The Third Law of Thermodynamics applied on open systems

The First Law of Thermodynamics is often applied to ecosystems, first of all when the energy balances of ecosystems are made. Also, the Second Law of Thermodynamics is applied to the ecosystem when we consider the entropy production of ecosystems as a consequence of the maintenance of the system far from thermodynamic equilibrium. This section is concerned with the application of the Third Law of Thermodynamics to ecosystems (see also Chapter 2).

The lesser-known Third Law of Thermodynamics states that the entropies, So, of pure chemical compounds are zero, and that entropy production, AS0, by chemical reactions between pure crystalline compounds is zero at absolute temperature, 0 K. The Third Law implies, since both S0 = 0 (absolute order) and AS0 = 0 (no disorder generation), that disorder does not exist and cannot be created at absolute zero temperature. But at temperatures higher than zero of Kelvin, disorder can exist (S > 0) and be generated (AS > 0). The Third Law defines the relation between entropy production, AS, and the Kelvin temperature, T:

where Acp is the increase in heat capacity by the chemical reaction. Since order is absolute at absolute zero, its further creation is precluded there. At higher temperatures, however, order can be created.

Entropy production implies degradation of energy from a state of high utility (large T) to a state of low utility (small T); compare also Carnot's Cycle (see Chapter 5). Ecosystems have, in other words, a global attractor state, the thermodynamic equilibrium, but will never reach this state as long as they are not isolated and receive exergy (energy that can do work; see Chapter 5) from outside to combat the decomposition of their compounds. As ecosystems have an energy through-flow, the attractor becomes the steady state, where the formation of new biological compounds is in balance with the decomposition processes. As seen from these perspectives of the Second Law of Thermodynamics for open (non-isolated) systems, it is vital for ecosystems to be non-isolated.

It has been stated a few times that it is necessary for an ecosystem to transfer the generated heat (entropy) to the environment and to receive low-entropy energy (solar radiation) from the environment for formation of dissipative structure. The next obvious question would be: will energy source and sink also be sufficient to initiate formation of dissipative structure, which can be used as source for entropy combating processes?

The answer to this question is "Yes." It can be shown by the use of simple model systems and basic thermodynamics; see Morowitz (1968, 1978). He shows that a flow of energy from sources to sinks leads to an internal organisation of the system and to the establishment of element cycles. The type of organisation is, of course, dependent on a number of factors: the temperature, the elements present, the initial conditions of the system, and the time available for the development of organisation. It is characteristic for the system, as pointed out above, that the steady state of an open system does not involve chemical equilibrium.

An interesting illustration of the creation of organisation (dissipative structure) as a result of an energy flow through ecosystems concerns the possibilities to form organic matter from the inorganic components which were present in the primeval atmosphere. Since 1897, many simulation experiments have been performed to explain how the first organic matter was formed on Earth from inorganic matter. All of them point to the conclusion that energy interacts with a mixture of gases to form a large set of randomly synthesised organic compounds. Most interesting is perhaps the experiment performed by Stanley Miller and Harold Urey at the University of Chicago in 1953, because it showed that amino acids can be formed by sparking a mixture of CH4, H2O, NH3 and H2; corresponding approximately to the composition of the primeval atmosphere.

Prigogine and his colleagues have shown that open systems that are exposed to an energy through-flow exhibit coherent self-organisation behaviour and are known as dissipative structures. Formations of complex organic compounds from inorganic matter as mentioned above are typical examples of self-organisation. Such systems can remain in their organised state by exporting entropy outside the system, but are dependent on outside energy fluxes to maintain their organisation, as was already mentioned and emphasised above. Glansdorff and Prigogine (1971) have shown that the thermodynamic relationship of far from equilibrium dissipative structures is best represented by coupled non-linear relationships, i.e. autocatalytic positive feedback cycles.

Given this necessary condition, simple energy flow through a system provides a sufficient condition. Creation of order is inevitable. On Earth, the surface temperature difference between sun and planet guarantees this. Morowitz (1968, 1978) showed, as mentioned above, that energy through-flow is sufficient to produce cycling, a prerequisite for the ordering processes characteristic of living systems.

A system at 0 K, on the other hand, is without any creative potential, because no dissipation of energy can take place at this temperature. A temperature greater than 2.726 ± 0.01 K, where 2.726 K is the temperature of deep space, is therefore required before order can be created. At 0 K, the world is dead and still as the temperature is a measure of the velocity of atoms. The so-called Bose-Einstein condensate is formed, where all atoms are the same and behave like one single atom. This was predicted by Bose and Einstein in the 1920s, but has recently been shown experimentally at temperatures very close to 0 K.

The velocity is zero at 0 K by definition and therefore determined without uncertainty. This explains that the position is undetermined according to Heisenberg's uncertainty equation. At 0 K there is therefore no structure, no gradients, and no complexity. No entropy can be formed because all mass is everywhere and nowhere and without form and structure. There is no disorder to create and therefore no entropy to produce. The system is trapped between complete order because all the mass occupies all the space, and complete disorder because all the space is occupied by mass—a complete dissipation has taken place. At 0 K, no creativity is possible, no differences (gradients), no structure and even no physical activity, because all velocities are zero. Everything is dull and dead. Even the light has stopped. Time has no meaning because time is determined by the rate of changes.

These extreme conditions at 0 K elucidate the meaning of the concept entropy. Entropy is, on the one side, the price we have to pay for order, structure, organisation and creativity, but without entropy there would be no order, structure, organisation and creativity.

Moreover, it explains the meaning behind the Second Law. Because heat is an energy form which is generated by transformation of all other energy forms and because a 100% effective transformation of heat to work cannot take place because in the Carnot Cycle the cold reservoir can never be maintained at 0 K. Energy that can do work is inexorably lost to energy that cannot do work. This is the condition which is imposed on us: time and all reactions are irreversible.

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