Do you think GMOs should be restricted or banned as they are in most of Europe and other parts of the ?????world? ?Why or why not? ?Refer to film. ?You can address ?????economic and/or health
In this post you can choose one of the suggested topics or add your own, as long as it addresses organic food and/or GMO. Follow the same format.
topics for your main post:
- Do you think GMOs should be restricted or banned as they are in most of Europe and other parts of the world? Why or why not? Refer to film. You can address economic and/or health concerns as you wish
- What are the ways in which you would identify the presence GMO in food, as obvious labeling is not required for them. For some foods, all you have to do is look for ingredients.
- There are certain labeling practices that can identify packaged products that are reasonably sure to be free of GMO. You can do an internet search for this one – cite reference.
- How can you identify loose fresh produce that is organically grown?
1 page
Spectrophotometric Determination of an Equilibrium Constant
CHM 153L 04B
ABSTRACT
A characteristic of every chemical reaction is its condition of equilibrium at any given temperature. When two or more reactants are mixed, they tend to keep forming products until a state of equilibrium is reached where the amounts of reactants converted to products is equal to the amount of products converted to reactants. Under such conditions, the system is in chemical equilibrium and will remain so until it is altered in one way or another. In relation to this is a number called the equilibrium constant, Kc, which expresses the ratio of the products to the reactants when the reaction is at equilibrium.
The principle of spectrophotometry in measuring light absorption is divided into the ultraviolet, infrared and visible regions of the electromagnetic spectrum. In this experiment, the spectrophotometric technique used focuses on the ultraviolet-visible region that makes use of the wavelength 800-200 nm in order to determine the equilibrium constant, Kc, of the Fe+3 + HSCN FeSCN+2 + H+ reaction from the absorbance of the complex ion FeSCN+2. The equilibrium system is set up by mixing known concentrations of Fe+3 and HSCN, producing a deep red FeSCN+2with an absorption maximum at about 447 nm. By determining the absorbance of FeSCN+2 spectrophotometrically, its concentration can also be subsequently determined. By deriving the initial concentrations of Fe+3 and HSCN and by measuring the FeSCN+2 equilibrium concentration using a calibration curve, the equilibrium concentrations of Fe+3 and HSCN can be determined and Kc for the system can be calculated.
INTRODUCTION
A chemical reaction can move in both a forward direction as well as reverse direction Aa + Bb Cc + Dd. In a reversible reaction, the forward rate, conversion of reactants to products (forward rate) occurs simultaneously with the conversion of products to reactants (reverse rate). When the forward rate and reverse rate are equal, the reaction is at dynamic equilibrium, and all reactant and product concentrations are constant. The equilibrium arrow which points in both directions reinforces this idea. The constant ratio called the equilibrium constant, Kc, can be formulated using the general formula.
One technique to determine equilibrium constant is spectrophotometry. In this method, the unknown concentration of the colored product, FeSCN+2, is calculated by comparing the absorbance of a solution of known concentration with that of a known standard. The aim of this experiment is to determine the equilibrium constants of the reactants and products in the chemical reaction, Fe+3 + HSCN FeSCN+2 + H+, by first measuring the equilibrium concentration of the reactants, Fe+3 and HSCN, and the overall equilibrium concentration by plotting absorbance response at 447nm to the varying concentrations in a calibration curve. This experiment specifically uses 447nm since Fe+3 and HSCN do not absorb light at this wavelength, thus eliminating any interferences. According to Beer’s Law, “the path length and concentration of a chemical are directly proportional to its absorbance of light” (Beer, 1852), expressed as: A = bc, where ‘A’ is absorbance (unitless), ‘’ representing molar absorption coefficient (mol-1cm-1) and ‘c’ representing concentration of the compound in solution (M). Hence, absorbance will be directly proportional to the concentration of FeSCN+2 in this experiment.
A spectrophotometer is an instrument used to measure how much light is absorbed by a sample. It finds common application in the industrial and commercial field in monitoring color accuracy throughout the manufacturing process. Through the use of a spectrophotometer, the chosen color of liquids, plastics, paper, metals and fabrics remains consistent throughout the original conception up to the final, finished product. Other uses of this machine include semiconductor manufacturing and analyzing gas pollutants in the air.
