The chemical reaction between violet-colored thionin and iron (II) ions produces a leucothionin, a colorless compound (Equation 1).1,2
Equation 1
Because DGo = + 75 kJ mol-1 for this reaction (see below), it is not spontaneous. However, light energy can be used to power the reaction. Thus, the purple color of a solution of thionin and iron (II) ions is observed to fade when exposed to light. When the light is turned off, the color returns as the reaction proceeds spontaneously in the reverse direction. These facts allow for the setup of some fascinating chemical reactions (Video 1).3
Video 1: Chemistry lights up my life, @pchemstud on TikTok, accessed April 18, 2022.
The absorption spectrum of thionin indicates that it absorbs yellow light very well (Figure 1).
Figure 1: Absorption spectrum of thionin
Because of this, I wondered if it could be demonstrated that yellow light drives this reaction (Equation 1). Check out the experiments I did to test for this (Video 2).4
Video 2: A Chemical Reaction that is Powered by Light, Tommy Technetium YouTube Channel, April 10, 2022
Connections to the chemistry curriculum
This experiment may be used to connect to topics in electrochemistry, thermodynamics, and quantum chemistry.
Connections to electrochemistry
The overall reaction (Equation 1) involves the oxidation of Fe2+ and the reduction of thionin. Therefore, we analyze Equation 1 by considering the following half reactions:
thionin + H+(aq) + 2e- → leucothionin Eo = 0.38V Equation 2
Fe3+(aq) + e- → Fe2+(aq) Eo = +0.77V Equation 3
Notice that Equation 1 results if we subtract twice Equation 3 from Equation 2. The overall cell potential (Eocell = Eored – Eoox) for Equation 1 is therefore -0.39V. This negative cell potential indicates that the reaction should be non-spontaneous, in accordance with the observation that the reaction requires light energy to proceed.
Connections to thermodynamics
From the calculated cell potential, the change in the standard Gibbs energy (DGo) can be calculated using:
DGo = - nFEo Equation 4
Where n is the number of electrons transferred in the reaction, F is Faraday’s constant (96485 C mol-1), and Eo is the cell potential for the reaction described in Equation 1. For Equation 1, n = 2 and Eo = -0.39 V. Substitutions of these values into Equation 4 yields +75 kJ mol-1. Once again, the positive Gibbs energy for this reaction is consistent with the non-spontaneity of Equation 1, and the fact that light energy is required to drive the formation of leucothionin from thionin and iron (II) ions. On the other hand, DGo = -75 kJ mol-1 for the reverse reaction (Equation 1). Therefore, the color returns spontaneously when the light is turned off.
Connection to quantum chemistry
As indicated in Video 2, thionin absorbs yellow light quite well. Indeed, the absorption spectrum of thionin peaks at about 600 nm (Figure 1). The energy, E, of a yellow photon can be calculated using:
E = hc/l Equation 5
Where h is Planck’s constant (6.626 x 10-34 J s), c is the speed of light in a vacuum (3.0 x 108 m s-1), and l is the wavelength of the photon in question. Substitution of these constants and l = 600 x 10-9 m into Equation 5 yields 3.31 x 10-19 J for a yellow photon. Multiplication of this energy by 6.02 x 1023 photons per mole means that 200 kJ of energy accompanies every mole of yellow photons. The energy per mole of yellow photons clearly exceeds the 75 kJ mol-1 necessary to drive Equation 1 to the right. This is consistent with the observation that yellow light absorbed by the thionin reaction mixture caused the thionin to fade (Video 2).
Conclusion
The reaction between thionin and iron (II) is very easy to set up, connects seamlessly to a variety of chemical topics, and is a real crowd pleaser. It is definitely worth performing for your students. Drop me a line in the comments if you try out this experiment. If you have already conducted this experiment, let me know how you include it in your chemistry classes.
Happy Experimenting!
References
1. Thionin – The Two Faced Solution, Flinn Scientific, Inc., 2017. https://www.flinnsci.com/api/library/Download/1e347e52d1fa4253b324d388aea826d1 (accessed April, 2022).
2. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: 2011; Vol. 5, pp 249−255.
3. Chemistry lights up my life, Tommy Technetium, TikTok video. https://www.tiktok.com/@pchemstud/video/7083104858265488686 (accessed April, 2022).
4. A Chemical Reaction that is Powered by Light, Tommy Technetium, YouTube Video. https://www.youtube.com/watch?v=j_ZnD71zbSM (accessed April, 2022).
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
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.
Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.
Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?
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 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 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 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 does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.
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 design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.
*More information about all DCI for HS-PS3 can be found at https://www.nextgenscience.org/topic-arrangement/hsenergy.
Students who demonstrate understanding can design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.
Assessment for quantitative evaluations is limited to total output for a given input. Assessment is limited to devices constructed with materials provided to students.
Emphasis is on both qualitative and quantitative evaluations of devices. Examples of devices could include Rube Goldberg devices, wind turbines, solar cells, solar ovens, and generators. Examples of constraints could include use of renewable energy forms and efficiency.
Students who demonstrate understanding can use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
*More information about all DCI for HS-PS4 can be found at https://www.nextgenscience.org/topic-arrangement/hswaves-and-electromagnetic-radiation.
Students who demonstrate understanding can use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
Assessment is limited to algebraic relationships and describing those relationships qualitatively.
Examples of data could include electromagnetic radiation traveling in a vacuum and glass, sound waves traveling through air and water, and seismic waves traveling through the Earth.
Students who demonstrate understanding can evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
*More information about all DCI for HS-PS4 can be found at https://www.nextgenscience.org/topic-arrangement/hswaves-and-electromagnetic-radiation.
Students who demonstrate understanding can evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
Assessment does not include using quantum theory.
Emphasis is on how the experimental evidence supports the claim and how a theory is generally modified in light of new evidence. Examples of a phenomenon could include resonance, interference, diffraction, and photoelectric effect.