Teacher Notes

Synthesis of a Coordination Compound

Student Laboratory Kit

Materials Included In Kit

Acetone, CH3COCH3, 250 mL
Ethyl alcohol, CH3CH2OH, 50%, 250 mL
Ethyl alcohol, CH3CH2OH, 95%, 250 mL
Ferrous ammonium sulfate, Fe(NH4)2(SO4)2•6H2O, 140 g
Hydrogen peroxide solution, H2O2, 6%, 250 mL
Oxalic acid, H2C2O4, 100 g
Potassium oxalate, K2C2O4, 75 g
Sulfuric acid solution, H2SO4, 2 M, 25 mL
Pipets, Beral-type, 36

Additional Materials Required

Water, distilled or deionized
Aspirators, 12
Aspirator trap assemblies, 12
Balance, 0.001-g precision
Beakers, 50-mL, 24
Beakers, 150-mL, 12
Beakers, 400-mL, 12
Büchner funnels and adapters, 12
Ceramic fiber squares, 12
Erlenmeyer flask, 250-mL
Filter flasks, 250-mL, 12
Filter funnels, 12
Filter paper, quantitative
Graduated cylinder, 10-mL, 12
Graduated cylinder, 50-mL, 12
Heat-resistant gloves, 12
Hot plates, 12
Stirring rods, 12
Thermometers, 0–100 °C
Vacuum tubing, 24 pieces
Watch glasses, 12
Wash bottles, 12

Prelab Preparation

Oxalic Acid Solution, H2C2O4, 10%: Add 100 g of oxalic acid to a 2-L beaker containing 900 mL of deionized water. Stir to dissolve. Transfer to a clean 1-L capped container and label as 10% oxalic acid.

Potassium Oxalate Solution, 25%: Add 75 g of potassium oxalate to a 400-mL beaker containing 225 mL of distilled water. Stir to dissolve. Transfer to a clean 500-mL capped container and label as 25% potassium oxalate.

Safety Precautions

The sulfuric acid solution is corrosive to eyes, skin and other tissue. Always add acid to water, never the reverse. The oxalic acid solution is a skin and eye irritant and moderately toxic by ingestion. The 6% hydrogen peroxide solution is an oxidizer and a skin and eye irritant. The 95% ethyl alcohol solution and the acetone are both flammable and dangerous fire risks. Keep both away from open flames and other sources of ignition. The addition of denaturants makes the 95% ethyl alcohol solution and the 50% ethyl alcohol solution poisonous; they cannot be made nonpoisonous. The acetone is slightly toxic by ingestion and inhalation. Wear chemical splash goggles and chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The sulfuric acid solution and the oxalic acid solution may be disposed of according to Flinn Suggested Disposal Method #24b. The filtrate containing ethyl alcohol, acetone, and water may be disposed of according to Flinn Suggested Disposal Method #26b. The potassium trioxalateferrate(III) trihydrate product may be disposed of according to Flinn Suggested Disposal Method #26a; its solution according to Flinn Suggested Disposal Method #26b.

Lab Hints

  • This lab work for this experiment can be reasonably completed in two 50-minute class periods. Carry out Part 1 and Part 2 synthesis steps in one laboratory period. Store the mixture in a fume hood away from light. The filtration and measurements of percent yield may then be carried out during the next laboratory period.
  • Caution the students to be sure to stir the hot mixtures containing solids to prevent “bumping.” Stirring should continue until the solution has cooled significantly.
  • Also caution students not to use thermometers to stir the solutions.
  • Review with students how to properly set up and use a Büchner funnel. Review proper transfer techniques.

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Asking questions and defining problems
Planning and carrying out investigations
Using mathematics and computational thinking
Analyzing and interpreting data

Disciplinary Core Ideas

MS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
HS-PS1.A: Structure and Properties of Matter
HS-PS1.B: Chemical Reactions

Crosscutting Concepts

Scale, proportion, and quantity
Systems and system models
Energy and matter
Cause and effect

Performance Expectations

MS-ESS2-4: Develop a model to describe the cycling of water through Earth’s systems driven by energy from the sun and the force of gravity.
MS-ESS2-5: Collect data to provide evidence for how the motions and complex interactions of air masses results in changes in weather conditions.