MATERIALS AND METHOD
Preparing the solution
One cuvette, five 6-inch test tubes, three small beakers and a stirring rod were first cleaned with liquid detergent and washed five times with tap water and twice with distilled water to eliminate any residue from previous experimentations and to remove interfering ions. With the exception of the cuvette, the washed glassware was then dried with paper towels. Care must be taken when handling the cuvette since grime, fingerprints, or scratches from drying with paper towel will be interpreted as more light being absorbed by the spectrophotometer, thus making readings inaccurate. One beaker was labelled “2.00 x 10-3 Fe(NO3)3” and rinsed twice with a few mL of the Fe(NO3)3 solution. After discarding the rinses in the sink, 50mL of Fe(NO3)3 was added into the beaker. Another beaker was labelled “2.00 x 10-3 HSCN” and rinsed twice with a few mL of the HSCN solution. After discarding the rinses in the sink, 30mL of the HSCN was added into the beaker. A third beaker was labelled “0.500M HNO3” and rinsed twice with a few mL of HNO3 solution. After discarding the rinses in the sink, 20mL of HNO3 was added into the beaker. Caution should be taken when handling HNO3 because it can cause chemical burns. Once all three beakers had been prepared, a pipet was rinsed with two small portions of Fe(NO3)3 solution, coating the walls of the pipet with the solution to remove any moisture adhering to the glass. Additionally, rinsing the pipette with the same solution being used is one way of ensuring the concentration of the solution remains the same even if a tiny amount of it remains in the pipette. The five 6-inch test tubes were then labelled #1 to #5 and pipetted with exactly 5.00mL of 2.00 x 10-3 Fe(NO3)3. The pipet was rinsed once with tap water and distilled water first, then twice with two small portions of HSCN solution. 1.00mL of the HSCN solution was pipetted into test tube #1, 2.00mL of the HSCN solution was pipetted into test tube #2, 3.00mL of the HSCN solution was pipetted into test tube #3, 4.00mL of the HSCN solution was pipetted into test tube #4 and 5.00mL of the HSCN solution was pipetted into test tube #5. The pipette was cleaned with tap water and deionized water first, then twice with two small portions of HNO3 solution. 4.00mL of the HNO3 solution was pipetted into test tube #1, 3.00mL of the HNO3 solution was pipetted into test tube #2, 2.00mL of the HNO3 solution was pipetted into test tube #3 and 1.00mL of the HNO3 solution was pipetted into test tube #4. No HNO3 solution was pipetted into test tube #5. The solutions in each of the five test tubes were stirred with a stirring rod, care being taken to clean and dry the rod when moving from one test tube to another. While leaving the solutions in the test tubes to stand for 10 minutes, the spectrophotometer was calibrated using a cuvette with a “blank” solution of deionized water. A second cuvette was prepared by rinsing with a few mL of the solution from test tube #1. The rinse was then discarded, and the cuvette was filled three-fourths with the solution from test tube #1. Absorbance was read and recorded. After the absorbance was read, the solution was poured back into test tube #1. Then after, the cuvette was rinsed with tap water and deionized water first, then with a few mL of the solution from test tube #2. The rinse was then discarded, and the cuvette was filled three-fourths with the solution from test tube #2. Absorbance was read and recorded. The same steps were repeated for tubes #3, #4 and #5. Once absorbance had been obtained for all five test tubes, a calibration curve was used to determine the concentration of FeSCN+2. The set of standard solutions was prepared according to the table below.
Table 1. Composition of the set of standard solutions for preparing the calibration curve.
|
Solution |
2.00 x 10-3 M Fe(NO3)3 in 0.500 M HNO3 |
2.00 x 10-3 M HSCN in 0.500 M HNO3 |
0.50000 M HNO3 |
|
1 |
5.00 |
1.00 |
4.00 |
|
2 |
5.00 |
2.00 |
3.00 |
|
3 |
5.00 |
3.00 |
2.00 |
|
4 |
5.00 |
4.00 |
1.00 |
|
5 |
5.00 |
5.00 |
0.00 |
Using the spectrophotometer
The wavelength was set to 447nm since Fe+3 and HSCN do not absorb light at this wavelength, thus eliminating interferences. The zero control (left-hand knob) was then set to 0%T, and the cuvette containing deionized water was wiped with a “Kimwipe” and served as the “blank” solution used to calibrate the spectrophotometer. The cuvette was placed into the solution holder, aligning the white line on the cuvette with the raised line on the solution holder. The cover on the solution holder was then closed. The spectrophotometer was set to read in “Absorbance” by pressing the “Mode” button once and adjusted until it reads zero (“0”) using the 100% adjust dial (right hand dial). By doing this, any absorbance contribution from the “blank” solution is “zeroed” out, preventing its interference with the reading of subsequent absorbances in the experimentation. After setting up the spectrophotometer, the cuvette was removed.
