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7/7/12 Determination of Equilibrium Constant The purpose of this experiment is to determine the equilibrium constant, K., of the following equilibrium reaction. Duc 10 A CIL Fe³+ (aq) + SCN- (aq) = FeNCS²+ (aq) (yellow) (colorless) (red) This will be accomplished using spectrophotometry and utilizing Beer's Law. As was learned in the previous lab, when doing spectrophotometry, the extinction coefficient () must be determined in order to find concentrations of unknown chemicals. To accomplish this, standards (chemicals of known concentration) must be created and tested using a spectrophotometer to find their absorbances. This data is then graphed so that the slope of the straight line will be the extinction coefficient, so the absorbance will be on the y-axis and concentration of the x-axis. Beer's Law A = ebc Case Gy. VS M reac A series of standards will be created with known concentrations of FeNCS to be tested by the spectrophotometer so a calibration curve can be constructed in Part A. Making standards for this lab feslos is trickier than most, as the instrument will be testing the absorbance of the product, FeNCS, but SCAT this reaction typically goes to equilibrium. At equilibrium the concentration would not be immediately known. In order to overcome this hurdle, a concentrated solution of Fe(NO₂), will be mixed with a much weaker solution of NaSCN, allowing LeChatler's principle to push the equilibrium reaction to the products side and to completion. This will cause the NaSCN to act as a limiting reactant so the concentration of the product, FeNCS, can be calculated before actually testing the chemicals. In Part A, the reaction does not truly go to equilibrium, but instead can be considered a normal limiting reactant reaction. Since that is the case, if the concentrations of the limiting reactant is determined in the standards, the mole ratio shows us it is a 1:1 ratio to the product, so the "equilibrium" concentration of FeNCS is the same as the concentration of the SCN ion in the standard solutions. 1 1 Fe (aq) + SCN (aq) → FeNCS² (aq) For Part B, solutions will be created and allowed to reach equilibrium and then tested by the spectrophotometer. The equilibrium concentration of FeNCS will be calculated using Beer's Law. The extinction coefficient in Beer's Law is determined from the slope of the calibration curve made from the Part A data. The equilibrium concentration of the reactants must be calculated using an ICE table. A quick reminder of Beer's Law is below. Absorbance (A) is unitless, concentration (c) can be any concentration unit, pathlength (b) is the width of the sample the light passes through in the unit centimeters and extinction coefficient (also called the molar absorptivity) (e) is a constant specific to each compound at a specific wavelength. Extinction coefficient have the necessary units to cancel out the units of b and c, as absorbance is unitless and all units must cancel.
GRAPH Procedure Wear Gloves! Part A. Preparation of Standards and Calibration Curve Volumetric pipets will be used to transfer the appropriate volumes of the chemicals into 50.0 mL. volumetric flasks following the table below. Once the chemicals are accurately transferred to the volumetric flasks, they will be diluted to the line with 0.10 M HNO,. Deionized water is NOT used as the solvent in this reaction. Once the solutions are diluted to the line using nitric acid, cap and mix well by inverting the flasks many times. Calculate the concentration of SCN in each of these standard solutions. Remember, each of these solutions is a dilution. The spectator ions are being ignored in the calculations for this lab so don't be startled when one ion from the equations below is ignored. Standard Number Blank 1 2 3 4 5 - Volume of 0.200 M Fe(NO), (ml) 10.00 10.00 Solution Number Blank 1 2 3 4 5 10.00 10.00 10.00 10.00 These solutions should be saved until Part B is complete as well. Then the standards made in Part A and the solutions from Part B will both be tested together following the instructions in Part C. Create the calibration curve using Excel with absorbance on the y-axis and the [FeNCS] on the 1 axis Print the graph and submit as part of the lab report. Do not forget to include the blank! Volume of 2.00x10 M Total Volume NaSCN (mL) 0.00 1.00 2.00 3.00 5.00 5.00 5.00 5.00 Please notice that a new blank was created in Part B. The instrument will be re-blanked using the Part B blank after Part A solutions have been tested. Volume of 2.00x10 M Fe(NO₂), (mL) 5.00 5.00 4.00 5.00 5 Part B. Determination of Equilibrium Constant Volumetric pipets will be used to accurately transfer the appropriate volumes of chemicals into adorable 10.00 mL. volumetric flasks following the table below. Dilute to the line using 0.10 M nitric acid. Invert the flasks many times to guarantee even mixing of the solutions. Notice this table has a more dilute concentration of iron (III) nitratel Be very careful to use the correct concentrations of chemicals. (ml.) 50.0 50.0 50.0 50.0 50.0 50.0 Volume of 2.00x10 M NaSCN (mL) 0.00 1.00 2.00 3.00 4.00 5.00 10.00 10.00 Total Volume (mL) 10.00 10.00 10.00 10.00 64 2 MA S Sinin 3
