Teacher Notes

Investigating the Reaction of Nickel with Ethylenediamine

Guided-Inquiry Wet/Dry Experiment for AP® Chemistry

Materials Included In Kit

Ethylenediamine solution, 0.5 M, 1000 mL
Nickel(II) sulfate solution, 0.5 M, 1000 mL

Additional Materials Required

(for each lab group)
Water, distilled or deionized
Beakers, 100-mL, 2
Burets, 50-mL, 2
Buret clamp
Cuvets, 2
Funnel, 1
Spectrophotometer (or colorimeter)
Support stand
Test tube rack
Test tubes, 12

Safety Precautions

Ethylenediamine is a strong irritant to skin and eyes and is moderately toxic by inhalation and skin absorption. Nickel(II) sulfate is toxic by ingestion and inhalation; it causes skin irritation and may cause an allergic reaction in sensitized individuals. Nickel compounds are known human carcinogens by inhalation of dust. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or lab coat. Remind students to wash their hands thoroughly with soap and water before leaving the lab. Please review 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. Excess ethylenediamine solution may be risned down the drain with excess water according to Flinn Suggested Disposal Method #26b. Excess nickel(II) sulfate solution as well as all nickel containing solutions may be handled according to Flinn Suggested Disposal Method #27f.

Lab Hints

  • This laboratory activity was specifically written, per teacher request, to be completed in one 50-minute class period. It is important to allow time between the Prelab Homework Assignment and the Lab Activity. Prior to beginning the homework, show the students the chemicals and equipment that will be available to them on lab day. Alternatively, you could provide the students with a list of the chemicals and equipment. For more advanced groups, you could include additional supplies that are not required to successfully complete the lab.
  • Because [Ni(en)x]2+ absorbs so much stronger than the free Ni2+ ion, care needs to be taken to prevent cross-contamination.
  • Remind students that burets are read from the top down, and encourage them to take care when dispensing their sample volumes.
  • It is very likely that the burets will need to be topped up from time to time. Remind students to check that there is enough solution left in the buret before they dispense a sample. Teaching Tips|

Teacher Tips

  • You might choose to give students a hint as to the expected ratio so they can plan their experiments accordingly. This can be done by pointing out that the most common nickel ammonia complex has six nitrogen atoms, and that each ethylenediamine molecule has two.
  • Complexes with one, two and three ethylenediamine ligands coordinated to the nickel will form in solution during this experiment. The different colors of the solutions prepared by the students can be related to their different species.
  • In this experiment the IUPAC preferred term molar attenuation is used instead of molar absorptivity. The rational for using attenuation instead of absorptivity is because a decrease in transmittance can be due to effects other than absorbance, such a light scattering.

Further Extensions

Alignment to Curriculum Framework for AP® Chemistry 

Enduring Understanding and Essential Knowledge
Atoms are so small that they are difficult to study directly; atomic models are constructed to explain experimental data on collections of atoms. (1D)
1.D.3: The interaction of electromagnetic waves or light with matter is a powerful means to probe the structure of atoms and molecules, and to measure their concentration.

Chemical changes are represented by a balanced chemical equation that identifies the ratios with which reactants react and products form. (3A)
3.A.2: Quantitative information can be derived from stoichiometric calculations that utilize the mole ratios from the balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done only by chemists.

Chemical and physical transformations may be observed in several ways and typically involve a change in energy. (3C)
3.C.1: Production of heat or light, formation of a gas, formation of a precipitate and/or a color change are possible evidences that a chemical change has occurred.

Chemical equilibrium is a dynamic, reversible state in which rates of opposing processes are equal. (6A)
6.A.3: When a system is at equilibrium, all macroscopic variables, such as concentrations, partial pressures and temperature, do not change over time. Equilibrium results from an equality between the rates of the forward and reverse reactions, at which point Q=K.

Systems at equilibrium are responsive to external perturbations, with the response leading to a change in the composition of the system. (6B)
6.B.1: Systems at equilibrium respond to disturbances by partially countering the effect of the disturbance (Le Chatelier’s principle).

Learning Objectives
1.15 The student can justify the selection of a particular type of spectroscopy to measure properties associated with vibrational or electronic motions of molecules.
1.16 The student can design and/or interpret the results of an experiment regarding the absorption of light to determine the concentration of an absorbing species in solution.
3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
3.10 The student is able to evaluate the classification of a process as a physical change, chemical change, or ambiguous change based on both macroscopic observations and the distinction between rearrangement of covalent interactions and noncovalent interactions.

