Determination of the Equilibrium Constant for FeSCN2+ 1. Purpose Determine the equilibrium constant, K eq, for the formation of FeSCN2+ using a spectrometer. 2. Introduction Most chemical reactions are reversible, and at certain conditions the rate of forward reaction and reverse reaction can be the same. Under such conditions, the concentration of reactants and products remain constant. These systems are to be said to be at equilibrium. The equilibrium we study in this lab is the reaction between Fe3+ and SCN–. This reaction forms an intensely colored complex ion, iron(III) thiocyanide. FeSCN2+. One of the important parameters for an equilibrium is the equilibrium constant, K eq, which is expressed by the formula below. Fe3+( aq ) + SCN–( aq ) ? FeSCN2+( aq ) K eq = [FeSCN2+]/([Fe3+][SCN–]) In this experiment, we will determine the K eq for the FeSCN2+ using a visible spectrometer. At some wavelengths FeSCN2+ will absorb light intensely while at others it will be nearly completely transparent. Our goal is to tune the instrument to the wavelength that will give us the best signal. This will be accomplished by testing our standard solutions and selecting the wavelength of maximum absorbance for the complex ion. By changing [SCN–] while keeping [Fe3+] constant, and recording the absorbance, we can create a calibration curve using the Beer’s law. There are two common methods by which to measure the interaction of light with a sample: %transmittance, % T , (amount of light to pass through the sample) or Absorbance, A , (amount of light absorbed by the sample). The below equation shows you the relationship between % transmittance and absorbance. A = –log(% T /100) Beer’s law states that absorbance (A) is directly proportional to concentration in molarity. A = e lc e : molar absorptivity, l : path length, c : molarity Since the term e and l are constants, the formula can be simplified as follows. A = n c The relationship between A and c shown in the formula can be obtained by plotting the absorbance vs. [FeSCN2+] for this lab. 3. Procedure Two stock solutions, 0.200 M FeCl3 and 0.00200 M KSCN are provided. Part I. Six standard solutions are made by mixing an excess of Fe3+ ions with known amounts of SCN- ions. Measure out 25.0 mL of 0.200 M FeCl3 solution and add it into a 50 mL beaker. To the solution, add 1.00 mL of 0.00200 M KSCN solution and 9.00 mL of DI water, and stir well. Repeat this to make five more solutions using 2.00, 4.00, 6.00, 8.00, 10.00 mL of 0.00200 M KSCN solution, and 8.00, 6.00, 4.00, 2.00, 0 mL of DI water, respectively. (The total volume for all the solution should be 35.00 mL.) Calculate the molarity of FeSCN2+ in each solution. Calibrate the spectrometer with distilled water. Add a standard solution into the cuvette and measure the highest absorbance*. *The video shows %transmission (% T ). You can convert it to absorbance using the equations below. % T = 100 T Abs = –log T Make a table for the volumes of 0.00200 M KSCN solution used, [FeSCN2+], %T, and absorbances. Using Excel or Google Sheets, create the Beer’s law plot (absorbance vs. concentration). Include the trendline, the equation, and the R2 value. Part II. Five test solutions are made by mixing a constant amount of Fe3+ ions with varying amounts of SCN- ions, which contain an unknown concentration of FeSCN2+ ions. Prepare 100 mL of 0.00200 M FeCl3 solution by diluting the stock solution. Measure out 5.00 mL of 0.00200 M FeCl3 solution and add it into a 25 mL beaker. To the solution, add 1.00 mL of 0.00200 M KSCN solution and 4.00 mL, and stir well. Repeat this to make four more solutions using 2.00, 3.00, 4.00, 5.00 mL of 0.00200 M KSCN solution, and 3.00, 2.00, 1.00, 0 mL of DI water, respectively. (The total volume for all the solutions should be 10.00 mL.) Using the spectrometer, measure and record the highest absorbance for each solution. Using the equation obtained from the Beer’s law plot, calculate the molarity of FeSCN2+ in each solution. Create a table for volumes of 0.00200 M KSCN solution added, %T, absorbances, [Fe3+], [SCN–], and [FeSCN2+]. 