Part C. Operating the Spectrophotometer Bring all the solutions from both Part A and B to one of the spectrophotometers around the lab. There are not enough of these instruments for each group to get their own. Do not alternate between groups. Once a group begins running their samples they must finish. All samples must be run together at the same time on the same instrument. There are two types of spectrophotometers in the lab. You will be able to clearly tell the difference between the two. One model is new and will be referred to as the "New" spectrophotometer and the other was manufactured in 1988 and referred to as the "Old" spectrophotometer. Both types work equally well, but there are different instructions depending on the type of spectrophotometer you choose. Select below the correct set of instructions depending on your instrument. "Old" Spectrophotometer instructions: Check that the wavelength is set to 447 nm. This can be found on the dial on top of the instrument. If it is not set to the correct wavelength, turn the dial until it is. Now the instrument needs to be set to display values in the unit Absorbance. Next to the digital screen that displays the readings there is a dot and four words: Transmittance, Absorbance, Concentration and Factor. Push the Mode button until the dot is ne: to Absorbance. Now the instrument is ready to use. Before running the samples, the instrument must be blanked. The purpose of the blank is to set the instrument to detect none of our compound when there is none present. In the blank there should be no FeNCS as no NaSCN was added to the blank. Take one cuvet, which look like a small test tube, and fill it with the blank solution from Part A. Dump this solution in a waste beaker. Fill the cuvet with the blank solution again and pour into the waste beaker. 3west Repeat one more time. Then fill the cuvet with the blank solution, take one KimWipe and wipe the entire outside of the cuvet. Make sure there is no water remaining or smudges. Even fingerprints must be wiped off so it will not interfere with the measurement. Place the cuvet into the spectrophotometer. The indention on the spectrophotometer aligns with the white line on the cuvet. Close the lid over the cuvet. To set the instrument to zero, turn the knob found at the very bottom of the front panel on the instrument. Below the knob will be labeled "T Increase A". Turn this knob until the digital screen shows 0.000. Now the standards can be tested in this instrument. Using the same cuvet, rinse it three times with standard 1, dumping the solution into the waste beaker after each rinse. Fill the cuvet with the solution, wipe the outside of the cuvet with a KimWipe and place it into the spectrophotometer making sure to align the line on the cuvet to the indention on the spectrophotometer. Close the lid. Record the absorbance from the digital screen. Using the same cuvet repeat this procedure for the other four standard solutions, rinsing three times between standards 2, 3, 4 and 5. Make sure to rinse the cuvet three times with the new solution before testing, not rinsing with deionized water. After the standards have been tested, blank the instrument by rinsing the cuvet with the blank created in Part B three times, wiping it with a KimWipe and aligning the indentation with the line on the cuvet. Since such a small volume of the solutions in Part B were made, the best way to rinse is to obtain a clean disposable plastic pipet and rinse the solution in the cuvet without filling it completely. Try to condition (tinse) the cuvet 3 times using this method. Use the knob to set the digital screen to 0.000. Once the instrument has been blanked, rinse it three times with the solution 1. Fill the cuvet with the solution and place it into the spectrophotometer making sure to align the line on the cuvet to the indention. Close the lid. Record the absorbance on the digital screen. Using the same cuvet repeat this procedure with the remaining solutions from Part B. Use a new plastic pipet for each solution.