Science Practices
2.1 The student can justify the selection of a mathematical routine to solve problems.
2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
3.2 The student can refine scientific questions.
4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
4.2 The student can design a plan for collecting data to answer a particular scientific question.
4.3 The student can collect data to answer a particular scientific question.
5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
6.1 The student can justify claims with evidence.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Planning and carrying out investigations
Constructing explanations and designing solutions
Obtaining, evaluation, and communicating information
Analyzing and interpreting data
Using mathematics and computational thinking

Disciplinary Core Ideas

HS-PS1.A: Structure and Properties of Matter
HS-PS2.B: Types of Interactions
HS-ETS1.C: Optimizing the Design Solution
HS-PS3.D: Energy in Chemical Processes
HS-PS4.A: Wave Properties
HS-PS4.B: Electromagnetic Radiation

Crosscutting Concepts

Cause and effect
Patterns
Stability and change
Energy and matter

Performance Expectations

HS-PS1-4: Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

Answers to Prelab Questions

  1. The cation [Ru(C10H8N2)3]2+ is strongly colored in solution, with a molar attenuation coefficient of 14600 L/mol•cm at 452 nm. What would be the concentration of a solution of [Ru(C10H8N2)3]2+ with a measured absorption of 1.50 at 452 nm?
    {12332_PreLab_Equation_4}
  2. What effect, if any, would the following have on the measured absorbance of a solution? Explain your reasoning in each case.
    1. The cuvet into which the solution is placed had some water droplets on the inside.

      This would dilute the solution resulting in a decrease in absorbance.

    2. A cuvet has more solution added to it than in a previous measurement.

      No effect. The quantity or amount of a sample is not part of Beer-Lambert law.

    3. The wavelength is moved away from the maximum absorbance.

      This would decrease the molar attenuation coefficient and as such lower the absorbance.

    4. A student touches the clear glass side of the cuvet before placing it into the spectrophotometer.

      The fingerprint would obstruct the light increasing the absorbance.

  3. Explain why, when preparing a UV-visible absorption calibration curve, it is a good idea to work from the least concentrated to the most concentrated solution?

    Should any cross contamination occur during the recording of samples, the effect of adding a drop of a lower concentration sample to a higher concentration sample is less dramatic than adding a drop of a high concentration sample to a low concentration sample.

  4. Iron(III) is well known to react with the thiocyanate anion according to the following equilibrium.
    {12332_PreLab_Equation_3}
    The visible absorption spectrum for the iron(III) thiocyanate complex is shown in Figure 2.
    {12332_PreLab_Figure_2}
    What might be a good wavelength to use in monitoring the formation of the iron(III) thiocyanate ion? What factors influenced this choice?

    A wavelength of around 450 nm is ideal as this is an absorption maxima for the iron complex.

  5. The following spectroscopic data was recorded at 470 nm using 5 mM solutions of Fe3+ and SCN according to Job’s method of continuous variation. The data needs to be corrected for the absorbance associated with the uncoordinated Fe3+ and the uncoordinated SCN. This is done through the following formula: Acor = Aobs – (1 – χSCN) AFe – (χSCN)ASCN Calculate the corrected absorbance for the other samples.
    {12332_PreLabAnswers_Table_2}
  6. Using your corrected absorbance values, plot the absorbance against mole fraction and determine at which mole fraction the maximum absorbance would occur.
    {12332_PreLabAnswers_Figure_4}
    Solving for the intercept of the two straight lines gives an optimal mole fraction of 0.4995.
  7. Based on your mole fraction from Question 6, what is the correct formula of the [Fe(SCN)x(H2O)6 – x](3 – x)+ ion?

    The correct formula is [Fe(SCN)(H2O)5]2+.

  8. Based on the data collected in sample 6, what is the molar attenuation coefficient of the Fe3+ SCN complex at 470 nm?
    {12332_PreLabAnswers_Equation_5}

    If students use non-corrected data, they will find a molar attenuation coefficient of 906 L /mol•cm. If they use the corrected absorbance, the answer will be 874 L /mol•cm.

  9. Why is it a good idea to use a point where there is a large excess of one of the reactants when calculating the molar attenuation coefficient of the iron complex?

    A large excess of a reactant is used in case the equilibrium does not lie strongly in favor of the products. The large excess of one reactant will result in the majority of the other being used in the formation of the product. In this case, almost all the iron will have reacted with the thiocyanate due to it being present in excess.

  10. Nickel(II) sulfate is a highly soluble source of nickel(II) cations. It dissolves in water to give a teal solution. The nickel cations can readily react with other complexes, such as ethylenediamine. The UV-vis spectra for both the nickel(II) sulfate and the nickel(II) ethylenediamine complex are shown in Figure 3.
    {12332_PreLabAnswers_Figure_3}
    What would be a good wavelength to use in monitoring the formation of the nickel(II) ethylenediamine complexes formation? Explain your choice (depending on the type of spectrophotometer available, you might be restricted in choice of wavelengths).

