Time (minutes)
60
70
80
Figure 10.
First Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 38.5ºC 0.5 0 ln((Af-At)/(Af-Ai))
0
10
20
30
40
50
60
70
80
-0.5 -1
y = -0.0259x + 0.0584 R² = 0.9857
-1.5 -2 -2.5
Time (minutes)
Figure 11.
Second Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 38.5ºC 8 7 (Af-Ai)/(Af-At)
6 5 4
y = 0.0686x + 0.3633 R² = 0.9025
3 2 1 0 0
10
20
30
40 50 Time (minutes)
60
70
80
Figure 12.
Zero Order: Absorbance vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC 1.4 1.2
Absorbance
1 0.8 0.6 0.4 0.2 0 0
10
20
30
40 50 Time (minutes)
60
70
80
Figure 13.
First Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC 0.5
ln((Af-At)/(Af-Ai))
0 -0.5
0
10
20
30
40
50
60
70
80
-1 -1.5 -2 -2.5
y = -0.0486x + 0.1542 R² = 0.9964
-3 -3.5
Time (minutes)
Figure 14.
Second Order: Absorbance at 324.82 nm vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 44.0ºC 30 25
(Af-Ai)/(Af-At)
20 15 10
y = 0.2986x - 1.8777 R² = 0.7975
5 0 0 -5
10
20
30
40
50
60
70
80
Time (minutes)
Figure 15. Rates for isomerization were taken directly from the slope (k = -m) of the 1st order plots seen above. The rate constants are tabulated below in Table 4. Rate Constant for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 Temperature (ºC)
Rate constant = k
25.0
0.0047 min-1 = 7.83 x 10-5 s-1
34.0
0.013 min-1 = 2.17 x 10-4 s-1
38.5
0.0259 min-1 = 4.32 x 10-4 s-1
44.0
0.0486 min-1 = 8.10 x 10-4 s-1 Table 4.
An Eyring plot (of ln(k/T) vs 1/T) is shown below in Figure 16. Error was approximated by creating first order plots using the 44ºC data with both the maximum and minimum Af values
obtained. This resulted in an error in k of 0.0127 min-1, or 2.12 x 10-4 s-1. Error propagation of this error closely matched the standard error of the sample. Both were approximately +/- 0.5 s1
K-1, so that value is used for the error bars in the Eyring plot.
Eyring Plot -12 0.0031
0.00315
0.0032
0.00325
0.0033
0.00335
0.0034
-12.5
ln(k/T) (units s-1K-1)
-13 -13.5
y = -11420x + 23.114 R² = 0.9955
-14 -14.5 -15 -15.5 -16
1/T (units K-1)
Figure 16. The slope and y-intercept of the Eyring plot were used to determine the kinetic parameters ∆S‡ and ∆H‡ by the following equation, Equation 1. The slope of the plot was taken to equal -∆H‡/R and the y-intercept was taken to be ∆S‡/R - ln(h/(κkb)).
(1) ln(k/T) = ∆S‡/R - ∆H‡/(RT) – ln(h/(κkb)) Taking κ = 1, Equation 1 gives values for ∆S‡ and ∆H‡ as given in Table 5.
Thermodynamic Parameters Calculated from Eyring Equation ∆S‡
∆H‡
-5.373 +/- 130 J mol-1 K-1
94.951 +/- 42 kJ mol-1 K-1 Table 5.
Data showing (Af-At)/(Af-Ai) vs t at varying concentrations of [Co(NH3)5ONO]Cl2 are seen in Figures 17-19. Exponential fits were placed on the curves, and the half-life of each condition was calculated from the exponential equation. Half-lives are presented in Table 6.
Modified Absorbance vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 45.0ºC at 5.06 x 10-4 M 1.2 y = 1.0744e-0.037x R² = 0.9815
(Af-At)/(Af-Ai)
1 0.8 0.6 0.4 0.2 0 0
10
20
30 Time (minutes)
40
50
60
Figure 17.
Modified Absorbance vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 45.0ºC at 9.96 x 10-4 M 1.4
(Af-At)/(Af-Ai)
1.2 1 y = 1.2563e-0.051x R² = 0.9221
0.8 0.6 0.4 0.2 0 0
10
20
30 Time (min)
40
50
60
Figure 18.
Modified Absorbance vs. Time for Conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 at 45.0ºC at 1.50 x 10-3 M 1.2
y = 0.9704e-0.036x R² = 0.965
(Af-At)-(Af-Ai)
1 0.8 0.6 0.4 0.2 0 0
10
20
30 Time (minutes)
40
50
60
Figure 19.
