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
Spectrophotometer (or colorimeter)
Test tube rack
Test tubes, 12
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.
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.
- 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|
- 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.
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).
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.
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
Cause and effect
Stability and change
Energy and matter
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
- 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?
- What effect, if any, would the following have on the measured absorbance of a solution? Explain your reasoning in each case.
- 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.
- 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.
- The wavelength is moved away from the maximum absorbance.
This would decrease the molar attenuation coefficient and as such lower the absorbance.
- 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.
- 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.
- Iron(III) is well known to react with the thiocyanate anion according to the following equilibrium.
The visible absorption spectrum for the iron(III) thiocyanate complex is shown in 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.
- 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.
- Using your corrected absorbance values, plot the absorbance against mole fraction and determine at which mole fraction the maximum absorbance would occur.
Solving for the intercept of the two straight lines gives an optimal mole fraction of 0.4995.
- 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+.
- Based on the data collected in sample 6, what is the molar attenuation coefficient of the Fe3+ SCN– complex at 470 nm?
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.
- 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.
- 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.
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.
The two lines intersect at a mole fraction of 0.735, which corresponds to a formula of [Ni(en)2.8
. Please note that the expected formula is [Ni(en)3
Answers to Questions
- Place two burets in a buret clamp on a support stand.
- 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.
- Obtain 11 clean dry test tubes and a test tube rack.
- 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).
- Obtain a Vernier colorimeter and LabQuest.
- Set the colorimeter to 565 nm.
- Using distilled or deionized water calibrate the colorimeter by recording a blank.
- Record the absorbance of each solution at 565 nm.
- Correct for the absorbance of free nickel using the formula: Acor = Aobs – (1 – χen ) A0.
- Plot the corrected absorbance against the mole fraction.
- Fit straight lines to each side of the plot and determine their point of intersection.
- From the mole fraction at the point of intersection determine the formula of [Ni(en)x]2+.
AP® Chemistry Guided-Inquiry Experiments: Applying the Science Practices; The College Board: New York, NY, 2013.