The chemical reaction between violet-colored thionin and iron (II) ions produces a leucothionin, a colorless compound (Equation 1).^1,2

Equation 1

Because DG^o = + 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               E^o = 0.38V                   Equation 2

Fe³⁺(aq) + e^- → Fe²⁺(aq)                                   E^o = +0.77V                 Equation 3

Notice that Equation 1 results if we subtract twice Equation 3 from Equation 2. The overall cell potential (E^ocell = E^ored – E^oox) 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 (DG^o) can be calculated using:

DG^o = - nFE^o                 Equation 4

Where n is the number of electrons transferred in the reaction, F is Faraday’s constant (96485 C mol^-1), and E^o is the cell potential for the reaction described in Equation 1. For Equation 1, n = 2 and E^o = -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, DG^o = -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).