Concentration of [Co(NH3)5ONO]Cl2 (M)
Half-life (minutes)
5.06 x 10-4
18.73
9.96 x 10-4
13.59
1.50 x 10-3
19.25 Table 6.
Discussion The data supports Mechanism 1 as the mechanism for isomerization from [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2. As noted in the introduction, the expected results for Mechanism 1 were that the conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 be first order with respect to [Co(NH3)5ONO]Cl2 and that ∆S‡ be negative. The order of a reaction is dependent on the number of molecules that must interact during the rate determining step of the reaction. To determine the order of the isomerization reaction with respect to [Co(NH3)5ONO]Cl2, kinetic conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 was performed at four different temperatures – 25.0ºC, 34.0ºC, 38.5ºC, and 44.0ºC. From each of these temperature conditions, UV-Vis spectra of the reaction mixtures were obtained during the course of the reaction. To perform calculations of the concentration of product, absorbances measured at 324.82 nm were used. This wavelength was chosen because it gave the maximum absorbance for the nitro complex while having a minimal absorbance for the nitrito complex. Because of this, a large change in absorbance at 324.82 nm was noted during the reaction. Three plots were produced from the absorbance (at 324.82 nm) data for each temperature condition. (1) A simple plot of At vs t was created to test for a zero order reaction. In the case of a zero order reaction, the rate of change of product concentration is independent of the
concentration of the starting material, so a graph of At vs t will be linear. (2) A plot of ln((AfAt)/(Af-Ai)) vs t was created to test for a first order reaction. In the case of a first order reaction, the rate of change of product concentration is proportional to –k and the concentration of the starting material. As a consequence, the graph of ln((Af-At)/(Af-Ai)) vs t will be linear with a slope of –k. (3) A plot of (Af-Ai)/(Af-At) vs t was created to test for a second order reaction. A second order reaction has the rate of change of product concentration proportional to the square of the starting material concentration. This means that a plot of (Af-Ai)/(Af-At) vs t will be linear with a slope of –k. The three plots detailed above were produced for each of the four temperature conditions. In the three highest temperature conditions (34.0ºC, 38.5ºC, and 44.0ºC), the 1st order graphs had a higher R2 value than the 2nd order graphs (0.99772, 0.98566, and 0.9964 compared to 0.97401, 0.90252, and 0.7972), indicating a higher degree of linearity to the 1st order plots and suggesting the reaction is 1st order. The 25.0ºC condition, likely due to much slower reaction speed and failure of the reaction to proceed to completion, showed very high linearity for both 1st order and 2nd order plots, with a 1st order R2 of 0.99155 and a 2nd order R2 of 0.995. The above analysis supports a mechanism that is 1st order. As noted, this result suggests either Mechanism 1 with RDS 1 or Mechanism 2 with RDS 2. To separate these two options, an Eyring plot was created and a calculation of the thermodynamic properties of the transition state (∆S‡ and ∆H‡) was performed. ∆S‡ represents the change in entropy from the starting materials to the transition state created during the RDS. As such, we would expect that RDS 1, which promotes a more ordered transition state (due to the increase in the number of bonds coordinating the NO2 ligand to the Co atom), would give ∆S‡ < 0. A negative entropy accounts
for the increased order present in the transition state for RDS 1. Alternatively, we would expect RDS 2, which is a dissociative process consisting of the loss of an NO2 ligand, to give ∆S‡ > 0. An Eyring plot of ln(k/T) vs 1/T was created using k values calculated from the 1st order plots at each temperature. From the Eyring plot and calculation, the thermodynamic parameters were calculated to be ∆S‡ = -5.373 +/- 130 J mol-1 K-1 and ∆H‡ = 94.951 +/- 42 kJ mol-1 K-1. These values are consistent with the negative ∆S‡ expected from Mechanism 1. Thus, both the thermodynamic and kinetic parameters measured and calculated suggest Mechanism 1 is the mechanism of isomerization from [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2. Two isosbestic points may be seen on the overlay in Figure 1. They fall roughly at 290 nm and 370 nm. The existence of these isosbestic points, at which absorbance does not change with time during the isomerization from nitrito to nitro, suggests that no loss of Co was occurring during the reaction. That is, [Co(NH3)5ONO]Cl2 + [Co(NH3)5NO2]Cl2 = [Co(NH3)5ONO]Cl2i. The largest source of error in this data arises from the determination of Af. Because of this, the variance in Af was used to calculate the error present in k, ∆S‡, and ∆H‡. Theoretically, all solutions at the same concentration should give the same final absorbance