Light and Exothermic Reactions
Date: Spring 2012
My students are working on a demonstration that illustrates either an endothermic or exothermic process. They've decided on the Luminol reaction with 3% hydrogen peroxide. I have always thought (perhaps incorrectly) that "thermic" is used when discussing heat energy - and there is very little heat change in this reaction. In the practice demo that the students did, the temperature dropped only about a degree C. However, there was a significant amount of light energy given off. When light energy is released, is that also considered exothermic? Or would it be more correct to call it an endothermic reaction (because the temp dropped), but also an exergonic reaction (because light was released)?
Good question! The confusion lay hidden in the unfortunate language that is being used. So lets avoid the confusion of language and go back to our thermodynamic roots.
Thermodynamics regards any energy release as being equal; whether expressed as heat, light, mechanical, spin change, elevation to a higher energy state or any variation thereof. That is pretty much it. We really do not get too involved with the word “therm” being just a temperature elevation alone. We are interested and focused on the energy and its flow.
Entropy drives most reactions to achieve a lower energy level. Usually activation energy is required or overcome, but a lower energy level drives the normal equilibrium. In the Luminol/H2O2 reaction, the product is elevated to a spin configured excited electronic state, which releases a photon(energy) as the energy of the molecule relaxes to an electronic ground state. The energy is released as a photon, thereby leaving the reaction event so that the local environment may be cooled in terms of temperature.
In contrast, is a reaction we commonly think of as “heat releasing” where excitations are manifested as release of energy through physical vibrational modes and therefore, the local environment is warmed.
Both reactions above are exothermic, exergonic and energy releasing.
The reference below is a very well thought out, carefully composed lab exercise using the Luminol reaction and investigates the “thermic” nature of the reaction. This lab is conducive to 9-12 science education in assisting your students with investigation of energy.
May your light shine on!
Peter E. Hughes, Ph.D.
I think you are generally correct to refer to “thermic” as a term to describe whether heat is liberated (exo-) or absorbed (endo-) in a chemical reaction. But you have to be careful. It is often difficult to determine the “thermic” behavior of a chemical reaction if the thermal behavior is small. In addition, the “thermic” behavior is frequently temperature is often temperature dependent. It may be (exo-) at one temperature and (endo-) at another temperature. A classic example is the “thermic” behavior of the ionization of weak acids and/or bases. The light liberated in the chemical reaction is generally not used in determining the energy balance even though it is part of the energy liberated in the reaction.
Measuring the heat of reaction is not an easy experiment. It requires careful calorimetric measurement. This requires careful experimental procedures and apparatus.
Do not get too hung up on putting the thermal behavior of a chemical reaction in a particular “box” because many reactions can behave differently depending upon the experimental conditions.
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Update: June 2012