Name
Period
Date
27. Determine the Equilibrium Constant for a Chemical
Reaction*
Driving Question
What is the equilibrium constant of the formation of iron(III) thiocyanate?
Background
In this experiment, you use a colorimeter to help determine the equilibrium constant for the formation of FeSCN 2+.
In dilute solutions, iron (III) nitrate, Fe(NO3)3(aq), and potassium thiocyanate, KSCN(aq), are completely dissociated.
When these two solutions are mixed, the following equilibrium is established:
Fe3+(aq) + SCN–(aq) ⇌ FeSCN2+(aq)
Pre-Lab Activity
Setting the stage for the activity
Of the five ions in solution, K+(aq), NO3– (aq), and SCN– (aq) are colorless, Fe3+(aq) is nearly colorless, and
FeSCN2+(aq) is deep red. Changes in the concentration of FeSCN2+(aq) are indicated by changes in the intensity of
the color of the solution.
The equilibrium expression for the reaction is
K eq 
[FeSCN2+ ]
[Fe3  ][SCN  ]
To calculate Keq for this reaction, you need to determine the molar concentration of Fe3+, SCN–, and FeSCN2+ at
equilibrium.
The relationship between electromagnetic absorption and concentration of the absorbing species is given by Beer's
law. Absorption of light is directly proportional to the distance that the light travels through an absorbing medium
and the molar concentration of the absorbing species.
A=ε×l×c
(1)
where
A = absorption
ε = absorptivity coefficient (M–1cm–1)
l = path length that light travels through the solution (cm)
c = molar concentration of the absorbing species (M).
The absorptivity coefficient ε is a proportionality constant, and its value depends both on the nature of the absorbing
species and on the wavelength of light chosen for the measurement. At a given wavelength and using a sample cell
of constant path length, absorption is directly proportional to concentration. Thus, a measurement of A can be used
to determine concentration. The red FeSCN2+ ion absorbs blue light and will be analyzed at 468 nm.
*
This is an AP Chemistry course recommended experiment.
1
Determine the Equilibrium Constant for a Chemical Reaction
Materials and Equipment
For each student or group:
 Data collection system
 Rubber pipet bulb
 Colorimeter and cuvette
 0.01 M Iron (Fe3+), 20 mL
 Extension cable
 0.00300 M Potassium thiocyanate (KSCN), 20 mL
 Beaker (2), 50-mL
 Kimwipes®
 Test tube (5),15-mL
 Deionized water, 40 mL
 Test tube rack
 Marker
 Graduated pipet (2), 10-mL
Procedure
After you complete a step (or answer a question), place a check mark in the box () next to that step.
Set Up
1.  Start a new experiment on the data collection system.
2.  Connect the colorimeter to the data collection system using a sensor extension cable. Close data studio and
open Sparkview. Click on “build”.
3.  Display Blue absorbance (468 nm) in a digit display. Click on the button displaying
and then “ok”.
4.  Label the first test tube “Blank” and the others from “1” to “4”.
5.  Prepare the solutions in the labeled test tubes by combining the prescribed amounts of the reactants and
water (see Table 1)
Table 1: Composition of solutions to study chemical equilibrium
2
Test Tube
1.00 × 10–2 M
Fe(NO3)3, acidic
(mL)
3.00 × 10–3 M
KSCN (mL)
Water (mL)
Blank
1.00
0.00
9.00
1
1.00
1.00
8.00
2
1.00
2.00
7.00
3
1.00
3.00
6.00
4
1.00
4.00
5.00
PS-2897B
Student Inquiry Worksheet
6.  Calibrate the colorimeter with the blank solution. To do this, rinse a cuvette with “blank” solution & empty.
Next fill the cuvette about 2/3 full. Wipe clean and dry and place it into the colorimeter. Press the green
button and the light will go on.
Light-tight cover
Calibration button
Colorimeter
Extension cable (optional)
Important: Always make sure that the cell (cuvette) is clean and dry on the outside before placing it into the colorimeter.
Collect Data
7.  Measure the absorbance of the four solutions following the steps below.
a. Rinse the cell with a small portion of the first solution and fill the cuvette two-thirds full. Wipe the
cuvette clean and dry and place it into the colorimeter. To display absorbance, click on the green
button.
b. After the reading stabilizes, record the absorbance.
c. Dispose of the solution and rinse the cell thoroughly with water.
8.  Repeat with the three other solutions.
3
Determine the Equilibrium Constant for a Chemical Reaction
Data Analysis
For your calculations, consider the product of absorptivity ɛ and the cell thickness l to be
ɛ × l = 5900 M–1
1.  Calculate and record in Table 2 the initial concentrations of Fe 3+ and SCN– in each test tube. MV=MV
2.  Calculate the concentration of the Fe(SCN)2+ ions from the absorbance measurements using Beer’s Law and
the above value of ɛ × l. Record the values in Table 2. [FeSCN]2+ =
𝐴
5900
3.  Calculate the equilibrium concentration of the Fe3+ and SCN– ions from the initial concentration of the ions
and the amount of ions used to establish the equilibrium concentration of FeSCN 2+. Record the values in
Table 2. [Initial] – [FeSCN]2+ = [Final]
4.  Calculate the equilibrium constant from the equilibrium concentration of the Fe(SCN) 2+, Fe3+, and SCN–
[𝐹𝑒𝑆𝐶𝑁]
ions. Record the values in Table 2. K = [𝐹𝑒][𝑆𝐶𝑁]
Table 2: Calculation of the equilibrium concentrations and the equilibrium constant
#
[Fe3+]0
(M)
[SCN–]0
(M)
A
[FeSCN2+]
(M)
[Fe3+]
(M)
[SCN–]
(M)
1
2
3
4
5.  Calculate the average equilibrium constant and record the value below.
Average value of Keq:
4
PS-2897B
Keq