4. Pre-lab Questions Calculate the molarities of Fe3+ in six standard solutions. (Show your work for one example calculation. You may insert a photo of the handwritten calculations, however, make sure that it’s legible and crop the photo to show the necessary part only.) Write here: Propose a step-by-step procedure for the dilution of the stock solution to make 0.00200 M solution. Show the actual values you would use for the process. Write here: When making a calibration curve, the regression analysis value, R2 is very important. Explain the meaning of R2 and the reason for the importance. Write here:
The Correct Answer and Explanation is :
Pre-lab Questions:
1. Calculation of the Molarities of Fe3+ in Six Standard Solutions:
To calculate the molarity of Fe3+ in each of the six standard solutions, we need to use the dilution equation:
[
M_1 V_1 = M_2 V_2
]
Where:
- ( M_1 ) is the initial concentration of FeCl3 (0.200 M),
- ( V_1 ) is the volume of FeCl3 used,
- ( M_2 ) is the final concentration of Fe3+ in the solution,
- ( V_2 ) is the total final volume of the solution (35.0 mL for each solution).
For Example:
Let’s calculate the molarity of Fe3+ in the solution when 1.00 mL of 0.00200 M KSCN is added.
Given:
- ( M_1 = 0.200 \, \text{M} ),
- ( V_1 = 25.0 \, \text{mL} ) (since 25.0 mL of 0.200 M FeCl3 solution is used),
- ( V_2 = 35.0 \, \text{mL} ) (final total volume).
Using the dilution equation:
[
M_2 = \frac{M_1 V_1}{V_2} = \frac{(0.200 \, \text{M})(25.0 \, \text{mL})}{35.0 \, \text{mL}} = 0.143 \, \text{M}
]
Thus, the molarity of Fe3+ in the first solution is 0.143 M. Similarly, you would repeat this calculation for the other five solutions, using the volumes of KSCN and DI water specified in the procedure.
2. Proposed Procedure for the Dilution of the Stock Solution to Make 0.00200 M Solution:
To make a 0.00200 M solution from the 0.200 M stock solution of FeCl3, you will use the dilution equation:
[
M_1 V_1 = M_2 V_2
]
Where:
- ( M_1 = 0.200 \, \text{M} ),
- ( M_2 = 0.00200 \, \text{M} ),
- ( V_2 = 100.0 \, \text{mL} ) (desired final volume).
Rearranging the equation to find ( V_1 ) (the volume of stock solution needed):
[
V_1 = \frac{M_2 V_2}{M_1} = \frac{(0.00200 \, \text{M})(100.0 \, \text{mL})}{0.200 \, \text{M}} = 1.00 \, \text{mL}
]
Thus, you would take 1.00 mL of 0.200 M FeCl3 stock solution and dilute it with 99.0 mL of DI water to obtain 100.0 mL of 0.00200 M FeCl3 solution.
3. Explanation of R² in Regression Analysis:
The regression analysis value ( R^2 ), also known as the coefficient of determination, indicates how well the data fits the regression model, which in this case is a straight line for the Beer’s law plot (absorbance vs. concentration). The value of ( R^2 ) ranges from 0 to 1, where:
- An ( R^2 ) of 1 means a perfect fit, indicating that 100% of the variation in absorbance is explained by the variation in concentration.
- An ( R^2 ) close to 0 suggests that the data does not fit the linear model well, meaning the concentration and absorbance are not linearly related.
In this experiment, a high ( R^2 ) value (close to 1) is important because it shows that the Beer’s law relationship is valid for the concentrations being measured, meaning the absorbance is directly proportional to the concentration of FeSCN2+ ions in the solution. This ensures that the calculations of the equilibrium constant ( K_{\text{eq}} ) are accurate and reliable. The importance of ( R^2 ) lies in its ability to validate the experimental data and ensure that the analysis is based on a solid mathematical relationship.