Effect of Temperature on Chemical Equilibrium using Copper Complexes

Colorful copper equilibria

A popular method for demonstrating the effect of temperature on chemical equilibrium involves the use of Co2+ complexes. When Co2+ is dissolved in water it appears pink, but it forms a blue-colored complex in the presence of high concentrations of chloride ion:1-3

Co2+(aq) + 4 Cl-(aq) ßà CoCl42-(aq)                     Equation 1                  

The reaction that forms CoCl42- is endothermic. Therefore, the above reaction can be shifted to the right at high temperatures or to the left at low temperatures (Video 1).

Video 1: Cobalt equilibrium and thermodynamics, Tommy Technetium YouTube Channel (accessed 4/7/2021)

 

In this demonstration, concentrated HCl (12 M) is used as the source of Cl- to produce the blue colored CoCl42- species.1-3 Because concentrated HCl presents several hazards, I have instead begun using a system of Cu2+ complexes, which does not require the use of concentrated acid, to explore chemical equilibrium.4 This system of Cu2+ complexes has the added benefit of displaying a larger variety of colors: blue, green, yellow, and orange (Scheme I).

 

Scheme I: Putative compounds, reactions, and colors involved in the formation of various copper-chloro complexes in acetone. Each reaction is endothermic from left to right.

 

Note that each reaction listed in Scheme I is endothermic from left to right. Because of this, each reaction shifts to the right at increasing temperatures, but to the left at decreasing temperatures. Indeed, I have noted that solutions of CuCl2 in acetone are dirty-yellow at room temperature, green when incubated in a dry-ice acetone mixture, and orange-yellow when incubated in hot water (Video 2).

 

Video 2: Chemical Equilibrium: Colorful Demonstrations, Tommy Technetium YouTube Channel (accessed 4/7/2021)

 

Using this system of Cu2+ complexes, I have only been able to generate colors that range from green to yellow by varying temperature. However, you will also note in the video above that addition of salt to this system causes a shift to an orange color, and addition of silver nitrate causes a shift to a blue color. The presence of high Cl- concentration causes each reaction to shift to the right, ultimately forming CuCl42- (Scheme I). On the other hand, addition of silver nitrate causes the removal of Cl- ion because silver ions precipitate out AgCl (s):

Ag+ + Cl- à AgCl(s)              Equation 1

Because removal of Cl- ion has the opposite effect of Cl- addition, addition of silver nitrate shifts each equilibrium listed in Scheme I to the left, ultimately resulting in a color change to blue.

Because these experiments avoid the use of concentrated HCl, they are amenable to having students perform these copper-based equilibrium experiments as part of a laboratory-based exercise. I have indeed had my students use some of these experiments as part of a laboratory-based exercise, and on other occasions I have used these experiments as in-class demonstrations.

Do you have any ideas for how to extend or modify the experiments presented here? If so, I’d love to hear any suggestions you might have.

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. https://www.chemedx.org/blog/multi-colored-equilibrium-experiment

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

Safety: Video Demonstration

Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.

Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.

Anyone thinking about performing a chemistry demonstration should first read and then adhere to the  These guidelines are also available at ChemEd X.

NGSS

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.

Summary:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

Assessment Boundary:
Clarification:

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  and further resources at .

Summary:

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 Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Clarification:

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 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.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can 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.

Assessment Boundary:

Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.

Clarification:

Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.

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  and further resources at .

Summary:

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 Boundary:

Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.

Clarification:

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.