I’d like to describe a very colorful system you can use to explore many facets of chemical equilibrium. The experiment is extremely easy to prepare, and avoids the use of concentrated acid that is used in many equilibrium experiments.1-3 To prepare the experiment, simply mix about 0.3 grams of anhydrous copper (II) chloride into 100 mL of acetone, and swirl until a dark yellow-green solution has formed. It’s okay if all of the copper (II) chloride doesn’t dissolve. The resulting solution is able to produce a variety of copper complexes that display various shades of blue, green, yellow or orange depending upon the conditions (Scheme I).4-13
Scheme I: Putative compounds, reactions, and colors involved in the formation of various copper-chloro complexes in acetone.
Copper complex equilibria
Putative compounds, reactions, and colors involved in the formation of various copper-chloro complexes in acetone.
In video 1 below you can view a variety of possible experiments:
Video 1: Colorful Copper Equilibrium from Tommy Technetium's YouTube Channel, March 5, 2020 (accessed 3/6/2020).
Discussion
Copper (II) ion reacts with chloride ion to form several chloro-containing complexes including [Cu(H2O)5Cl]+, [CuCl2(H2O)2], [CuCl3(H2O)]-, and [CuCl4]2-.4-13 These complexes, which tend to form best in non-aqueous solvents,6,8-10,13 form an array of vivid colors that span from blue to orange.12 A complete description of the relationship between the various colors and structures is not fully understood. Further, the colors and structures involved seem to vary greatly with conditions and solvent. Nevertheless, the reactions, compounds, and colors proposed in Scheme I are fairly consistent with reports in the literature4-13 and also the experiments seen in video 1.
Abbreviated form of complex copper equilibria
Figure 1: Abbreviated form of equations displayed in Scheme I.
Figure 1: Abbreviated form of equations displayed in Scheme I.
Addition of chloride ion (by addition of NaCl from table salt) tends to shift the equations above to the right (Figure 1), causing color changes from blue to green, green to yellow-green, yellow-green to yellow, and yellow to orange. Such results are entirely consistent with Le Châtelier's Principle, which insures that the addition of a compound to one side of a system at equilibrium will cause that system to shift to the other side.
Addition of water shifted each equation to the left (Figure 1). Generally speaking, changes in water concentration do not affect equilibria in aqueous solution in which water is the solvent. Under such conditions the amount of water is so large that changes in water concentration display little effect. However, acetone is the solvent in the experiments shown in the video above. Because of this, additions of water strongly increase the concentration of water in the system, shifting all equations to the left.
In a future post I will share how addition of AgNO3 (for chloride removal) and temperature affect this system of non-aqueous copper (II) chloro complexes. Please do leave a comment if you have any other suggestions for further experimentation, or any other comments on this work. Happy experimenting!
References
1. Shakhashiri, B. Z. (1989). Chemical Demonstrations: a Handbook for Teachers of Chemistry Volume 1. Madison, WI: The University of Wisconsin Press.
2. Grant, A. W. Cobalt complexes and Le Chatelier, J. Chem. Educ. 1984, 61, 466.
3. DeGrand, M. J.; Abrams, M. L.; Jenkins, J. L.; Welch, L. E. Gibbs Energy Changes during Cobalt Complexation: A Thermodynamics Experiment for the General Chemistry Laboratory. J. Chem. Educ. 2011, 88, 634−636.
4. Helmholz, L.; Kruh, R. F. The Crystal Structure of Cesium Chlorocuprate, Cs2CuCl4, and the Spectrum of the Chlorocuprate Ion J. Am. Chem. Soc.1952, 74, 1176-1181
5. Khan, M. A.; Schwing-Weill, M. J. Stability and Electronic Spectra of the Copper (II) Chloro Complexes in Aqueous Solutions Inorg. Chem. 1976, 15, 2202-2205.
6. Elleb, M.; Meullemeestre, J.; Schwing-Weill, M. J.; Vierling, F. Stability, Electronic Spectra, and Structure of the Copper (II) Chloride Complexes in N, N-Dimethylformamide Inorg. Chem. 1980, 19, 2699-2704.
7. Ramette, R. W.; Fan, G. Copper (II) Chloride Complex Equilibrium Constants Inorg. Chem. 1983, 22, 3323-3326.
8. Khan, M. A.; Meullemeestre, J.; Schwing-Weill, M. J.; Vierling, F. Detailed Spectrophotometric Study of Copper (II) Halides in Anhydrous Methanol Inorg. Chem. 1989, 28, 3306-3309.
9. Benghanem, S. D.; Khan, M. A.; Meullemeestre, J.; Vierling, F. Halogenocomplexes of Copper (II) in Anhydrous Propan-2-ol Polyhedron, 1991, 10, 2529-2533.
10. Katzin, L. I. Ionization Differences Between Coordination Statsof a Cation. Octahedral-Tetrahedral Equilibrium of Transition-Element Chlorides in Dimethylformamide, J. Chem. Phys. 1962, 36, 3034-3041.
11. Yi, H.-B.; Xia, F.-F.; Zhou, Q.; Zeng, D. [CuCl3]- and [CuCl4]2- Hydrates in Concentrated Aqueous Solution: A Density Functional Theory and ab Initio Study J. Phys. Chem. 2011, 115, 4416-4426.
12. De Vreese, P.; Brooks, N. R.; Van Hecke, K.; Van Meervelt, L.; Matthijs, E. Binnemans, K.; Van Deun, R. Speciation of Copper (II) Complexes in an Ionic Liquid Based on Choline Chloride and in Choline Chloride/Water Mixtures Inorg. Chem. 2012, 51, 4972-4981.
13. Ohtaki, H. Structural studies on solvation and complexation of metal ions in nonaqeous solution Pure & Appl. Chem. 1987, 59, 1143-1150.
Safety
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds.
Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. Use a model to predict the relationships between systems or between components of a system.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Assessment is limited to chemical reactions involving main group elements and combustion reactions.
Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.
Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.
Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.