07
Mar
2019

Measuring Keq

categories: Grade 12

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 8.00 x 10-5 0.371
C

 

 10.0 mL 1.5 mL 13.5 mL 1.20 x 10-4 0.514
D

 

 10.0 mL 2.0 mL 13.0 mL 1.60 x 10-4 0.674
E

 

 10.0 mL 2.5 mL 12.5 mL 2.00 x 10-4 0.880
F

 

 10.0 mL 3.0 mL 12.0 mL 2.40 x 10-4 1.040

EQUATION:      y = 4338.9x                                                                               R2: 0.9983

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 1.00 x 10-3 0 0 0
II

 

 5.0 mL 1.0 mL 4.0 mL 1.00 x 10-3 2.00 x 10-4 0.145 3.34 x 10-5
III

 

 5.0 mL 2.0 mL 3.0 mL 1.00 x 10-3 4.00 x 10-4 0.323 7.44 x 10-5
IV

 

 5.0 mL 3.0 mL 2.0 mL 1.00 x 10-3 6.00 x 10-4 0.502 1.16 x 10-4
V

 

 5.0 mL 4.0 mL 1.0 mL 1.00 x 10-3 8.00 x 10-4 0.676 1.56 x 10-4
VI

 

 5.0 mL 5.0 mL 0.0 mL 1.00 x 10-3 1.00 x 10-3 0.810 1.87 x 10-4

 

* 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.

SAMPLE ICE CHART

Test Solution II

Keq = 208

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 1.00 x 10-3 2.00 x 10-4 0
C

 

 -3.34 x 10-5 -3.34 x 10-5 +3.34 x 10-5
E

 

 9.67 x 10-4 1.67 x 10-4 3.34 x 10-5
Test Solution III

Keq =247

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 1.00 x 10-3 4.00 x 10-4 0
C

 

 -7.44 x 10-5 -7.44 x 10-5 +7.44 x 10-5
E

 

 9.26 x 10-4 3.26 x 10-4 7.44 x 10-5

 

Test Solution IV

Keq = 271

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 1.00 x 10-3 6.00 x 10-4 0
C

 

 -1.16 x 10-4 -1.16 x 10-4 +1.16 x 10-4
E

 

 8.84 x 10-4 4.84 x 10-4 1.16 x 10-4

 

Test Solution V

Keq =287

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 1.00 x 10-3 8.00 x 10-4 0
C

 

 -1.56 x 10-4 -1.56 x 10-4 +1.56 x 10-4
E

 

 8.44 x 10-4 6.44 x 10-4 1.56 x 10-4

 

Test Solution VI

Keq = 282

 Fe3+               +                SCN–                    ⇄            FeSCN2+
I

 

 1.00 x 10-3 1.00 x 10-3 0
C

 

 -1.87 x 10-4 -1.87 x 10-4 +1.87 x 10-4
E

 

 8.13 x 10-4 8.13 x 10-4 1.87 x 10-4

 

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?

Our Keq that we got for our first two solutions were off by quite a bit, but our last 3 solutions are pretty close to the reported value of 280. Regardless of initial concentration the last 3 were very similar so we think that the Keq is a constant value even though initial concentration of the reactants differ.

  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:

 

Average value of Keq = 259

Average value of Keq  (only using accurate values) = 280

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

Reported value

7.47% = (259 –280)  x 100%        (all values used)

= 280

0% = (280-280) x 100%                 (only accurate values used)

=280

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