upon heating and completion of conversion from nitrito to nitro. However, this effect was not observed. A range of Af values was observed from 1.141 to 1.320. The variance in Af values suggests one of a number of reasons: (1) On heating of the reaction mixtures to 80ºC to complete the reactions, heating did not occur for long enough to fully complete the conversion from nitrito to nitro. This should give a lower Af than predicted by the initial absorbance seen on the nitro spectrum. (2)During the course of the reaction or heating to obtain Af, evaporation of some of the liquid in the mixture would change the concentration of [Co(NH3)5ONO]Cl2 and subsequently affect the final aborbance. This should give a higher Af than predicted by the initial absorbance seen on
the nitro spectrum. Thus, both of these sources of error seem likely, as the A324.82nm in the original [Co(NH3)5NO2]Cl2 was 1.300. As some Af values lie above 1.300 and others lie below it, both (1) and (2) are likely sources of error in Af. Another source of error in the experiment was the use of multiple cuvettes in the collection of the 38.5ºC data, perhaps leading to the stepwise pattern seen. Error propagation was performed on the uncertainty present in Af values obtained at the four temperatures to obtain uncertainty in k calculated at each temperature. Further, the uncertainties in ∆S‡, and ∆H‡ were estimated by performing calculations of ∆S‡, and ∆H‡ using extreme values of k. The uncertainty in these thermodynamic parameters is large enough to call into question the conclusion that ∆S‡ is negative. For the rather large amount of uncertainty present, though, it is unusual that the Eyring plot displays such a high degree of linearity, with R2 = 0.99551. This result suggests that the amount of calculated error far exceeds the actual error present in the data. Further confirmation for the order of the reaction with respect to [Co(NH3)5ONO]Cl2 would bolster the conclusion that Mechanism 1 the correct mechanism for isomerization from nitrito to nitro. By performing the isomerization reaction, varying the initial concentration of [Co(NH3)5ONO]Cl2, and comparing the half-lives, additional data regarding the order of the reaction can be obtained. For a first order reaction, t1/2 = ln(2) / k. Because the half life equation does not depend on the concentration of [Co(NH3)5ONO]Cl2, it is expected that the half life of the two solutions at different concentrations will be equal. Conversely, if the reaction were second order, the half life would depend on the concentration of starting material, and it would be different for the two solutions.
An additional study measuring the half-life of the reaction at various initial concentrations of [Co(NH3)5ONO]Cl2 was performed, but nothing was able to be concluded from this data. The half-lives presented above vary by approximately 6 minutes, or 30-50% of their value. This seems to suggest that half-life does change with concentration. However, there was an extremely large amount of noise present when collecting this data, leading to a greater uncertainty in the calculations. Further, approximating this uncertainty accurately was difficult, as Af was likely the biggest source of error and variations in Af could not be compared between different concentrations. As a rough estimate, though, we could take 0.0127 min-1 as the error in k (calculated above from the first experiment) and use that to estimate the error in the half-lives as ln(2)/(0.0127 min-1), or 55 minutes. This enormous error indicates that no further conclusion can be reached from the half-life data to support the conclusion from the first experiment.
Conclusion In this experiment, conversion of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2 was performed at four temperatures. By examining the absorbance at λmax for the nitro complex over time and creating plots of different functions of these absorbances, we determined that the reaction is first order. This eliminated Mechanism 2 with RDS 3 as a possible mechanism. To distinguish between Mechanism 1 with RDS 1 and Mechanism 2 with RDS 2, an Eyring plot was created, comparing the rate constant with temperature yielded ∆S‡, and ∆H‡. The negative ∆S‡ obtained suggests Mechanism 1 is the correct mechanism for isomerization of [Co(NH3)5ONO]Cl2 to [Co(NH3)5NO2]Cl2. Further study seeking to bolster the conclusion of first-order proved unsuccessful.
References 1
Buda, C.; Kazi, A. B.; Dinescu, A; Cundari, T. R. J. Chem. Inf. Model. 2005, 45, 965-970.
2
Heyns, A. M.; De Waal, D. Spectrochimica Acta 1989, 45, 905-909.
3
Phillips, W. M.; Choi, S.; Larrabee, J. A. J. Chem. Educ. 1990, 67, 267-269.
UV-Vis Spectrum of [Co(NH3)5Cl]Cl2 1
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UV-Vis Spectrum of [Co(NH3)5ONO]Cl2 1.2 1
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UV-Vis Spectrum of [Co(NH3)5NO2]Cl2 1.2 1
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