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GENERAL CHEMISTRY

GENERAL CHEMISTRY Page 1 of 5
DETERMINATION OF AN EQUILIBRIUM CONSTANT BY
SPECTROPHOTOMETRY
Determination of an equilibrium constant / July 2014
OBJECTIVE:
The objective of the experiment is to determine the equilibrium constant, Kc, for the reaction of iron with thiocyanate.
BACKGROUND:
Iron (III) nitrate and sodium thiocyanate dissociate in aqueous media. The resulting ions form an equilibrium with the complex ion, thiocyanatoiron(III), as follows:
Fe(NO3)3 (aq) + 3KSCN (aq) ï‚® Fe(SCN)3 (aq) + 3KNO3 (aq)
Fe+3 + SCN-1  FeSCN+2
The thiocyanatoiron(III) ion in solution presents a “rust color”, the intensity of which is measurable using spectrophotometry. At proper concentrations additional Iron(III) nitrate in measured portions drives the equilibrium reaction to the right further intensifying the “rust color”. Initial concentrations are known. Reactant concentrations are varied and measured by the absorbance of light as the rust color intensifies.
A spectrophotometer is an instrument used to study the response of a solution to light.
According to Beer’s Law, the intensity of a colored compound is directly proportional to its concentration:
Absorption (A) = bc
where  = molar absorptivity (proportionality constant with a specific
value for each absorbing species at a given wavelength)
b = path length (thickness of absorbing solution, width of cuvette)
c = concentration of absorbing species
Two scales are used, one displays % Transmittance (%T) and the other displays Absorbance(A). The relationship between transmittance and absorbance is as follows:
Absorbance = 2.00 – log10 (%Transmittance)
The wavelength at which percent transmittance is a minimum (absorbance is a maximum) is the wavelength at which the solution is the most sensitive. This wavelength is defined as the analytical wavelength.
GENERAL CHEMISTRY Page 2 of 5
DETERMINATION OF AN EQUILIBRIUM CONSTANT BY
SPECTROPHOTOMETRY
Determination of an equilibrium constant / July 2014
Absorbance is also related to the concentration of the absorbing complex ion, [FeSCN+2], by a simple relationship:
A
[ FeSCN+2 ] = 
b
By material balance the initial concentrations are related to the equilibrium concentrations of the ions as follows:
[ Fe+3 ] = [ Fe* ] – [ FeSCN+2 ]
[ SCN-1 ] = [ SCN* ] – [ FeSCN+2 ]
( Molarity of Fe(NO3)3 Solution) x (Volume (L) Fe(NO3)3 Solution Used )
[ Fe* ] = 
Total Volume(L) of Reaction Solution after Adding Fe(NO3)3
( Molarity of KSCN Solution) x (Volume (L) KSCN Solution Used )
[ SCN* ] = 
Total Volume(L) of Reaction Solution after Adding Fe(NO3)3
The Kc for the equilibrium reaction is:
[ FeSCN+2 ]
Kc = 
[ Fe+3 ] [ SCN-1 ]
By substitution:
( A / b )
Kc = 
{ [ Fe* ] – A / b } { [ SCN*] – A / b }
Although not an easy task,
A A { [ Fe*] + [ SCN* ] } Kc
 = –  + bKc
[ Fe* ] [ SCN* ] [ Fe* ] [ SCN* ]
is an acceptable mathematically solution for the equilibrium constant expression Kc. The relationship follows a linear format, y = mx + b, where the slope, m, is equal to Kc.
GENERAL CHEMISTRY Page 3 of 5
DETERMINATION OF AN EQUILIBRIUM CONSTANT BY
SPECTROPHOTOMETRY
Determination of an equilibrium constant / July 2014
CAUTIONS: Laboratory Safety Rules are to be followed. Material Safety Data Sheets (MSDS) are posted.
