Ap Lab #17 - The proper and detailed lab reports for buffered solution PDF

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Lab Report #17: Determination of Keq for FeSCN

Franklin Wang, Aron Huang AP Chem. Lab (Period A) 25 February 2020

Purpose: The purpose of the experiment is to calculate the equilibrium constant for the reaction iron (III) ions with thiocyanate ions.

Theory: In this experiment, several concepts and equations are used to help determine the Keq at the equilibrium, for example, chemical equilibrium, equilibrium constant, complex-ion reaction, and colorimetry. Chemical equilibrium occurs during the reaction proceeds essentially to completion. This can be shown by one equation: aA+bB ⇌ cC+dD. Mathematically, the equilibrium constant expresses it by the equation, where the square brackets refer to the molar concentrations of the concentrations and products at equilibrium, Keq=[C]c[D]d/[A]a[B]b. The equilibrium constant is the constant at a particular temperature for any reversible chemical reaction as the special ratio of reactants and products. Complex ions are ions with a central metal ion bonded to one or more molecules or ions, and FeSCN2+ is a good example of the complex ion in this reaction. By this way, the reaction equation can be written as Keq= [FeSCN2+] / [Fe3+][SCN-]. A colorimeter is a special sensor or instrument which can be used to measure the absorbance of the light by the red ions. The concentration of complex ions is proportional to the intensity of the red color. The more intense the red color, the greater the absorbance.

Procedure:

Data Table: Reference Solutions Temperature

22°C

Sample

[FeSCN2+]

Absorbance #1

Absorbance #2

Reference solution #1

0.00004

0.455

0.461

Reference solution #2

0.00006

0.799

0.696

Reference solution #3

0.00008

0.988

0.965

Reference solution #4

0.0001

1.265

1.220

Reference solution #5

0.0012

1.047

1.394

Test Solutions

Temperature

22°C

Sample

[Fe3+]*

[SCN-]*

Absorbance #1

Absorbance #2

Test Solution #6

0.001

0.0002

0.624

0.582

Test Solution #7

0.001

0.0004

0.357

0.352

Test Solution #8

0.001

0.0006

0.852

0.818

Test Solution #9

0.001

0.0008

1.097

1.07

Test Solution #10

0.001

0.001

1.356

1.336

Result Table Equations used: ● [Fe3+]eq= [Fe3+]initial - [FeSCN2+]eq ● [SCN-]eq= [SCN-] initial - [FeSCN2+]eq ● Keq= [FeSCN2+]/[Fe3+][SCN-] Sample

[FeSCN2+]eq

[Fe3+]eq

[SCN-]eq

Keq

Test Solution #6

4.96*10-5

9.5*10-4

1.504*10-4

347.1

Test Solution #7

2.68*10-5

9,732*10-4

3,732*10-4

73.78

Test Solution #8

6.91*10-5

9.309*10-4

5.309*10-4

139.8

Test Solution #9

9.0*10-5

9.1*10-4

7.1*10-4

139.3

8.88*10-4

8.88*10-4

142

Test Solution #10 1.12*10-4

Average value

168.4

Average deviation

71.28

Calculations: Reference solutions #1-#5 RS#1: M1V1=M2V2 M2=(M1V1)/V2

M1=0.0002M V1=2ml V2=10ml M2=[FeSCN2+]=(0.0002*2)/10 = 4*10-5M

Test Solutions #6-#10 TS#6: M1V1=M2V2 M1=0.0002M V1=5ml V2=10ml M2=[Fe3+]=1*10-3M M1=0.002M V1=1ml (add 1ml more every time) V2=10ml M2=[SCN-]=(0.002*1)/10ml = 2*10-4

Result #6-#10 (by using the linear function) y=11719x+0.0426 (y=absorbance, x=[FeSCN2+]eq) #6: 0.624=11719x+0.0426 x=4.96*10-5 [Fe3+]eq=[Fe3+]-[FeSCN2+]=0.001-4.96*10-5=9.5*10-4 [SCN-]eq=[SCN-]-[FeSCN2+]=0.0002-4.96*10-5 =1.504*10-4

Keq=[FeSCN2+] / [Fe3+][SCN-] = 4.96*10-5 / (9.5*10-4)(1.504*10-4) = 347.1 Average value= (347.1=73.78+139.8+139.3+142)/5 = 1684. Average deviation (the average of the difference between the mean and the each absorbance) =(178.7+94.6+28.6+28.1+26.4)/5 =71.28

Graphs:

Post Lab Questions: Questions #1 to #8 are answered in the sections of “graphs” and “calculations.” Questions #9 and #10 are answered in the conclusion (the second and the third paragraphs).

Conclusion: The purpose of this experiment “calculating the equilibrium constant for the reaction iron (III) ions with thiocyanate ions” is accomplished. The precision does indicate that the equilibrium constant is indeed a “constant” for this reaction.The calculated number is supposed to be a small percentage of the mean Keq constant. The calculated Keq for each solution is supposed to be very close to one another, and if it were not for experimental error, they would be the same number (Post Lab #9). As well as learning to use several laboratory equipment, there are several possible sources of error in this experiment. The first one is the volumes of the solutions. The second error is the molarity of the solutions. If the molarity of the solution is too big for the experiment, this could result in the wrong data. The third one is the cuvettes might not match, meaning they give different readings for the experimenters to observe and record. This error would lead to an inaccurate precision. The last error is During the experiment the spectrophotometer used to measure the absorbance of the solutions. Although the spectrophotometer was zeroed at the beginning of each test, the machine did not remain perfectly zeroed throughout the experiment. The absorbance measurements of the reference solutions and the test solutions may not have been accurate (Post Lab #10)....