Calculations
The equilibrium concentration of FeSCN+2 in each of the five test tubes by using the calibration curve prepared for the spectrophotometer and the absorbance values obtained from the spectrophotometric readings. Using the formula M1V1 = M2V2, the initial molarity of Fe+3 and HSCN present before the reaction occurred was calculated. For the equilibrium concentration of Fe+3 and HSCN, the formulas [Fe+3]eq = [Fe+3]0 – [FeSCN+2]eq and [HSCN]eq = [HSCN]0 – [FeSCN+2]eq were used, wherein [Fe+3]0 and [HSCN]0 are initial molarity of Fe+3 and HSCN calculated previously and [FeSCN+2]eq as the concentration from the calibration curve. Lastly, Kc was calculated using the equation.
RESULTS, INCLUDING FIGURES AND TABLES
Table 1. Initial concentrations (10-4 M) of Fe+3 and HSCN using the equation M1V1 = M2V2 for M2.
|
Solution |
[HSCN]0 |
[Fe3+]0 |
|
1 |
2.00 |
10.0 |
|
2 |
4.00 |
10.0 |
|
3 |
6.00 |
10.0 |
|
4 |
8.00 |
10.0 |
|
5 |
10.0 |
10.0 |
Table 2. Initial concentration (10-4 M) of HSCN equivalent to the equilibrium concentration of FeSCN+2 as the equilibrium shifts to the right by the addition of HSCN in increasing concentrations.
|
Solution |
[HSCN]0 |
[FeSCN2+]eq |
Absorbance |
|
1 |
0.100 |
0.100 |
0.064 |
|
2 |
0.200 |
0.225 |
0.134 |
|
3 |
0.300 |
0.325 |
0.195 |
|
4 |
0.400 |
0.450 |
0.270 |
|
5 |
0.500 |
0.525 |
0.315 |
Graph 1. The concentration of FeSCN+2 M on the x-axis versus absorbance on the y-axis.
Table 3. Equilibrium concentrations (10-4 M) of Fe+3 and HSCN calculated using the equation [Fe+3]eq = [Fe+3]0 – [FeSCN+2]eq and [HSCN]eq = [HSCN]0 – [FeSCN+2]eq.
|
Solution |
[Fe+3]eq |
[HSCN]eq |
|
1 |
9.90 |
1.90 |
|
2 |
9.78 |
3.78 |
|
3 |
9.68 |
5.68 |
|
4 |
9.55 |
7.55 |
|
5 |
9.48 |
9.48 |
Table 4. The Kc value (10-4) calculated for each solution as well as average Kc for all 5 solutions.
|
Solution |
Kc |
|
1 |
26.6 |
|
2 |
30.5 |
|
3 |
29.6 |
|
4 |
31.2 |
|
5 |
29.2 |
|
Average Kc |
29.4 |
DISCUSSION
The calibration curve is reliable in calculating the equilibrium concentration of FeSCN+ as the absorption is linear to the concentration of the unknown analyte, thus increasing absorption also increases concentration. Similarly, decreasing absorption also decreases concentration. The concentration (M) and absorbance were graphed against each other to create the calibration curve, with the line passing through the origin where FeSCN+2 and absorbance is zero. According to Beer’s Law, the absorbance of a solution is proportional to the concentration of the absorbing species, therefore; if FeSCN+ is zero then absorption is also zero. The calculated R2 value for the line was 0.9951 (on a range of 0 to 1) which indicates a progressive correlation between absorbance and concentration (M). The principle of spectrophotometry can be understood through its two main components: (1) a spectrometer with lens that send a straight beam of light through a prism, splitting the prism into its individual wavelengths and (2) a wavelength selector that picks out the specific wavelength and sends it to the photometer that detects the number of photons absorbed and displays the value on a digital display. Using the specific wavelength (nm) selected for the compound being measured, a spectrophotometer quantifies the concentration of the compound by the amount of light the compound absorbs. By using a specific technique to determine absorbance and color, it decreases the chances of random error such as visual perception to color which can be subjective. Ultimately, determining the maximum absorbance of the compound in question determines the specific wavelength needed.