PROCEDURES:
Solutions and Equipment:
0.002M KSCN 10 mL Transfer Pipet
2.0M HNO3 25 mL Transfer Pipet
0.10M Fe(NO3)3 Glass Stirring Rod
Vernier SpectroVis Calibrated Droppers
Cuvettes Cuvettes Rack
Calibrated Droppers 100 mL Graduated Cylinder
Beakers
I. PREPARATORY: Vernier SpectroVis
1. Connect the Vernier SpectroVis Plus to a powered USB port.
2. Start the Logger Pro software.
II. DETERMINATION OF ANALYTICAL WAVELENGTH
Add to a clean 50 ml beaker the following solutions:
5 mL of 0.002M KSCN
10 mL of 0.10 M Fe(NO3)3
5 mL of 2.0 M HNO3
1. Fill a cuvette 3/4 full of 2M HNO3 solution. Calibrate the Vernier SpectroVis Plus. Click “Experiment”, then click “Calibrate”. Allow SpectroVis to warm-up, insert HNO3 solution, “Finish Calibration”.
2. Fill a cuvette 3/4 full of the analytical wavelength calibration solution prepared above. Place curvette in SpectroVis, click “Collect”. When graph appears, click “Stop”.
3. Click “Stat” and read the analytical wavelength (maximum value).
III. DETERMINATION OF EQUILIBRIUM CONSTANT
GENERAL CHEMISTRY Page 4 of 5
DETERMINATION OF AN EQUILIBRIUM CONSTANT BY
SPECTROPHOTOMETRY
Determination of an equilibrium constant / July 2014
1. Add to a clean 250 ml beaker the following solutions:
10 ml of 0.002M KSCN
25 ml of 2.0 M HNO3
65 ml of DI water
2. Click “Config Spec Data” icon and set to “Measurement versus Concentration”. Set the Analytical Wavelength determined above and click “OK.”
3. A dialogue box will appear asking if latest run is to be saved, click “YES.”
4. Go to Experiment, select “change data”, then “spectrometer”, set collection units to “% Transmission.”
5. Add to the beaker of solutions prepared in step 1, 1.0 mL of 0.10 M Fe(NO3)3 and mix with glass stirring rod. Total volume = 101 mL.
6. Fill a cuvette 3/4 full of reaction solution from step 5.
7. Click “Collect”, click “Keep” and enter the number “1”. Return solution to the original 250 ml beaker holding the initial reaction solution.
NOTE: DO NOT DRY THE CUVET.
8. Add another 1 mL sample of the 0.10M Fe(NO3)3 solution to the 101 mL solution giving a total volume of 102 mL. Stir and mix with a glass rod.
9.
10. Again fill a cuvette 3/4 full of reaction solution and measure the % Transmittance, click “Keep” and enter the number “2”. Return this sample to the original beaker and repeat 1mL additions of Fe(NO3)3 until 10 measurements have been made.
11. Repeat Steps 1 through 6 to obtain a total of FIVE sets of data.
GENERAL CHEMISTRY Page 5 of 5
DETERMINATION OF AN EQUILIBRIUM CONSTANT BY
SPECTROPHOTOMETRY
Determination of an equilibrium constant / July 2014
REQUIREMENTS: Follow the Laboratory Notebook format for report writing. Use regression analysis to determine the experimental values for Kc. Using your statistical package, report the Average and Range of (Kc) to a 99% probability.
Compare your sample average value of Kc with the published value of 136.2  3.5*.
* Value for Kc is not corrected for hydrolysis of Fe+3
REFERENCES:
“The Stability and Light Absorption of the Complex Ion FeSCN++”; Hanry S. Frank and Robert L. Oswalt, University of California, 1947
“Determining an Equilibrium Constant Using Spectrophotometry”; Norman J. Hudak, Willamette University
“Determination of an Equilibrium Constant by Spectrophotometry, The Iron (III) Thiocynate Complex Ion”; Roberts, Hollenberg and Postma, General Chemistry in the Laboratory

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