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Experiment 9: Determination of an Equilibrium Constant Aiza Kesodia General Chemistry II (CHM-152-AC41) Community College of Allegheny County August 4, 2020

2 Introduction: A chemical reaction is started with reactants which react to yield products. Most of the time the reactants are completely used up to make products, however, sometimes they do not completely turn into products. There is an equilibrium between the concentration of reactants and products. At equilibrium, the reactants turn into product and the products decompose into reactants at the same rate. This ratio of the products to reactants at equilibrium is represented by the equilibrium constant (Kc). Given a general chemical reaction: aA + bB ⇌ cC + dD, the equilibrium constant can be found by: �� = [�]�[�]� / [�]�[�]�. In this experiment, iron(III) ion reacted with thiocyanate ion which produced iron (III) thiocyanate: ��(��)�+ + ���(��)− ⇄ ����� (��)�+. FeSCN2+ is a complex ion, which consists of a central atom/ion attached to a group called ligands. The ligands can be neutral atoms or anions. The central ion is the Fe3+ ion, and the ligand is the SCN - ion (thiocyanate). Since the complex ion was dissociated reversibly, this process achieved equilibrium. Using a spectrophotometer, the absorbance of FeSCN2+ was measured at different concentrations. The volumes and concentrations of the solutions containing Fe3+ and SCN when mixed together, would be used to calculate their initial concentrations. Since FeSCN2+ ion is strongly colored, the complementary colors must be absorbed by the ion and the ion absorbed a specific wavelength of light. The amount of light absorbed by the ion in solution was proportional to the concentration of the ion. In order to measure the amount of light absorbed by a solution, a spectrophotometer was used to allow light of a chosen wavelength to pass through the solutions and it compared the amount of light exiting the solution to the amount of light entering it. From this, the absorbance was found. The relationship between absorbance and concentration is: �=�∙�, where � is the absorbance, � is a proportionality constant, and � is the concentration of

3 the solution. The value of k was determined by measuring the absorbance of a solution when the concentration of FeSCN2+ was known. Stoichiometry was used to calculate the amount of FeSCN2+ that would be formed from the amount of SCN - ; then it was converted to the concentration of FeSCN2+ . These values were plugged these into the equation to solve for k. Once the value of k was found, the absorbance of other solutions was determined. Afterwards, all the values for A and k were plugged to solve for C, which was the equilibrium concentration of the complex ion in these solutions. Finally, from the values of the initial concentrations of Fe3+/SCN- and the value of the equilibrium concentration of FeSCN 2+, the value of the equilibrium constant was calculated. Procedure: To begin the lab, 5 x 10-2 M 50 mL of Fe(NO3)3 solution was added to a flask and labeled Standard Fe(NO3)3. Four containers and four Erlenmeyer flasks were labeled A, B, C, and D; while the volumetric flasks were labeled dilution1, dilution2, and dilution3. Each of the labeled Erlenmeyer flasks had 10.0mL 5.00x10-4M of KSCN. In flask A, 10.0mL of the standard was mixed with the KSCN and it was gently swirled. A 10.0mL portion of the standard was added into the first volumetric flask. 5mL of 2.5M HNO3 was also added. Distilled water was added exactly to the line on the flask. The stopper was placed on the flask and the product was shaken a couple times. The pipette with the dilution was rinsed 3 times. 10.0mL of dilution 1 was added to flask B and swirled. These steps were repeated for the next 2 dilutions. Afterwards, all 4 flasks were filled with the new dilution mixtures and 3 dilutions have been created. After the spectrophotometer was turned on and a set of sample tubes were obtained, a blank was prepared: a tube filled with distilled water and which is then “set” to read the increase in absorbance. Then a sample tube was prepared by filling another sample tube halfway with solution A. On the

4 spectrophotometer the desired wavelength was set at 450 nm. The display was set to 0% transmittance and the blank was placed the sample compartment. So then, the Transmittance/Absorbance Control was adjusted until the display shows 100% T. After, the blank was removed and the sample tube A was inserted. The Mode was set on absorbance and the value of the absorbance was recorded. These steps were repeated for flasks B to D. They all were recorded, however, after each sample the blank was inserted to zero it out. For the cleanup, the sample tubes were washed with distilled water and returned to the box. Data and Calculation: Calculation of Reactant Concentrations: Solution A Total Volume 20.00mL 2.5x10-2 M Fe(NO3)3 2.5x10-4 M KSCN

of

moles FeSCN −2 =M ×V ( ¿ L ) =5 ×10−4 × 0.01 L=5 ×10−6 mole s

5 −2 Molarity ( M ) FeSCN =

A=kC → k=

5 ×10−6 moles moles −4 = =2.5 ×10 M 0.02 L totalVolume ( L )

A 0.85 = =340 0 C 2.5 ×10−4 M C B=

Calculation of the Concentration of the Complex Ion:

A 0.73 = =2.15 ×10−4 M k 3400

A 0.54 CC = = =1.59 ×10−4 M k 3400 A 0.37 =1.09 ×10−4 M CD= = k 3400 Calculation of Kc: Sample B:

