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Determine the Value of an Equilibrium Constant by Complex Ion Formation

Module by: Mary McHale. E-mail the author

Summary: Determine the Value of an Equilibrium Constant by Complex Ion Formation by reacting Fe(III) with the thiocyanate ion

Determine the Value of an Equilibrium Constant by Complex Ion Formation

Objectives

In this laboratory you will:

  • Use MicroLab to take colorimetric measurements
  • Use Beer’s Law to measure the equilibrium concentration of a complex ion
  • Review Le Chatelier’s Principle
  • Calculate the equilibrium constant for the formation of a complex ion

 Grading

Your grade will consist of the following:

  • Pre-lab (10%)
  • Correctness and thoroughness of your observations and the answers to the questions on the report form (80%)
  • TA evaluation of lab procedure (10%)

Before Coming to Lab . . .

  • Complete the pre-lab exercise
  • Read the introduction and any related materials provided to you

Introduction

When two reactants are mixed, the reaction typically does not go to completion. Rather, they will react to form products until a state is reached whereby the concentrations of the reactants and products remain constant at which point the rate of formation of the products is equal to the rate of formation of the reactants. The reactants and products are then in chemical equilibrium and will remain so until affected by some external force. The equilibrium constant KcKc size 12{K rSub { size 8{c} } } {} for the reaction relates the concentration of the reactants and products.

In our experiment we will study the equilibrium properties of the reaction between iron (III) ion and thiocyanate ion:

Fe3+(aq)+SCN(aq)FeSCN2+(aq)Fe3+(aq)+SCN(aq)FeSCN2+(aq) size 12{ ital "Fe" rSup { size 8{3+{}} } \( ital "aq" \) + ital "SCN" rSup { size 8{ - {}} } \( ital "aq" \) rightarrow ital "FeSCN" rSup { size 8{2+{}} } \( ital "aq" \) } {} Equation 1

When solutions containing Fe3+Fe3+ size 12{ ital "Fe" rSup { size 8{3+{}} } } {} ion and thiocyanate ion are mixed, the deep red thiocyanatoiron (III) ion ( FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {}) is formed. As a result of the reaction, the starting concentrations of Fe3+Fe3+ size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and SCNSCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} will decrease: so for every mole of FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} that is formed, one mole of Fe3+Fe3+ size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and one mole of SCNSCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} will react. The equilibrium constant expression KcKc size 12{K rSub { size 8{c} } } {}, according to the Law of Chemical Equilibrium, for this reaction is formulated as follows:

[FeSCN2+]/[Fe3+][SCN]=Kc[FeSCN2+]/[Fe3+][SCN]=Kc size 12{ \[ ital "FeSCN" rSup { size 8{2+{}} } \] / \[ ital "Fe" rSup { size 8{3+{}} } \] \[ ital "SCN" rSup { size 8{ - {}} } \] =K rSub { size 8{c} } } {} Equation 2

Remember, square brackets ([]) are used to indicate concentration in mol/liter, i.e., molarity (M).

The value of KcKc size 12{K rSub { size 8{c} } } {} is constant at a given temperature. This means that mixtures containing Fe3+Fe3+ size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and SCNSCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} will react until the above equation is satisfied, so that the same value of the Kc will be obtained no matter what initial amounts of Fe3+Fe3+ size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and SCNSCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} were used. Our purpose in this experiment will be to find KcKc size 12{K rSub { size 8{c} } } {} for this reaction for several mixtures made up in different ways, and to show that KcKc size 12{K rSub { size 8{c} } } {} indeed has the same value in each of the mixtures.

The reaction is a particularly good one to study because KcKc size 12{K rSub { size 8{c} } } {} is of a convenient magnitude and the red color of the FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} ion makes for an easy analysis of the equilibrium mixture using a spectrophotometer. The amount of light absorbed by the red complex is measured at 447 nm, the wavelength at which the complex most strongly absorbs. The absorbance, A, of the complex is proportional to its concentration, M, and can be measured directly on the spectrophotometer:

A = kM Equation 3

We know it as the Beer-Lambert law which relates the amount of light being absorbed to the concentration of the substance absorbing the light and the pathlength through which the light passes:

A = εε size 12{ε} {}bc Equation 4

In this equation, the measured absorbance (A) is related to the molar absorptivity constant ( εε size 12{ε} {}), the path length (b), and the molar concentration (c) of the absorbing species. The concentration is directly proportional to absorbance.

Thiocyanate ( SCNSCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {}) is an interesting ion and is widely used in a variety of industrial processes such as the manufacturing of thiourea, photofinishing, metal separation, and electroplating. It is also found in gold mining wastewater as a result of treating the cyanide rich ore with sulfur dioxide in order to produce the less toxic thiocyanate( SCNSCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {}) ion. Iron, as you will see later on in the semester, has the unique ability to inexpensively clean up and produce drinking quality water in Third World countries.

 Procedure

Equipment

  • 5 test tubes
  • Burette (0.1 ml graduations) filled with 0.0200M Fe(NO3)3Fe(NO3)3 size 12{ ital "Fe" \( ital "NO" rSub { size 8{3} } \) rSub { size 8{3} } } {} in 0.5 M  HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {}
  • Neighboring partners’ burette filled with 6.00×1046.00×104 size 12{6 "." "00" times "10" rSup { size 8{ - 4} } } {} M KSCN
  • stirring rod
  • small labels or china clay pencil
  • 5 cuvettes for the spectrophotometers
  •  250 ml bottle acetone for rinsing

Hazard: As always wear Safety glasses while performing this experiment

Contamination Notes: If your flask is wet before you prepare your standard/sample solutions ensure that the flask is wet with dilutant (in this case it is 0.5 M HNO3).