"New" Spectrophotometer instructions: First check that the wavelength is set to 447 nm. The wavelength will be displayed on the digital screen of the instrument. If it is not set to the correct wavelength use the two buttons that are labeled with "nm" to increase or decrease the value appropriately. Now the instrument is ready to use. Before running the samples, the instrument must be blanked. The purpose of the blank is to set the instrument to detect none of our compound when there is none present. In the blank there should be no FeNCS" as no NaSCN was added to the blank. Take one cuvet, which look like a rectangular prism, and fill it with the blank solution from Part A. Dump this into a waste beaker. Fill the cuvet with the blank solution again and pour into the waste beaket. Repeat one more time. Then fill the cuvet with the blank solution, take one KimWipe and wipe the entire outside of the cuvet. Make sure there is no liquid remaining or smudges. Fingerprints must be wiped off so it will not interfere with the measurement. Place the cuvet into the spectrophotometer. Make sure the clear panels in the cuvet are facing front and back and the cloudy sides are to the left and right. Close the lid over the cuvet. Now we must set the instrument to zero. To do this press the button labeled "0 Abs, 100 %T". Now the standards can be tested in this instrument. Using the same cuvet, rinse standard solution 1 three times dumping the waste into the waste beaker. Fill the cuvet with the solution, wipe the outside of the cuvet with a KimWipe and place it into the spectrophotometer making sure the clear panels in the cuvet are facing front and back and the cloudy sides are to the left and right. Close the lid. The digital screen will show the absorbance of the solution. Record this. Using the same cuvet, repeat this procedure for the other four standard solutions but only rinse 3 times between standards 2, 3, 4 and 5. Make sure to rinse the cuvet three times with the new solution before testing, not rinsing with deionized water. After the standards have been tested, blank the instrument with the blank solution from Part B. Rinse the cuvet three times before filling it and, wiping it with a KimWipe. Since such a small volume of the solutions in Part B were made, the best way to rinse is to obtain a clean disposable plastic pipet and rinse the solution in the cuvet without filling it completely. Try to condition (rinse) the cuvet 3 times using this method. Place the cuvet into the spectrophotometer. Make sure the clear panels in the cuvet are facing front and back and the cloudy sides are to the left and right. Close the lid over the cuvet. Press the button labeled "0 Abs, 100 %T". Once the instrument has been blanked, rinse it three times with solution 1, dumping into the waste beaker. Fill the cuvet with the solution, wipe with a KimWipe and place it into the spectrophotometer making sure the clear panels in the cuvet are facing front and back. Close the lid. Record the absorbance from th digital screen. Using the same cuvet, repeat this procedure with the remaining solutions in Part B. Use a new plastic pipet for each solution. Part D. Waste Disposal The chemicals in this lab are hazardous so make sure to wear gloves the entire time. Nitric acid can burn and discolor skin. Combine all solutions used for the entire lab into a 1 L beaker. Set the beaker on a provided absorbent pad. Carefully pour a small amount of sodium bicarbonate, NaHCO,, into the waste beaker and stir with a glass stir rod. When the solid comes in contact with the solution, it will create CO(g) and fizz. When the fizzing calms down, check the pH of the solution using a pH strip. If the pH is no lower than 6 and no higher than 9 it can safely be poured down the drain with running water. If the pH is below 6, slowly add more NaHCO, until the pH is in the correct range. Any excess NaHCO, solid in the beaker can be poured down the drain as long as the pH of the solution is between 6 and 9.