    Approximately 530 nm would be an ideal wavelength to measure at, as this is both near the maximum absorbance of the nickel ethylenediamine complex and the minimum absorbance of the unreacted nickel. However, depending on the spectrophotometer available this wavelength might not be possible. For example the Vernier Colorimeter (available from Flinn Scientific, Catalog No. TC1504) can only record at 430 nm, 470 nm, 565 nm and 635 nm of which the best choice is 565 nm.

Sample Data

{12332_Data_Table_4}
{12332_Data_Figure_5}
The two lines intersect at a mole fraction of 0.735, which corresponds to a formula of [Ni(en)2.8]2+. Please note that the expected formula is [Ni(en)3]2+.

Answers to Questions

Example Procedure

  1. Place two burets in a buret clamp on a support stand.
  2. Fill one of the burets with 50 mL of a 0.5 M NiSO4 solution and the other with 50 mL of a 0.5 M ethylenediamine solution.
  3. Obtain 11 clean dry test tubes and a test tube rack.
  4. Prepare 11 samples by adding the volumes in the table below to clean dry test tubes (top up the burets with additional solution when needed).
    {12332_Answers_Table_3}
  5. Obtain a Vernier colorimeter and LabQuest.
  6. Set the colorimeter to 565 nm.
  7. Using distilled or deionized water calibrate the colorimeter by recording a blank.
  8. Record the absorbance of each solution at 565 nm.
  9. Correct for the absorbance of free nickel using the formula: Acor = Aobs – (1 – χen ) A0.
  10. Plot the corrected absorbance against the mole fraction.
  11. Fit straight lines to each side of the plot and determine their point of intersection.
  12. From the mole fraction at the point of intersection determine the formula of [Ni(en)x]2+.

References

AP® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013.

Student Pages

Investigating the Reaction of Nickel with Ethylenediamine

Introduction

Capture the concepts and hit the ground running on exam day with this lab! Encompassing Big Ideas 1 and 3, this lab involves the use of Job’s method of continuous variation to spectroscopically investigate the equilibrium reaction between nickel(II) and ethylenediamine. Nickel(II) complexes have been observed to exhibit a range of products while in solution. For example, when reacted with ammonia, the complex ions [Ni(NH3)(H2O)5]2+, [Ni(NH3)2(H2O)4]2+, [Ni(NH3)3(H2O)3]2+, [Ni(NH3)4(H2O)2]2+, [Ni(NH3)5(H2O)]2+ and [Ni(NH3)6]2+ have all been observed in a complicated equilibria. Job’s method of continuous variation enables a researcher to identify which mix of reactants results in the largest yield through the use of UV-visible spectroscopy. A prelab homework assignment guides you through the necessary concepts to ensure success on lab day. You will find it fun, engaging and challenging!

Concepts

  • Coordination chemistry
  • Stoichiometry
  • Spectroscopy
  • Equilibria
  • Beer-Lambert Law

Background

Nonmetal compounds that are able to coordinate and bond to transition metals are known as ligands. These ligands can coordinate to transition metals in various quantities and coordination geometries, the color of the resulting complex is directly related to the type and number of ligands coordinated to the metal. Through the use of UV-visible spectroscopy, the formation of these complexes can be monitored and the stoichiometric ratio of the metal to the ligand determined.

When light is shone through a colored solution, some of the light will be absorbed by the various species in the solution. The ratio of light that does manage to pass through the solution to the amount of light that entered the solution is called the transmittance. More commonly, in UV-visible spectroscopy, the absorbance is reported. Absorbance is defined as the negative log of the transmittance. Beer-Lambert law (Equation 1) relates the absorption of a sample to its concentration and the path length of the sample cell. For the sake of simplicity, most spectrophotometers have a path length of 1 cm. The value of the molar attenuation coefficient is specific to each wavelength of light. It is quite common for the wavelength at which measurements are being taken to be written as a subscript when reporting the molar attenuation coefficient.

