Daily Archives: March 7, 2019

Measuring Keq Chemistry Lab

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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 0
B

 

 10.0 mL 1.0 mL 14.0 mL 0.00008 0.350
C

 

 10.0 mL 1.5 mL 13.5 mL 0.00012 0.514
D

 

 10.0 mL 2.0 mL 13.0 mL 0.00016 0.702
E

 

 10.0 mL 2.5 mL 12.5 mL 0.00020 0.895
F

 

 10.0 mL 3.0 mL 12.0 mL 0.00024 1.130

EQUATION:                 y=4529x                                                          R2__0.9957_____

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 0 0 0
II

 

 5.0 mL 1.0 mL 4.0 mL 0.001 0.0002 0.162 0.000036
III

 

 5.0 mL 2.0 mL 3.0 mL 0.001 0.0004 0331 0.000073
IV

 

 5.0 mL 3.0 mL 2.0 mL 0.001 0.0006 0.508 0.000112
V

 

 5.0 mL 4.0 mL 1.0 mL 0.001 0.0008 0.612 0.000135
VI

 

 5.0 mL 5.0 mL 0.0 mL 0.001 0.0010 0.796 0.000176

 

* To be determined from the standard graph equation.

ANALYSIS:

  1. Use your graph equation to calculate the equilibrium concentrations of FeSCN+2.
  2.  Prepare and ICE chart for each test solution (II – VI) and calculate the value of Keq for each of your 5 tests solutions.

 ICE CHART II

Test Solution

Keq =227.7

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0002 0
C

 

 -0.000036 -0.000036 0.000036
E

 

 0.000964 0.000164 0.000036

ICE CHART III

Test Solution

Keq =240.8

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0004 0
C

 

 -0.000073 -0.000073 0.000073
E

 

 0.000927 0.000327 0.000073

 

ICE CHART IV

Test Solution

Keq =258.5

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0006 0
C

 

 -0.000112 -0.000112 0.000112
E

 

 0.000888 0.000488 0.000112

 

ICE CHART V

Test Solution

Keq =234.7

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.0008 0
C

 

 -0.000135 -0.000135 0.000135
E

 

 0.000865 0.000665 0.000135

 

ICE CHART VI

Test Solution

Keq =259.2

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 0.001 0.001 0
C

 

 -0.000176 -0.000176 0.000176
E

 

 0.000824 0.000824 0.000176

 

CONCLUSION AND EVALUATION:

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

Yes.  Although there is some difference in the Keq, they are close enough to assume that equilibrium concentration does not affect the Keq.

  1. Calculate the average value of Keq from 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:

Avg Keq=244.2

% difference = (|experimental value – reported value|) x 100%

Reported value

 

% difference = (|244.2-280|)  x 100 = 12.8%

280