A exergonic reaction It is one that occurs spontaneously and that, in general, is accompanied by a release of energy, either in the form of heat, light or sound. When heat is released, it is said that we are facing an exothermic and exergonic reaction.
That is why the terms 'exothermic' and 'exergonic' are confused, coming to be mistakenly treated as synonyms. This is because many exothermic reactions are also exergonic. Therefore, if a great release of heat and light is observed, such as that caused by lighting a fire, it can be assumed that it consists of an exergonic reaction.
However, the energy released may go unnoticed and may not be so surprising. For example, a liquid medium can heat up slightly and still be the result of an exergonic reaction. In some exergonic reactions that proceed too slowly, not even the smallest increase in temperature is observed.
The central and characteristic point of this type of thermodynamic reactions is the decrease in the Gibbs free energy in the products with respect to the reactants, which translates into spontaneity.
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The main characteristic of an exergonic reaction is that the products have lower Gibss free energies than those of the reactants or reactants (upper image). This fact is usually associated with the fact that the products are chemically more stable, with stronger bonds, more dynamic structures or more “comfortable” conditions..
Therefore, this energy difference, ΔG, is negative (ΔG < 0). Al ser negativa, la reacción en teoría debe ser espontánea. Sin embargo, otros factores también definen dicha espontaneidad, como lo son la energía de activación (la altura de la colina), la temperatura, y los cambios de entalpía y entropía.
All these variables, which respond to the nature of the phenomenon or chemical reaction considered, make it possible to determine whether or not a reaction will be exergonic. And it will also be seen that it does not necessarily have to be an exothermic reaction..
When the activation energy is very high, the reactants require the help of a catalyst to lower said energy barrier. That is why there are exergonic reactions that occur at very low speeds, or that do not occur at all in the first place..
The following mathematical expression encompasses the aforementioned:
ΔG = ΔH - TΔS
The ΔH term is positive if it is an endothermic reaction, and negative if it is exothermic. If we want ΔG to be negative, the TΔS term must be very large and positive, so that when subtracting from ΔH the result of the operation is also negative.
Therefore, and this is another special characteristic of exergonic reactions: they involve a large change in the entropy of the system.
Thus, taking into account all the terms, we can be present before an exergonic reaction but at the same time endothermic; that is, with positive ΔH, a very high temperature, or a large entropy change.
Most exergonic reactions are also exothermic, because if ΔH is negative, and by subtracting another term that is even more negative, we will consequently have a ΔG with a negative value; unless TΔS is negative (entropy decreases), and therefore the exothermic reaction would become endergonic (not spontaneous).
It is important to highlight that the spontaneity of a reaction (whether it is exergonic or not), depends enormously on thermodynamic conditions; while the speed with which it passes, is due to kinetic factors.
From what has been said it is already known that an exergonic reaction is spontaneous, whether or not it is exothermic. For example, a compound can be dissolved in water by cooling it together with its container. This dissolution process is endothermic, but when it happens spontaneously, it is said to be exergonic.
There are “more exergonic” reactions than others. To find out, have the following expression handy again:
ΔG = ΔH - TΔS
The most exergonic reactions are those that occur spontaneously at all temperatures. That is, regardless of the value of T in the above expression, ΔH is negative and ΔS positive (ΔH < 0 y ΔS > 0). They are therefore very exothermic reactions, which does not contradict the initial idea.
Likewise, there may be exothermic reactions where the entropy of the system decreases (ΔS < 0); tal como sucede en la síntesis de macromoléculas o polímeros. En este caso, son reacciones exergónicas solamente a bajas temperaturas, ya que de lo contrario el término TΔS sería muy grande y negativo.
On the other hand, there are reactions that are only spontaneous at high temperatures: when ΔH is positive and ΔS positive (ΔH> 0 and ΔS> 0). We are talking about endothermic reactions. That is why decreases in temperature can occur spontaneously, since they carry with them an increase in entropy.
Meanwhile, there are reactions that are not exergonic at all: when ΔH and ΔS have positive values. In this case, no matter what the temperature is, the reaction will never occur spontaneously. We therefore speak of a non-spontaneous endergonic reaction.
Chemistry is usually characterized by being explosive and bright, so it is assumed that most reactions are exothermic and exergonic.
Exergonic reactions are the combustions of alkanes, olefins, aromatic hydrocarbons, sugars, etc..
Likewise, metal oxidations are exergonic, although they take place more slowly..
However, there are other processes, more subtle, that are also exergonic and very important: the catabolic reactions of our metabolism. Here macromolecules are broken down that act as energy reservoirs, releasing themselves in the form of heat and ATP, and thanks to which the body performs many of its functions.
The most emblematic of these reactions is cellular respiration, as opposed to photosynthesis, where carbohydrates are “burned” with oxygen to transform them into small molecules (COtwo and HtwoO) and energy.
Among other exergonic reactions we have the explosive decomposition of nitrogen triiodide, NI3; the addition of alkali metals to water, followed by an explosion; polymeric syntheses of ethoxylated resins; acid-base neutralizations in aqueous solution; and chemo-luminescent reactions.
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