7. Experiments II :
Substance Conversions

In the second experimental chapter, we shall turn to those phenomena in which the energy storage system itself changes. The storage systems for the thermal energy of translation, rotation and vibration differ in their structure and knowledge of these differences is helpful in understanding the different material properties in the solid, liquid and gaseous state.

The storage systems can change in that the energy level distances do not remain constant in a process. It may be associated with chemical reactions that the lowest energy levels of the various storages are not at the same energy height. It may also happen that during the course of a reaction, storages of the translation, rotation and vibration are added or disappear.

Classical thermodynamics of the 19th century was and is an isothermal theory in its dominant parts. Although virtually no chemical reaction actually occurs isotherm and under standard conditions, it has of course been known that both reactants and products can exist under standard conditions. A temperature compensation with the environment automatically follows a reaction if an isolation is removed.

After this preliminary work, we want to turn to an endothermic chemical process in the real experiment.

Video: endothermic reaction in isolated system

After removing the partition wall and shaking, a solid reacts with a liquid. The temperature drops by 17K in about 10s. However, we do not yet know which substances have actually been used here. It is also still unclear which changes to the storage system are responsible for the temperature drop. Both are to be clarified after we have also seen an exothermic reaction.

Video with sound: exothermic reaction in an isolated System

The individual experimental steps are completely analogous to the endothermic process. This time, two fluids react with each other. After removing the partition wall and shaking, the temperature rises in 10s around 16K.

Interpretation for both attempts:

In the first experiment, solid ammonium chloride reacted endotherm with distilled water and formed hydrated ions. A neutral substance splits endotherm into opposite-charged ions.

The endothermic core of this reaction consists of the cleavage of the ionic bond. Although the ions of the solid salt become exothermically hydrated here, the ion-dipole interaction in the hydrate shell is weaker than the ion-ion interaction in the solid salt, so that overall the endothermic influence predominates.

The ion-dipole bonds between the hydrate water dipoles and the hydrogen or hydroxide ions are split and the two central particles form a polar atomic bond. The water molecules of the hydrate shells are written in the reaction equation on the product side separately from the newly formed water molecule. We must be aware, however, that all water particles are indistinguishably inserted into the water clusters. The exothermic nucleus of this reaction consists in the formation of a polar atomic bond from two far-distant ions with opposing charges. Although the bonds of the hydrate shells must be destroyed endothermically, they are partly re-formed in the exothermic cluster formation. It is, therefore, obvious to attribute the temperature change mainly to the formation of the atomic bond.

The endothermic reaction in the second video is practically the reverse reaction of the above process: A neutral substance is decomposed by water into hydrated ions. However, the bond which now has to be splitted is not a polar atomic bond but the ionic bond in the solid ammonium chloride:

Both processes behave like reaction and backreaction:

1. endotherm: A neutral substance is decomposed by water into hydrated, opposite-charged ions.

2. exotherm: Opposite-charged ions neutralize each other exotherm to a neutral molecular substance.

Let us now turn to the quantum-chemical questions and investigate the changes in the storage system which are exothermic, ie, temperature-increasing, or endothermic, ie temperature-decreasing, in a chemical reaction. In this investigation, we can restrict ourselves to the estimation according to the force rule because the law of conservation of mass applies to the course of a reaction.
When dealing with thermal inertia above we have already pointed out that a storage system with small energy level distances, up to a certain temperature, can store more thermal energy than a system with large level distances. If the particles are displaced in the course of a reaction together with their constant thermal energy into stores with more dense energy levels, the temperature in the system drops. A high level density means a large storage capacity. At the same time, denser levels also indicate that the particles are exposed to weaker restoring forces.
From the first experiment we can conclude that in the case of the exothermic neutralization, on account of the formation of the strong atomic bond on the one hand, and the embedding of the new water molecule by strong H-bonds into the cluster structure on the other, the energy level density on the product side decreased. With constant energy, the temperature must rise. In the fluid model, the filling height (temperature) increases because the storage capacity (cross-sectional area) decreases. The following figure shows this model representation. Since the filling level is the absolute temperature in the fluid model, the changes we observe in conventional experiments must always be relative to the overall scale. Therefore, the temperature changes in the model image appear quite small, but perceptible. Here one reads on the lateral scale about half a model temperature unit 0.5 tu, which would however correspond in the real experiment probably about 50K. This was of course not achieved in the above-mentioned case. The following sketch is intended to illustrate the fact that in equilibrium the entropy, ie, the cross-sectional area in the fluid model, becomes maximal. The Thermulation software - unlike in reality - allows us to push all reactant particles beyond the equilibrium state to the product side. As a result, the fluid model reveals that the entropy decreases again and consequently the entropy maximum exists in equilibrium state.

The interpretations in this section are kept deliberately simple by discussing the temperature changes only with the fluid model. In this model, the energy heights of the lowest energy levels of reactant and product are not visible even though they have been taken into account. In both cases, however, a satisfactory visualization of the experimental results was achieved. This second "set screw" is discussed in section 7.2 dealing again with the chemical equilibrium.

As an interpretation for the second experiment, we can say that the water molecules break up the ionic bonds in the solid ammonium chloride and bring the oppositely charged ions at a great distance. The large distance of the hydrated ions on the one hand and the large dielectric constant of the water on the other reduce the restoring forces, thus increasing the storage capacity (cross-sectional area) and lowering the temperature (filling level) with constant energy. From the following picture, it can be seen that the temperature change is also about half a model unit - however endothermic.

The spontaneity of a process does not depend on the exo- or endothermia of the process. After the vigorous endothermic process in the cleavage of an ionic bond in the preceding section, in the following we are dealing with an equally strong exothermic and spontaneous process.
A supersaturated solution of sodium acetate is crystallized with a seed crystal. For comparison, an approximately equal portion of seed crystals is added to a dilute aqueous solution.

Video: Spontaneous Crystallization

The kinetic inhibition allows the solution to be supersaturated. The cause of the inhibition is that the formation of a new surface of the solid sodium acetate is inhibited. However, a seed crystal already brings this new surface with, so that only the surface of the crystals has to be enlarged during crystallization. When considering the thermodynamic drive in section 7.4 we again come back to the kinetic inhibition.