please help solve these questions especially number 2 and 3
2. Determining the equilibrium constant Determining k in Beer's Law Test Tube No. Absorbance Absorbance Test Tube No. 6 .160 1 080 . 7 .283 2 148 8 3 .436 ,510 9 4 .219 284 239 10 .732 5 o productore and absorptinity molar concentration A =éck verkoncentration or AKI- ecl Post Laboratory Questions 1. Complete the following table and calculate k-value for the Beer's Law. All concentrations should be in M. c2 CA [Fe(SCN)*2) Absorbance A Test Tube No. Diluted Fe(NO3)3 (mL) с 0 0 0 0 0 Reference .080 8.00 *10-7 1 1.0 2 2.0 1.0x10-5 2.0*10-5 3,0x18-s 4.0x10-5 . 148 :219 284 3 3.0 2.96*10-6 6, 57*10-6 1. 14 x 10-5 1,20x10-5 4 4.0 .239 5 5.0 5.0+165 T C&A k = A/C -43-
Sample calculation for obtaining [Fe(SCN)*2): Calculate k Sum of CA (ECA) = Sum of c? (Ec) = k = ΣcΑ / Σc2 = M-1 Beer's Law plot: Absorbance vs. Concentration of Fe(SCN)+2.
"ppe 2. Determining the equilibrium constant by filling the following table. Test Tube No. Starting [Fe') Starting [SCN] [Fe(SCN)) from A Eq. [Fe*') Eq. [SCN1 Kog 6 7 8 9 10 Sample calculation for obtaining initial [Fe**) and (SCN): Sample calculation for obtaining [Fe(SCN)*2) from absorbance: Sample calculation for the equilibrium [Fe3+]: Sample calculation for the equilibrium (SCN): Sample calculation for keq
3. Determine the precision of your results for Ko by calculating the standard deviation Keg di (Keq - mean) d2 Test Tube No. 6 7 8 9 10 Mean value of Keg Sum of d? (Ed) Σ(d,) Standard deviation n-1 My Keg value should be reported as 4. Performing this experiment at 450 nm will guarantee the maximum sensitivity because the absorption spectrum of Fe(SCN)*2 peaks at 450 nm. Let's assume, for some reason, you have performed the experiment at 400 nm or 500 nm where the absorbance is not zero, and determined the Keg value according to the procedure provided for this experiment. Would the same value of Keq be obtained at one of these wavelengths? If it can be, what will be the effect on its precision?
no Experiment 1084–06: Determination of an Equilibrium Constant out o de OS og Bolni nolu s volensi Sood aeontheb sta Purpose ons ronisl sito ES edil ola bue Determine the equilibrium constant for the formation of Fe(SCN)2 using tot ilme 6 9 anool) ligo OS spectroscopic measurement. A Beer's Law plot will be constructed to determine SE - nieze the concentration of the complex by exploiting Le Chatelier's principle. noise alusot edini osce bobivog Background Information Let us exploit Le Chatelier's principle which Chemical equilibrium can be established in two states that a reaction can be pushed to the forward different settings: homogeneous equilibria among direction if more reactants are added. If we add an substance in solution and heterogeneous equilibria overwhelming amount of SCN-, we can make between substances in different phases (solid, almost all Fet to form Fe(SCN)2 If enough SCN- gas, and solution). In any circumstances, a is present, the amount of Fe(SCN)2 shall be chemical reaction rarely proceeds to completion, essentially identical to the starting amount of Fe** rather many reactions are reversible. Now we know how to prepare the known concentration of Fe(SCN)*2 to construct the Beer's In a reversible reaction, the initial high reactant Law plot. concentrations cause rapid forward reaction. As Once we have determined the Beer's Law the forward reaction proceeds, concentration of constant, k, we can determine the equilibrium reactant(s) decrease and the rate of the forward concentration of Fe(SCN)2 in solutions containing reaction decreases. At the same time, the substantial amount of both reactants. We know increasing concentration of product(s) causes the the initial concentrations of reactants from the rate of reverse reaction to increase. Eventually, concentrations and volumes of the stock solutions the rate of forward reaction becomes the same as you used and the final volume. We also know the the reverse reaction: the reaction has reached the concentration of the product from the Beer's Law state of dynamic equilibrium. plot. Now we have all the information to calculate the equilibrium constant Keg which doesn't change The equilibrium constant, Keg, for a given with concentrations. This should become apparent reaction stays constant regardless of initial as the experiment is performed using several conditions, but it changes with temperature. different initial concentrations. Therefore, the temperature should be specified whenever an equilibrium constant is given. In this experiment, the reaction between Fe+3 and SCN- will be examined. The product is a complex ion that has a coordinate covalent bond (both electrons for the bond are provided by one substance) between Fe+3 and SCN- Fe+3(aq) + SCN-(aq) 5 Fe(SCN)*2(aq) [Fe(SCN)*21 With Keq = [Fe+'][SCN) Procedure Getting started 1. Work with a partner. 