The purpose of this section is to show that the drive for a chemical reaction can be traced back to the same basic phenomenon as the drive which leads to the temperature compensation between a hot and a cold substance, as presented in section 6.3. The four basic questions to the drive remain relevant in this section as well, and at the end of this chapter we will see that the answers are in principle the same.

On the basis of a model experiment with the shelf model, we analyze the drive of an endothermic forward reaction to a chemical equilibrium.

Video with sound: model attempt to establish an equilibrium, forward reaction

The following two pictures show the initial state of the reactants and the final state of the set equilibrium. Since the lines of the half-value energies are sketched, it is easy to see that the equilibrium has set itself on an endothermic path.
On the product side, the shelf is somewhat darker. It stands as in the video behind the reactant shelf.

On the reactant side, 6 levels are occupied and in equilibrium the particles are distributed to 9 levels. This is an indication that the entropy has increased. The way in which the product shelf is filled up in the individual steps and removed from the reactant shelf corresponds precisely to the basic rule. Each new distribution is "Boltzmann-like". With the Thermulation-I program, it can be shown that the half-value energy of reactants and products is the same in every step.

If at constant energy all the particles are brought to the product side, only two levels are occupied and the temperature has become even colder. This shows the next picture.

Since now the lowest level is clearly above the zero level of the total system, the particles are compressed to the two lowest levels of the product side because the total energy is too small to occupy even higher levels. This state would not spontaneously and voluntarily be reached from the reactants. There only two energy levels would be occupied, indicating a low entropy. Our experience teaches us that the natural processes end at maximal entropy.
The half-value energy, ie, the temperature, has decreased even further because the process is also endothermic even beyond the equilibrium. However, the cause of the endothermics of the overall process is not the change in the level distances between reactant and product side, but the difference in energy height of the lowest energy levels on the reactant and product side.

But once the final substance has been obtained by other means, it can be cooled to this temperature. Then one can establish that this substance spontaneously and voluntarily takes the exothermic reaction, of course only if there is no kinetic inhibition. This case can be discussed with the following picture:
It can be seen that now even the particles from the lowest level of the product side can emit photons if they "fall" into the lower levels of the reactant side. The number of occupied levels, ie the entropy, increases, and this will continue until the entropy becomes maximum again.

If particles fall down during the reaction, some have to be raised to higher levels because of energy conservation. This is achieved by the photons that are emitted during the transition to the lower levels. The photons do not leave the system because of the isolation, but instead bring their own system particles to higher levels. The particles are distributed to more than two levels, and the half-value energy, ie the temperature rises. The back reaction is exothermic.

At the end of the section on the drive for chemical reactions we want to check again how we can now answer the same four questions about the drive from section 6.3.

1. Spontaneous start?
   

   The reactants are present in a macrostate with a certain dominant distribution. By abolishing the kinetic inhibition, however, a different distribution of the thermal total energy in the overall system becomes dominant because other, previously unoccupied levels are now suddenly attainable. If these levels are at a lower energy level, particles change with the emission of photons into these levels. If the system is isolated, the photons remain in the system and other particles can also change to higher sub-occupied levels by absorbing these photons.


2. Decrease of process speed?
   

   After the start, more and more reactant particles enter the product side during the process. Both envelopes have the same half-value energy, but the envelopes are not yet at the same level. The picture above shows such an intermediate state with drawn half-value energies. The emission intensity of the reactants decreases steadily, possibly (with endothermia) by the sinking temperature. As a result, the process slows down and finally the drive vanishes when the dominant distribution of the total energy in the overall system and thus the entropy maximum is reached.

3. "Standstill halfway"?
   

   A system can not remove itself from a dominant distribution by its own efforts. The "chemical" equilibrium is determined by the type of substances and the initial conditions.

4. Drive for the reversal process?
   

   In this case, the reversal process would mean that the products again form reactants. This phenomenon is known and occurs in two cases:
   1. Starting from the pure product and removing the kinetic inhibition,
   2. When a set chemical equilibrium is disturbed. Depending on the nature of the disturbance, the equilibrium reacts with the spontaneous onset of the forward reaction or the back reaction. This case is discussed in detail in the accompanying booklet on the Thermulation-I program (see also section 8.2 / 8).

We can now summarize the answers to the four questions on the thermodynamic drive in temperature equilibrium and chemical reaction:
Spontaneous emission and absorption are very helpful interpretive patterns in both cases. The difference essentially consists in the fact that during the chemical reaction, the transitions between different storage systems are carried out, whereas during the temperature compensation the transitions are possible only within the individual storage shelves.
In both cases, a dominant distribution of the total energy is found in the total storage system. In both cases, the entropy maximum occurs. The difference is that in thermal equilibrium the total entropy is additive composed of the different individual entropies, whereas in the chemical equilibrium the total entropy of the equilibrium state is greater than the sum of the reactant and product entropies. In the known additive property of entropy, it is often overlooked that it applies only to systems which are insofar independent of each another that they can only interact by the exchange of photons. In contrast, in the case of chemical equilibrium, the reactant particles can as well attain the energy levels of the product side and vice versa. (see section 8.2 / 2)