{12332_Background_Equation_1}

Beer-Lambert Law
A is absorbance
a is the molar attenuation coefficient
b is the path length (usually 1 cm)
c is the concentration in mol/L

Job’s method of continuous variation involves mixing solutions of metal ions and ligands at multiple different concentrations and measuring the absorption of the resulting solution at a specific wavelength. In order to accurately determine the correct ratio of metal to ligand in the complex ion, the absorbance is plotted against the mole fraction of the ligand. Whichever mole fraction gives the largest absorbance must have the highest concentration of product and be the same as the ratio of metal to ligand in the formula. In the case of a system where the equilibrium constant is very much larger than 1, and hence favors the products, the plot will give two straight lines. However, in the case where the equilibrium does not lie as strongly in favor of the products, the plot will give a more rounded maximum and a linear fit will need to be performed in order to accurately determine the position of the maximum value (see Figure 1).
{12332_Background_Figure_1}
One final consideration that needs to be taken into account when using Job’s method is that the other components of the equilibrium system might also be colored. This can often be mitigated in two ways. Firstly, by selecting a wavelength where the product absorbs strongly but the reactant only absorbs weakly. And secondly, by applying a correction to the absorbance. This correction is made by subtracting the absorbance of the reactant multiplied by its mole fraction from the measured absorbance (Equation 2).
{12332_Background_Equation_2}

Experiment Overview

The purpose of this activity is to complete the homework assignment prior to lab to promote understanding of UV-visible spectroscopy, Beer-Lambert law and Job’s method of continuous variation. You will need to consider the equipment and chemicals that are being made available for you, and then using the Prelab Questions as a guide to design an experiment that will enable you to experimentally determine the formula of the nickel(II) ethylenediamine (en) complex [Ni(en)x]2+.

Prelab Questions

Complete the following homework set and write a lab procedure to be approved by your instructor prior to performing the lab. When writing your procedure be mindful of the chemicals, quantities and equipment that will be available to you on lab day. Along with your procedure you will turn in any graphs or figures you were asked to create in this homework set and the answers to the questions on a separate piece of paper, if needed.

  1. The cation [Ru(C10H8N2)3]2+ is strongly colored in solution, with a molar attenuation coefficient of 14600 L/mol•cm at 452 nm. What would be the concentration of a solution of [Ru(C10H8N2)3]2+ with a measured absorption of 1.50 at 452 nm?
  2. What effect if any would the following have on the measured absorbance of a solution? Explain your reasoning in each case.
    1. The cuvet into which the solution is placed had some water droplets on the inside.
    2. A cuvet has more solution added to it than in a previous measurement.
    3. The wavelength is moved away from the maximum absorbance.
    4. A student touches the clear glass side of the cuvet before placing it into the spectrophotometer.
  3. Explain why, when preparing a UV-visible absorption calibration curve, it is a good idea to work from the least concentrated to the most concentrated solution?
  4. Iron(III) is well known to react with the thiocyanate anion according to the following equilibrium.
    {12332_PreLab_Equation_3}
    The visible absorption spectrum for the iron(III) thiocyanate complex is shown in Figure 2.
    {12332_PreLab_Figure_2}
    What might be a good wavelength to use in monitoring the formation of the iron(III) thiocyanate ion? What factors influenced this choice?
  5. The following spectroscopic data was recorded at 470 nm using 5 mM solutions of Fe3+ and SCN according to Job’s method of continuous variation. The data needs to be corrected for the absorbance associated with the uncoordinated Fe3+ and the uncoordinated SCN. This is done through the following formula: Acor = Aobs – (1 – χSCN) AFe – (χSCN)ASCN For example, for sample 3, Acor = 0.913 – (1 – 0.2)0.034 – (0.2)0.025 = 0.8608 Calculate the corrected absorbance for the other samples.
    {12332_PreLab_Table_1}
  6. Using your corrected absorbance values, plot the absorbance against mole fraction and determine at which mole fraction the maximum absorbance would occur.
  7. Based on your mole fraction from Question 6, what is the correct formula of the [Fe(SCN)x(H2O)6 – x](3 – x)+ ion?
  8. Based on the data collected in sample 6, what is the molar attenuation coefficient of the Fe3+ SCN complex at 470 nm?
  9. Why is it a good idea to use a point where there is a large excess of one of the reactants when calculating the molar attenuation coefficient of the iron complex?
  10. Nickel(II) sulfate is a highly soluble source of nickel(II) cations. It dissolves in water to give a teal solution. The nickel cations can readily react with other complexes, such as ethylenediamine. The UV-vis spectra for both the nickel(II) sulfate and the nickel(II) ethylenediamine complex are shown in Figure 3.
    {12332_PreLab_Figure_3}
    What would be a good wavelength to use in monitoring the formation of the nickel(II) ethylenediamine complexes formation? Explain your choice (depending on the type of spectrophotometer available, you might be restricted in choice of wavelengths).
  11. Write a detailed lab procedure for determining the formula of [Ni(en)x]2+ through Job’s method of continuous variation.

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