"New" Spectrophotometer instructions: First check that the wavelength is set to 447 nm. The wavelength will be displayed on the digital screen of the instrument. If it is not set to the correct wavelength use the two buttons that are labeled with "nm" to increase or decrease the value appropriately. Now the instrument is ready to use. Before running the samples, the instrument must be blanked. The purpose of the blank is to set the instrument to detect none of our compound when there is none present. In the blank there should be no FeNCS" as no NaSCN was added to the blank. Take one cuvet, which look like a rectangular prism, and fill it with the blank solution from Part A. Dump this into a waste beaker. Fill the cuvet with the blank solution again and pour into the waste beaket. Repeat one more time. Then fill the cuvet with the blank solution, take one KimWipe and wipe the entire outside of the cuvet. Make sure there is no liquid remaining or smudges. Fingerprints must be wiped off so it will not interfere with the measurement. Place the cuvet into the spectrophotometer. Make sure the clear panels in the cuvet are facing front and back and the cloudy sides are to the left and right. Close the lid over the cuvet. Now we must set the instrument to zero. To do this press the button labeled "0 Abs, 100 %T". Now the standards can be tested in this instrument. Using the same cuvet, rinse standard solution 1 three times dumping the waste into the waste beaker. Fill the cuvet with the solution, wipe the outside of the cuvet with a KimWipe and place it into the spectrophotometer making sure the clear panels in the cuvet are facing front and back and the cloudy sides are to the left and right. Close the lid. The digital screen will show the absorbance of the solution. Record this. Using the same cuvet, repeat this procedure for the other four standard solutions but only rinse 3 times between standards 2, 3, 4 and 5. Make sure to rinse the cuvet three times with the new solution before testing, not rinsing with deionized water. After the standards have been tested, blank the instrument with the blank solution from Part B. Rinse the cuvet three times before filling it and, wiping it with a KimWipe. Since such a small volume of the solutions in Part B were made, the best way to rinse is to obtain a clean disposable plastic pipet and rinse the solution in the cuvet without filling it completely. Try to condition (rinse) the cuvet 3 times using this method. Place the cuvet into the spectrophotometer. Make sure the clear panels in the cuvet are facing front and back and the cloudy sides are to the left and right. Close the lid over the cuvet. Press the button labeled "0 Abs, 100 %T". Once the instrument has been blanked, rinse it three times with solution 1, dumping into the waste beaker. Fill the cuvet with the solution, wipe with a KimWipe and place it into the spectrophotometer making sure the clear panels in the cuvet are facing front and back. Close the lid. Record the absorbance from th digital screen. Using the same cuvet, repeat this procedure with the remaining solutions in Part B. Use a new plastic pipet for each solution. Part D. Waste Disposal The chemicals in this lab are hazardous so make sure to wear gloves the entire time. Nitric acid can burn and discolor skin. Combine all solutions used for the entire lab into a 1 L beaker. Set the beaker on a provided absorbent pad. Carefully pour a small amount of sodium bicarbonate, NaHCO,, into the waste beaker and stir with a glass stir rod. When the solid comes in contact with the solution, it will create CO(g) and fizz. When the fizzing calms down, check the pH of the solution using a pH strip. If the pH is no lower than 6 and no higher than 9 it can safely be poured down the drain with running water. If the pH is below 6, slowly add more NaHCO, until the pH is in the correct range. Any excess NaHCO, solid in the beaker can be poured down the drain as long as the pH of the solution is between 6 and 9.
Part A. Preparation of Standards and Calibration Curve Blank Standard 1 Standard 2 1.00 Volume of NaSCN (ml) [SCN-] [FeNCS+] Absorbance Determination of Equilibrium Constant A7₁-N₂ SCN 2.00e**M = Ind 1 Extinction Coefficient: 34 FC Show calculation for one standard for the (SCN). MY 0.001 C-T € Partner Name D L 0.021 2.00 0.055 0.087 -Mal= 2.000 ³ (0.001). Le mil sex Standard 31 3.0 +X Standard 4 0.121 SCA [FeSEN]²+ keaz [Fe sol]" [Fe] [Sear] Standard 5 0.155 Okn
76 SF P3 38 M₁ V₂ not Y Part B. Determination Temperature: a Use three significant figures for the equilibrium concentrations and K, values in the below table. Solution 1 Solution 31 Solution 41 Solution 51 Volume of Fe(NO₂) (mL) Volume of NaSCN (mL) Initial [Fe] Initial [SCN:] Absorbance Equil [FeNCS] Equil [Fe³+] Equil [SCN-] K 1 C -Y € cuy Blank CALL 5.00 D CALL of O of Equilibrium Constant °C Fe³+ SCN-> Fe Scu D tv вкары copp 9.00 1.00 Solution 21 5.00 2.00 5.00 3. 0.049 0.085 0.109 4.00 0.144 5150 0.167
Date 2/24/22 Average K (do not include the blank) Standard Deviation (Show calculation) Name беру маклабо Show all the calculation for one solution, including the initial concentrations of both reactants, the equilibrium [FeNCS], and the ICE table and work for the equilibrium [Fe] and [SCN].