Answers to Prelab Questions

  1. Define the terms Lewis acid and Lewis base.

    A Lewis acid is a substance that can accept a pair of electrons from another atom to form a new bond. A Lewis base is a substance that contains an atom with a lone pair of electrons that can be donated to another atom to form a new bond.

  2. Define the terms ligand and coordination number.

    A ligand is an atom, ion, or molecule that acts as a Lewis base to bond to the central metal atom or ion in a complex ion. The coordination number of the central metal atom of a complex ion refers to the number of ligand atoms bonded to it.

  3. What are the oxidation numbers of the metal atoms in each of the following coordination compounds?
    1. [Ni(NH3)6](NO3)2

      NO3 has –1 charge; therefore the Ni(NH3)6 complex ion has a +2 charge. Since each NH3 molecule is neutral, the Ni atom must have a +2 oxidation number.

    2. K3[Co(CN)6]

      K has a +1 charge; therefore the complex ion Co(CN)6 has a –3 charge. Each CN ligand has a –1 charge, giving the Co atom an oxidation number of +3.

    3. [Pt(NH3)3Br]Cl

      Cl has a –1 charge; therefore the complex ion Pt(NH3)3Br has a +1 charge. Each NH3 ligand is neutral and the Br ligand has a –1 charge. The Pt atom has, therefore, a +2 oxidation number.

  4. For each of the following ligands, draw the Lewis structures and indicate the atom that donates an electron pair for complex ion formation.
    {13896_PreLabAnswers_Figure_2}
  5. What is the coordination number of the metal in each of the following compounds?
    1. [FeCO(CN)5](NO3)3

      The coordination number of the Fe atom equals the number of ligands in the complex ion. Coordination number of Fe = 6.

    2. [Ag(CN)2]Cl

      Coordination number of Ag = 2.

    3. [Cr(H2O)2Cl2]Br

      Coordination number of Cr = 4.

  6. Suppose a student synthesizes potassium trioxalatoferrate(III) trihydrate, K3[FeC2O4)3]•3H2O, by starting with 11.356 g of ferrous ammonium sulfate, Fe(NH4)2SO4•6H2O.
    1. What is the theoretical yield, in grams, for K3[FeC2O4)3]•3H2O?

      Mass of Fe(NH4)2SO4•6H2O = 11.356 g

      {13896_PreLabAnswers_Equation_12}
      The theoretical moles of K3[FeC2O4)3]•3H2O are the same, 0.02896 moles.

      The theoretical yield, in grams, is:

      Yield = 0.02896 moles x 491.16 g/mole = 14.22 g

    2. If 9.376 g of K3[FeC2O4)3]•3H2O were actually synthesized, what is the percent yield?
      {13896_PreLabAnswers_Equation_13}

Sample Data

  1. Mass of Fe(NH4)2(SO4)2•6H2O, g        ___9.879___ g
  2. Mass of K3[FeC2O4)3]•3H2O, g            ___9.347___ g

Answers to Questions

  1. Calculate the theoretical yield of K3[FeC2O4)3]•3H2O, based on the sample weight of Fe(NH4)2(SO4)2•6H2O.

    Mass of Fe(NH4)2(SO4)2•6H2O = 9.879 g

    {13896_Answers_Equation_14}
    The theoretical moles of K3[FeC2O4)3]•3H2O are the same as Fe(NH4)2(SO4)2•6H2O, or 0.02519 moles

    Theoretical yield of K3[FeC2O4)3]•3H2O;

    Yield = 0.02519 moles x 491.16 g/mole = 12.37 g

  2. Calculate the percent yield for the K3[FeC2O4)3]•3H2O product.
    {13896_Answers_Equation_15}

Student Pages

Synthesis of a Coordination Compound

Introduction

Coordination compounds are interesting substances, usually highly colored and containing a complex ion in their structures. In this lab, green crystals of the coordination compound potassium trioxalatoferrate(III) trihydrate are synthesized. These crystals are then analyzed to determine the percent yield of the experiment.

