Part I: Preparation of a standard absorption curve for FeSCN+2

Standard 0.20M Fe(NO3)3 0.0020 M KSCN 0.100M HNO3 [FeSCN+2] Absorbance
A

 

 10.0 mL 0.0 mL 15.0 mL 0 M 0
B

 

 10.0 mL 1.0 mL 14.0 mL 0.00008 M 0.293
C

 

 10.0 mL 1.5 mL 13.5 mL 0.00012 M 0.482
D

 

 10.0 mL 2.0 mL 13.0 mL 0.00016 M 0.650
E

 

 10.0 mL 2.5 mL 12.5 mL 0.0002 M 0.845
F

 

 10.0 mL 3.0 mL 12.0 mL 0.00024 M 1.040

EQUATION:                  y = 4353.9x – 0.0289                                         R2: 0.9964

Part 2: Measuring Equilibrium

Test Solution 0.0020 M Fe(NO3)3 0.0020 M

KSCN

 0.10 M

HNO3

 Initial [Fe+3] Initial [SCN–] Absorbance Equilibrium

[FeSCN+2]*
I

 

 5.0 mL 0 5.0 mL 0.001 M 0 0 0
II

 

 5.0 mL 1.0 mL 4.0 mL 0.001 M 0.0002 0.204 0.0000535
III

 

 5.0 mL 2.0 mL 3.0 mL 0.001 M 0.0004 0.342 0.0000852
IV

 

 5.0 mL 3.0 mL 2.0 mL 0.001 M 0.0006 0.518 0.000126
V

 

 5.0 mL 4.0 mL 1.0 mL 0.001 M 0.0008 0.802 0.000191
VI

 

 5.0 mL 5.0 mL 0.0 mL 0.001 M 0.001 1.025 0.000242

 

* To be determined from the standard graph equation.

ANALYSIS:

  1. Use your graph equation to calculate the equilibrium concentrations of FeSCN+2            

ii) 0.0000535

iii) 0.0000852

iv) 0.000126

v) 0.000191

vi) 0.000242

 

  1.  Prepare and ICE chart for each test solution (II – VI) and calculate the value of Keq for each of your 5 tests solutions.
ii)

Keq = 384

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0002 0
C

 

 – 0.0000535 – 0.0000535 0.0000535
E

 

 0.000947 0.000147 0.0000535

 

iii)

Keq = 296

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0004 0
C

 

 – 0.0000852 – 0.0000852 0.0000852
E

 

 0.000915 0.000315 0.0000852

 

iv)

Keq = 304

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0006 0
C

 

 – 0.000126 – 0.000126 0.000126
E

 

 0.000874 0.000474 0.000126

 

v)

Keq = 387

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0008 0
C

 

 – 0.000191 – 0.000191 0.000191
E

 

 0.000810 0.000610 0.000191

 

vi)

Keq = 421

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.001 0
C

 

 – 0.000242 – 0.000242 0.000242
E

 

 0.000758 0.000758 0.000242

 

CONCLUSION AND EVALUATION:

  1. Comment on your Keq values.   Do your results convince you that Keqis a constant value regardless of the initial concentrations of the reactants?  Why or why not?

Even though all the Keqvalues of the five solutions weren’t exactly the same they were close enough to assume that Keq  is a constant value.

 

  1. Calculate the average value of Keqfrom your five trials. The actual value of Keq  for this reaction at 25oC is reported as 280.  Calculate (should you use all of your values?) the percent difference of your average value from the reported value:

% Difference = (experimental value – reported value)  x 100%

Reported value

 

Average Keq: 358.4

% Difference = (358.4 – 280)x 100% = 28%

280