Initial Change Equilibrium

Fe3+ (aq) 2x10-2 -x 1x10-2 -x

SCN- (aq) 2.5x10-4 -x 2.5x10-4-x

FeSCN2- (aq) 0 +x x

2+¿ FeSCN ¿ ¿ 3+¿ ¿ Fe ¿ −¿ ¿ SCN ¿ ¿ ¿ K C=¿ ¿

2.15 ×10−4 ( 2×10−2−2.15 × 10−4) ( 2.5 ×10−4− 2.15 × 10−4 )

K C =310.5

Sample C:

Initial

Fe3+ (aq) 8x10-3

SCN- (aq) 2.5x10-4

FeSCN2- (aq) 0

6 Change Equilibrium

-x 8x10-3 -x

-x 2.5x10-4-x

+x x

SCN- (aq) 2.5x10-4 -x 2.5x10-4-x

FeSCN2- (aq) 0 +x x

2+¿ FeSCN ¿ ¿ 3+¿ ¿ Fe ¿ −¿ ¿ SCN ¿ ¿ ¿ K C=¿ −4

¿

1. 59× 10 −3 ( 8× 10 −1.59 ×10−4)(2.5 ×10−4−1 .59 × 10−4 )

K C =¿

222.8

Sample D:

Initial Change Equilibrium

Fe3+ (aq) 3.2x10-3 -x 3.2x10-3 -x

2+¿ FeSCN ¿ ¿ 3+¿ ¿ Fe ¿ −¿ ¿ SCN ¿ ¿ ¿ K C=¿ ¿

1.0 9 ×10−4 ( 3.2 × 10−3−1.0 9 × 10−4)( 2.5 ×10−4−1.0 9 × 10−4)

K C =¿ 250.1

Average K c =

310.5 + 222.8 +250. 1 =261.1 3

7 Absorption versus Wavelength for the FeSCN2+ complex ion:

Results and Discussion: The name of the formula that relates absorbance to concentration of a solution is called the Beer-Lambert Law and there are alternate formulations: a=k*c. The Erlenmeyer flask was initially rinsed with Fe(NO3)3 when preparing the dilute solutions because it needed to maintain its concentration. The Wavelength Control was set for 450 nm because the amount of light absorbed by the ion in solution needed to be proportional to the concentration of the ion. Also, 450 nm is the wavelength of maximum absorbance by FeSCN2+(aq) .The value of K indicated the equilibrium ratio of products to reactants. In a mixture, a "favored side" meant that one side of the equation has higher numbers of moles and higher concentrations than the other side. 10.00 mL of 5.00 x 10-2 M Fe(NO3)3 solution is added to a 25.00 mL volumetric flask. 5 mL of 2.5 M HNO3 is then added, followed by distilled water to the line on the neck of the flask. The solution is then mixed by inverting the flask several times. The concentration of the Fe(NO3)3 in the flask

8 is 2.0 x 10-4 M. 10.00 mL of the solution prepared was then placed in an Erlenmeyer flask; 10.00 mL of 5.00 x 10-4 M KSCN is also added to the flask. A source of error could have been not calibrating the spectrophotometer in between each reading of the dilution. This would have led to higher or lower readings of absorption. Another possible error could be not cleaning the cuvette well enough before it was placed in the spectrophotometer, it would have given inaccurate percent transmittance values. The data from this lab showed that as concentration of FeNCS 2+ increased, the absorbance also increased. After calculating the absorbance values and the concentration of FeNCS2+, a graph was made comparing the concentration of FeNCS 2+ to the absorbance values. The equilibrium constant was determined to be 261.1 Conclusion: Using spectrometry, the equilibrium constant for the formation of iron(III) thiocyanate complex ion was determined. The objectives of this lab was to find the equilibrium constant based on the calculations and absorption readings from a spectrophotometer. If the lab were to be done again, the measurements should have been more carefully measured. LeChatelier’s principle took effect in this experiment because when more of a substance was added to the solution it tried to stabilize its reactions. Overall, the equilibrium constant, K, was the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their respective

stoichiometric

coefficients.

Measurement

of K involves

determination

of

concentrations in chemical equilibrium. Initial reactant concentrations were based on the known molarities of Fe3+ and SCN- in the reactant solutions and the total volume. The equilibrium concentration of Fe(SCN)2+ was determined from the measured absorbance of Fe(SCN)2+. Since all of the products were formed from the 1:1 ratio of Fe3+ and SCN-, the equilibrium concentration of Fe(SCN)2+ matched to the decrease in concentration of the reactants.

9

10 References: “Lab Experiment #13: The Equilibrium Constant.” YouTube, uploaded by Ali Hayek, 4 Feb. 2016, www.youtube.com/watch?v=xAcfl7tNnS0. Lyle, Ken. “Calculating Equilibrium Constants.” Chem.purdue, www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Calculating_Equilibrium_Const ants.htm....