Concepts

  • Complex ions
  • Coordination compound and number
  • Lewis acids and bases
  • Ligands

Background

Coordination compounds are compounds that contain a metal atom or ion bonded to a group of molecules or ions. While these compounds may be neutral molecules, most are ionic compounds consisting of a complex ion (a metal ion with its attached molecules or ions) and a counter ion or ions to balance the charge. The molecules or ions attached to the central metal atom are called ligands.

FeCl2•6H2O is a typical coordination compound. The formula as written reflects the compound makeup, but not its structure. When writing the formula for coordination compounds, brackets are put around the metal and its ligands, indicating the actual structure and bonds in the compound. For FeCl2•6H2O the formula becomes [Fe(H2O)6]Cl2.

The number of ligands atoms bonded to the metal ion is called the coordination number of the metal ion. This value is usually, but not always, 2, 4 or 6 and depends on the particular metal ion.

Ligands have lone pairs of electrons that can be used to form a bond with the metal ion. The ligand acts as a Lewis base and the metal ion acts as a Lewis acid.

{13896_Background_Equation_1}
The bonding in coordination compounds involves the overlap of the metal d-orbitals and the ligand lone pair orbitals. If two atoms in a ligand donate lone pair electrons to form separate single bonds with the metal ion, the ligand is said to be a bidentate ligand. As many as six atoms in an individual ligand can be bonded with the metal ion.

In Part 1, ferrous ammonium sulfate, Fe(NH4)2(SO4)2•6H2O, is reacted with oxalic acid to form an intermediate compound, iron(II) oxalate dihydrate, FeC2O4•2H2O. This compound is isolated and then converted in Part 2 to potassium trioxalato ferrate(III) trihydrate, K3[Fe(C2O4)3]•3H2O, by oxidation with hydrogen peroxide in the presence of potassium oxalate and oxalic acid.

In Part 1, the reaction to form the intermediate compound is:
{13896_Background_Equation_2}
In Part 2, the iron(II) oxalate dihydrate is first reacted with potassium oxalate, forming an orange complex of iron(II) and oxalates.
{13896_Background_Equation_3}
It is this iron(II) compound that is oxidized by the hydrogen peroxide. Initially, the K2[Fe(C2O4)2]•2H2O is oxidized to iron(III) hydroxide, Fe(OH)3, a brown precipitate.
{13896_Background_Equation_4}
and
{13896_Background_Equation_5}
When more oxalic acid is added, the iron(III) hydroxide dissolves. The iron(III) ion in solution forms a complex ion with the oxalate ligands, yielding a clear green solution of the complex ion, Fe(C2O4)33–.
{13896_Background_Equation_6}
{13896_Background_Equation_7}
The potassium salt of this ion is soluble in water, but only slightly soluble in ethanol. The addition of ethanol forces the precipitation of the product salt.
{13896_Background_Equation_8}

Experiment Overview

The purpose of this experiment is to synthesize the coordination compound potassium trioxalatoferrate(III) trihydrate, K3[Fe(C2O4)2]•2H2O. Once produced, the percent yield of the product will be determined.

Materials

Part 1
Ferrous ammonium sulfate, Fe(NH4)2(SO4)2•6H2O, 10 g
Oxalic acid solution, H2C2O4, 10%, 65 mL
Sulfuric acid solution, H2SO4, 2 M, 1 mL
Water, distilled or deionized
Beaker, 150-mL
Beaker, 400-mL
Ceramic fiber square
Erlenmeyer flask, 250-mL
 Graduated cylinder, 50-mL
Heat-resistant gloves
Hot plate
Pipet, Beral-type
Stirring rod
Thermometer, 0–100 °C