As shown in Graph 1, concentration (M) and absorbance is basically proportional. The UV-visible spectrophotometer is a choice of equipment to use for the experiment as it utilizes light (wavelengths) to calculate absorbance and, subsequently, the equilibrium concentrations of the solutions. Wavelengths between 200-800nm are in the ultraviolet and visible range of the electromagnetic spectrum and include the colors red (longest wavelength) to violet (shortest wavelength). If values other than 447nm are used, the experiment may not be accurate as some analytes react in other wavelengths. In Table 1 of the experiment, the initial molarity of Fe3+ and HSCN present before the reaction occurs was shown. Because both concentrations of Fe3+ and HSCN have been diluted by being mixed with each other and HNO3, the dilution equation M1V1 = M2V2 representing molarity (M1) and volume (V1) of the initial solutions pipetted into the test tubes for a total of 10.0mL (V2) to derive the unknown M2. In Table 2, equilibrium was forced to shift to the right side of the equation, favoring the products due to the addition of HSCN.
The experiment aims to determine the constant at which the reaction is in equilibrium, neither shifting to the left nor right, thus, neither favoring the reactants nor products. This constant, Kc, is dependent on the equilibrium concentrations of the reactants using their relative ratios which stay constant as can be seen in Table 3, with Fe3+ maintaining a constant concentration (M) throughout the five solutions in the test tubes with increased concentrations of HSCN being added.
In Table 4, the Kc values were shown, with a calculated Kc average value of 29.4 x 10-4 . Kc values for the five solutions centered around the mean value with no extreme outliers present. Consequently, if the value of Kc is > 1, then the products is greater than the reactants, and thus the reaction favors the formation of products (forward reaction). In contrast, if Kc is < 1, then there are more reactants than products, and the reaction favors the formation of reactants (reverse reaction). If Kc = 1, the products = the reactants. In all solutions of the experimentation, the Kc was greater than one, demonstrating that the reaction favored the formation of products and is thus a forward reaction.
Overall, the results despite being accurate, is not free from possible imprecision bias on the part of the experimenter. An important aspect to consider is the variability and bias due to manual pipetting in the preparation of analyte concentrations when interpreting results.
CONCLUSION
As Beer’s law states that A = εbc, the concentration (M) of a substance and its absorbance are directly proportional under specific conditions. This can be observed in Graph 1, as shown by the strong correlation between Concentration (M) and absorbance with an R2 value of 0.9951. For the linearity of Beer’s Law to be maintained, the value of absorbance must range between 0.064 and 0.315 ( Table 2). Values outside this range will result in a loss of linearity and, thus, a lower R2 value. With the objective of determining the equilibrium constant, Kc, of the reaction: Fe+3 + HSCN FeSCN+2 + H+, the average Kc was calculated to be 29.4 x10-4 using a variety of techniques to determine equilibrium concentrations of reactants and products such as the use of a spectrophotometry and mathematical equations. The Kc values of all five samples centered around the mean of 29.4 x10-4, with no outliers present in the data. Overall, the experimentation was successful as the graph showed a very strong correlation between concentration and absorbance, supporting the accuracy of Beer’s law. If the lab was to be repeated, proper pipetting techniques would help make the dilution solutions more precisely prepared. Additionally, more solutions with varying reactant volumes (mL) could be added to produce more data points, which would contribute to increasing the R2 value of the graph and result in a more precise and accurate result.
REFERENCES
Determination of an Equilibrium Constant. General Chemistry II Labs
(n.d.). Retrieved November 5, 2022, from https://www.webassign.net/question_assets/ncsugenchem202labv1/lab_5/manual.html
Spectrophotometric Determination of an Equilibrium Constant. Chemistry 112.
(n.d.). Retrieved November 5, 2022, from https://www.cerritos.edu/chemistry/_includes/docs/Chem_112/Lab/SpectrophotometricDetermination10-2018.pdf
#7-Determination of an Equilibrium Constant. General Chemistry II Lab.
(n.d.). Retrieved November 5, 2022, from https://www.ccri.edu/chemistry/courses/chem_1100/terezakis/labs/CHEM-1100_Exp%207_Determination_of_an_Equiibrium_Constant.pdf
Huang, V., & Kubin, J. (Eds.). (2020). Laboratory Manual for CHM-153: General Chemistry Principles – Part II (Spring 2020 edition).
Absorbance
1.0000000000000001E-5 2.2500000000000001E-5 3.2499999999999997E-5 4.5000000000000003E-5 5.2500000000000002E-5 6.4000000000000001E-2 0.13400000000000001 0.19500000000000001 0.27 0.315
Concentration of FeSNC2+ (M)
Absorbance
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