Calibration Of MicroLab/Spectrophotometer

N.B. Do not use test tubes from your drawer!

Find and open the MicroLab program.

  1. Find and open the MicroLab program. This brings up a box that will enable you to select from a list of experiments. Select colorimeter. Check that the accompanying box has power and is turned on, and that it is connected to the laptop via the USB plug.
  2. In the tab labeled “New” you will find the icon for the “Spectrophotometer”, please double click this. Make sure that you click on the absorbance tab.
  3. This brings up the program, at which point you should take a reading of a blank, this is done by filling a vial with 15 mL of 0.5 M  HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {}(not deionized water in this case) and placing in the appropriate slot. Covering with the film case to exclude ambient light from entering the system. When the blank sample is in place, click the button “Read Blank”. This will generate a series of data points.
  4. A solution of 0.0200M Fe(NO3)3Fe(NO3)3 size 12{ ital "Fe" \( ital "NO" rSub { size 8{3} } \) rSub { size 8{3} } } {} in 0.5 M  HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {}has been prepared for you.
  5. Dilute 1.5, 3.0, 4.5 and 6.0 mL portions of 6.00×1046.00×104 size 12{6 "." "00" times "10" rSup { size 8{ - 4} } } {} M KSCN to 20 mL with the 0.0200M Fe(NO3)3Fe(NO3)3 size 12{ ital "Fe" \( ital "NO" rSub { size 8{3} } \) rSub { size 8{3} } } {} in 0.5 M  HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {}. For this you can use some of the smaller beakers in your lab drawer. These need to be rinsed and dried thoroughly before each use. Use two clean, dry burettes to dispense the two solutions.
  6. This will give you 4 solutions which can be assumed to be 6.0×1056.0×105 size 12{6 "." 0 times "10" rSup { size 8{ - 5} } } {}M, 1.2×1041.2×104 size 12{1 "." 2 times "10" rSup { size 8{ - 4} } } {} M,1.8×1041.8×104 size 12{1 "." 8 times "10" rSup { size 8{ - 4} } } {} M and 2.4×1042.4×104 size 12{2 "." 4 times "10" rSup { size 8{ - 4} } } {} M in FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {}
  7. Take the most concentrated solution and collect a visible spectrum from 400 to 800 nm to determine λmaxλmax size 12{λ rSub { size 8{"max"} } } {}.
  8. Measure the absorbance of all the solutions at λmaxλmax size 12{λ rSub { size 8{"max"} } } {}, using 0.5 M  HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {} as the blank/reference solution.
  9. Measure the absorbance of these solutions again at 430nm, under the same conditions.
  10. Plot absorbance vs. [ FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {}] for both wavelengths using Excel and add a linear trendline passing through the origin (under Options, set intercept equal to 0). Using Beer’s law and the equation of the trendline, find the molar absortivity.

Experimental Determination of Kc

The mixtures will be prepared by mixing solutions containing known concentrations of iron (III) nitrate, Fe(NO3)3Fe(NO3)3 size 12{ ital "Fe" \( ital "NO" rSub { size 8{3} } \) rSub { size 8{3} } } {}, and potassium thiocyanate, KSCN. The color of the FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} ion formed will allow us to determine its equilibrium concentration. Knowing the initial composition of a mixture and the equilibrium concentration of FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {}, we can calculate the equlibrium concentrations of the rest of the pertinent species and then determine KcKc size 12{K rSub { size 8{c} } } {}.

  1. Label five regular test tubes 1 to 5, with labels or by noting their positions in the test tube rack.
  2. Dilute 0.02 M Fe(NO3)3Fe(NO3)3 size 12{ ital "Fe" \( ital "NO" rSub { size 8{3} } \) rSub { size 8{3} } } {} in 0.5 M HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {}by a factor of 10 using the volumetric flask that you have in your drawer. Remember to do the dilution with 0.5 M HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {} not water.
  3. Pour about 25 mL 0.002 M Fe(NO3)3Fe(NO3)3 size 12{ ital "Fe" \( ital "NO" rSub { size 8{3} } \) rSub { size 8{3} } } {} in 0.5 M HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {} into a clean, dry burette.
  4. Dispense 5.00 mL of that solution into each test tube.
  5. Then pour about 20 mL 6.00×1046.00×104 size 12{6 "." "00" times "10" rSup { size 8{ - 4} } } {} M KSCN into another clean, dry burette.
  6. Dispense 1,2,3,4, and 5 mL from the KSCN burette into each of the corresponding test tubes labeled 1 to 5.
  7. Using a small graduated cylinder, dispense the proper number of milliliters of 0.5 M HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {} into each test tube to bring the total volume in each tube to 10.00 mL.
  8. The volumes of reagents to be added to each tube are summarized in the table.
Table 1
TestTube # 1 2 3 4 5
Reagents (mL) 1 2 3 4 5
Fe(NO3)3 5.00 5.00 5.00 5.00 5.00
KSCN 1.00 2.00 3.00 4.00 5.00
0.5 M HNO3 4.00 3.00 2.00 1.00 0.00
  1. Mix each solution thoroughly with a glass stirring rod. Be sure to dry the stirring rod after mixing each solution to prevent cross-contamination.
  2. Place the mixture in tube 1 in a spectrophotometer cell and measure the absorbance of the solution at λmaxλmax size 12{λ rSub { size 8{"max"} } } {}.
  3. Determine the concentration of FeSCN2+FeSCN2+ size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} from your calibration curve. Record the value on your Report form. Repeat the measurement using the mixtures in each of the other test tubes.

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