Part 2
Acetone, CH3COCH3, 20 mL
Ethyl alcohol, CH3CH2OH, 50%, 20 mL
Ethyl alcohol, CH3CH2OH, 95%, 20 mL
Hydrogen peroxide solution, H2O2, 6%, 17 mL
Oxalic acid solution, H2C2O4, 10%, 65 mL
Potassium oxalate solution, K2C2O4, 25%, 18 mL
Water, distilled or deionized
Aspirator trap assembly
Balance, 0.001-g precision
Beakers, 50-mL, 2
Beaker, 150-mL
Büchner funnel and adapter
Ceramic fiber square
Erlenmeyer flask, 250-mL
Filter flask, 250-mL
Filter funnel
Filter paper, quantitative
Graduated cylinder, 10-mL
Graduated cylinder, 50-mL
Heat-resistant gloves
Hot plate
Pipets, Beral-type, 2
Stirring rod
Thermometer, 0–100 °C
Vacuum tubing, 2 pieces
Wash bottle
Watch glass

Prelab Questions

  1. Define the terms Lewis acid and Lewis base.
  2. Define the terms ligand and coordination number.
  3. What are the oxidation numbers of the metal atoms in each of the following coordination compounds?
    1. [Ni(NH3)6](NO3)2
    2. K3[Co(CN)6]
    3. [Pt(NH3)3Br]Cl
  4. For each of the following ligands, draw the Lewis structures and indicate the atom that donates an electron pair for complex ion formation.
    1. NH3
    2. CN–
    3. C2O42–
  5. What is the coordination number of the metal in each of the following compounds?
    1. [FeCO(CN)5](NO3)3
    2. [Ag(CN)2]Cl
    3. [Cr(H2O)2Cl2]Br
  6. Suppose a student synthesizes potassium trioxalatoferrate(III) trihydrate, K3[Fe(C2O4)3]•2H2O, by starting with 11.356 g of ferrous ammonium sulfate, Fe(NH4)2SO4•6H2O.
    1. What is the theoretical yield, in grams, for K3[Fe(C2O4)3]•2H2O?
    2. If 9.376 g of K3[Fe(C2O4)3]•2H2O were actually synthesized, what is the percent yield?

Safety Precautions

The sulfuric acid solution is corrosive to eyes, skin and other tissue. Always add acid to water, never the reverse. The oxalic acid solution is a skin and eye irritant and moderately toxic by ingestion. The 6% hydrogen peroxide solution is an oxidizer and a skin and eye irritant. The 95% ethyl alcohol solution and the acetone are both flammable and dangerous fire risks. Keep both away from open flames and other sources of ignition. The addition of denaturates makes the 95% ethyl alcohol solution and the 50% ethyl alcohol solution poisonous; they cannot be made nonpoisonous. The acetone is slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. Preparation of Iron(II) Oxalate Intermediate