2. Obtain 5 large test tubes and 5 matching rubber stoppers. Wash, rinse and dry the tubes and the stoppers. 3. Obtain 5 Mohr pipets and 100 mL volumetric flask. Rinse them with distilled water. +3 4. Mark each tube with an ID number (1 to 5). 5. All solutions will be discarded into the waste container in the fume hood. The Fe(SCN)*2 solution displays an intense bloody red color against almost colorless reactants. We can take advantage of this color to determine the concentration of Fe(SCN)*2 in the solution using spectroscopic technique (Beer's Law, A = kc). To construct the Beer's Law plot, we must measure the absorbance of a series of solutions with known amount of Fe(SCN)2. However, we will have to keep in mind that Fe(SCN)*2 is an active participant in the equilibrium. Even if we dissolve known amount of Fe(SCN)2, it will dissociate to reach equilibrium causing the concentration of Fe(SCN)*2 to be less than what was originally intended. Using Spec 20 Spectrometer 1. Turn on Spec 20 spectrometer to warm it (at least 15 minutes). 2. Set wavelength knob to desired wavelength. 3. Make sure the sample chamber is closed. 4. Set Mode to % transmittance. -39-
4 Insert the rubber stoppers. Mix each solution thoroughly. 5. Transfer a solution into a Spec 20 cell (fill up to 2/3 of the length) and measure the absorbance of the solution at 450 nm. Record and plot the data. 6. Calculate the k value by linear regression - use provided space in the results section. 5. Using the zero knob, set the transmittance to *0* Now, Spec 20 will consider this situation as the complete darkness. 6. Insert a Spec 20 cell (looks like a small test tube) filled with distilled water (reference) in the sample chamber 7. Using 100 % knob, set the transmittance to 100. The concentration of the substance in the solution at this point is O and the light is 100% transmitted. 8. Remove the reference. 9. Set Mode to Absorbance, insert your sample and read the number displayed. This is your absorbance for the sample. 10. You have to perform steps 2-8 whenever the wavelength setting is changed. Otherwise, keep the Mode at Absorbance and just replace your sample to read off the absorbance value. 2.0 Determining k in Beer's Law CAUTION: Do NOT put the pipet directly into the stock bottle. Such practice will change the concentration of the stock solution and make it unusable. Instead, obtain a small beaker to take some stock solution to your bench. 1. Rinse a pipet with a 0.0025 M solution of Fe(NO3)2 (in 0.1 M HNO3). Use this Mohr pipet to transfer exactly 4 mL of the Fe(NO3)3 solution to a 100-mL volumetric flask. Add distilled water till the bottom of the meniscus coincides with the marking on the flask. Use a disposable dropper if needed. Mix the solution thoroughly. 2. Label 3 pipets with diluted Fe(NO3)3, 1 M KSCN and 0.1 M HNO3 and rinse with corresponding solutions. 3. Using the pipets, prepare following 5 solutions (Table 6.1) in the 5 cleaned and dried large test tubes. All amounts are given in mL. Table 6.1 Composition of solutions for Beer's Law. Test Tube Diluted 0.1 M 1 M KSCN number Fe(NO3)3 HNO3 1 1.0 5.0 4.0 Determining the equilibrium constant NOTE: This part of experiment requires. 1. 0.0025 M Fe(NO3)3 solution, NOT the diluted solution you prepared. 2. 0.0025 M KSCN solution, NOT 1 M. 1. Wash and dry pipets, test tubes and rubber stoppers. Renumber the test tubes (6 to 10). 2. Prepare the five solutions shown in Table 6.2. Use a properly rinsed Mohr pipet for each solution. Table 6.2 Composition of solutions for determining the equilibrium constant Test Tube 0.0025 M 0.0025 M 0.1 M number Fe(NO3)3 KSCN HNO3 6 1.0 1.0 5.0 7 1.0 2.0 4.0 8 1.0 3.0 3.0 9 2.0 2.0 3.0 10 3.0 2.0 3. Stopper the tube and mix each solution thoroughly. 4. Measure the absorbance of each solution at 450 nm. Record your data. 5. Calculate the concentration of Fe(SCN)2 in each solution using the absorbance and the Beer's Law. 6. Calculate the initial concentration of Fe(NO3)3 and KSCN using the table given above. 7. Calculate the final concentration of Fe(NO3)3 and KSCN using the data from step 6 and stoichiometry of the reaction. 8. Calculate the equilibrium constant for each solution. Calculate the mean value of Ke with its standard deviation. 9. Discard all solutions into the proper waste container. 2 2.0 5.0 3.0 2.0 3 5.0 3.0 4 4.0 5.0 1.0 5 5.0 5.0 0 -40-