  1. On an analytical balance, accurately mass between 10.0 and 10.5 g of ferrous ammonium sulfate, Fe(NH4)2(SO4)2•6H2O. Record the exact mass in the data table.
  2. Obtain 30 mL of deionized water in a clean 50-mL graduated cylinder and transfer the deionized water to a clean 150-mL beaker.
  3. Add the 10 g of ferrous ammonium sulfate to the 150-mL beaker and stir to dissolve.
  4. Using a Beral-type pipet, transfer 6 drops (1 mL) of 2 M H2SO4 to the solution of ferrous ammonium sulfate. Stir to mix.
  5. Obtain 50 mL of the 10% oxalic acid solution in a clean 50-mL graduated cylinder. Add this solution slowly, while stirring, to the 150-mL beaker. Yellow iron(II) oxalate now precipitates.
  6. Place the 150-mL beaker on a hot plate and carefully heat the mixture to boiling. Be sure to continuously stir the contents to prevent bumping.
  7. Turn off the hot plate. Carefully remove the 150-mL beaker using heat-resistant gloves and place the beaker on a ceramic fiber square to cool.
  8. Continue stirring the beaker contents as they cool to prevent “bumping.”
  9. Obtain a 400-mL beaker and decant the supernatant liquid in the 150-mL beaker into the 400-mL beaker. The supernatant liquid is the liquid above the precipitate.
  10. Add approximately 100 mL of deionized water to a clean 250-mL Erlenmeyer flask. Place the Erlenmeyer flask on the hot plate and heat water to approximately 40 °C.
  11. Wash the yellow precipitate remaining in the 150-mL beaker with three successive portions of approximately 30 mL of hot, deionized water. Decant the liquid into the 400-mL beaker between each washing.
Part 2. Synthesis of K3[Fe(C2O4)3]•3H2O
  1. Obtain 18 mL of the 25% potassium oxalate solution in a clean 50-mL graduated cylinder.
  2. Add the 18 mL of 25% potassium oxalate solution to the 150-mL beaker containing the yellow iron(II) oxalate solid.
  3. Heat the iron(II) oxalate/potassium oxalate mixture to 40 °C on a hot plate.
  4. Add 17 mL of 6% hydrogen peroxide solution to a clean 50-mL beaker. Use a clean graduated cylinder to transfer.
  5. Obtain 15 mL of the 10% oxalic acid solution in a clean 50-mL beaker. Transfer 8 mL of this solution to a clean 10-mL graduated cylinder.
  6. Monitor the temperature of the iron(II) oxalate/potassium oxalate solution. When the mixture is around 40 °C, slowly and carefully, while stirring, add the 17 mL of 6% hydrogen peroxide solution drop-wise using a Beral-type pipet. Continuously stir the solution while adding the 6% hydrogen peroxide. A brick red solid forms.
  7. Once all the 6% hydrogen peroxide solution has been added to the oxalate solution, heat the solution in the beaker to its boiling point. Be sure to continuously stir the solution as it heats.
  8. When the solution is boiling, quickly add the 8 mL of 10% oxalic acid solution. Add the remaining 7 mL of 10% oxalic acid in the 50-mL beaker drop-wise to the solution using a Beral-type pipet. The mixture will change from brick red solids in suspension to an olive color and finally to a bright green solution with few, if any, solids in suspension.
  9. Turn off the hot plate. Remove the 150-mL beaker using heat-resistant gloves and place the beaker on a ceramic fiber square to cool.
  10. Continue stirring the beaker contents as they cool to prevent “bumping.”
  11. Once the beaker contents have cooled, filter any remaining solid by gravity using a filter flask. The solution now contains K3[Fe(C2O4)3]•2H2O in solution.
  12. Obtain 20 mL of 95% ethyl alcohol in a clean 50-mL graduated cylinder.
  13. Add the 95% ethyl alcohol to the solution of K3[Fe(C2O4)3]•2H2O. This will produce a green precipitate of K3[Fe(C2O4)3]•2H2O.
  14. Cover the beaker with a watch glass and store the mixture as directed by the instructor.
  15. Set up the filtration assembly as shown in Figure 1.
    {13896_Procedure_Figure_1_Filtration assembly}
  16. Turn on the water to start the suction in the Büchner funnel. Add the quantitative filter paper to the Büchner funnel and wet the paper with deionized water from a wash bottle.
  17. Transfer the contents of the 150-mL beaker to the Büchner funnel. Wash the beaker and green precipitate with 20 mL of 50% ethyl alcohol, then 20 mL of acetone. Transfer each washing to the Büchner funnel.
  18. Allow the precipitate to air dry under suction for five minutes.
  19. Disconnect the hosing from the aspirator, then turn off the faucet.
  20. Carefully remove the filter paper and precipitate. On an analytical balance, tare a piece of weighing paper or a weigh boat.
  21. Transfer the precipitate to the weighing paper or weight boat. Record the mass of the precipitate in the data table as mass of K3[Fe(C2O4)3]•2H2O.
  22. Dispose of the Part 2 filtrate solution, the product and the product solution as directed by the instructor.

Student Worksheet PDF

13896_Student1.pdf

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