ENDERGONIC REACTION: CHARACTERISTICS, EXAMPLES - CHEMISTRY - 2025

An endergonic reaction is one that cannot occur spontaneously, and also requires a high supply of energy. In chemistry, this energy is generally caloric. The best known of all endergonic reactions are endothermic reactions, that is, those that absorb heat to occur.

Why aren't all reactions spontaneous? Because they go uphill to the laws of thermodynamics: they consume energy and the systems formed by the species involved decrease their entropy; that is, for chemical purposes, they become more molecularly ordered.

Source: Pxhere

Building a brick wall is an example of an endergonic reaction. Bricks alone do not compact enough to form a solid body. This is because there is no energy gain that promotes their unions (also reflected in their possible low intermolecular interactions).

So, to build the wall you need cement and a labor force. This is energy, and the non-spontaneous reaction (the wall will not be built automatically) becomes possible if an energy benefit is perceived (economic, in the case of the wall).

If there is no benefit, the wall will collapse under any disturbance, and its bricks will never be able to hold together. The same is true of many chemical compounds, whose building blocks cannot spontaneously come together.

Characteristics of an endergonic reaction

What if the wall can be built spontaneously? To do this, the interactions between the bricks must be very strong and stable, so much so that no cement or a person will need to order them; while the brick wall, although it is resistant, is the hardened cement that holds them together and not properly the material of the bricks.

Therefore, the first characteristics of an endergonic reaction are:

-It is not spontaneous

-Absorbs heat (or another type of energy)

And why does it absorb energy? Because their products have more energy than the reactants involved in the reaction. This can be represented by the following equation:

ΔG = G _Reactive -G _Products

Where ΔG is the change in Gibbs free energy. Since G _Product is greater (because it is more energetic) than G _Reagents, the subtraction must be greater than zero (ΔG> 0). The following image further summarizes what has just been explained:

Source: Gabriel Bolívar

Note the difference between the energy states between the products and the reactants (purple line). Therefore, the reactants do not become products (A + B => C) if there is no heat absorption first.

Increases the free energy of the system

Every endergonic reaction is associated with an increase in the Gibbs free energy of the system. If for a certain reaction it is true that ΔG> 0, then it will not be spontaneous and will require a supply of energy to be carried out.

How to know mathematically if a reaction is endergonic or not? Applying the following equation:

ΔG = ΔH – TΔS

Where ΔH is the enthalpy of the reaction, that is, the total energy released or absorbed; ΔS is the entropy change, and T is the temperature. The TΔS factor is the energy loss not used in the expansion or arrangement of the molecules in a phase (solid, liquid or gas).

Thus, ΔG is the energy that the system can use to do work. Since ΔG has a positive sign for an endergonic reaction, energy or work must be applied to the system (the reactants) to obtain the products.

Then, knowing the values ​​of ΔH (positive, for an endothermic reaction, and negative, for an exothermic reaction), and TΔS, it is possible to know if the reaction is endergonic. This means that although a reaction is endothermic, it is not necessarily endergonic.

The ice cube

For example, an ice cube melts into liquid water, absorbing heat, which helps separate its molecules; however, the process is spontaneous, and therefore it is not an endergonic reaction.

And what about the situation where you want to melt the ice at a temperature well below -100ºC? In this case, the TΔS term in the free energy equation becomes small compared to ΔH (because T decreases), and as a result, ΔG will have a positive value.

In other words: melting ice below -100ºC is an endergonic process, and it is not spontaneous. A similar case is that of freezing the water around 50ºC, which does not happen spontaneously.

Your product links are weaker

Another important characteristic, also related to ΔG, is the energy of the new bonds. The bonds of the products formed are weaker than those of the reactants. However, the decrease in the strength of the bonds is compensated by a gain in mass, which is reflected in the physical properties.

Here the comparison with the brick wall begins to lose meaning. According to the above, the bonds within the bricks must be stronger than those between them and the cement. However, the wall as a whole is more rigid and resistant due to its greater mass.

Something similar will be explained in the examples section but with sugar.

It is coupled with exergonic reactions

If endergonic reactions are not spontaneous, how do they take place in nature? The answer is due to the coupling with other reactions that are quite spontaneous (exergonic) and that somehow promote their development.

For example, the following chemical equation represents this point:

A + B => C (endergonic reaction)

C + D => E (exergonic reaction)

The first reaction is not spontaneous, so it naturally could not occur. However, the production of C allows the second reaction to occur, causing E.

Adding the Gibbs free energies for the two reactions, ΔG ₁ and ΔG ₂, with a result less than zero (ΔG <0), then the system will present an increase in entropy and therefore will be spontaneous.

If C did not react with D, A could never form it, because there is no energy compensation (as in the case of money with the brick wall). It is then said that C and D "pull" A and B to react, even though it is an endergonic reaction.

Examples

Source: Max Pixel

Photosynthesis

Plants use solar energy to create carbohydrates and oxygen from carbon dioxide and water. CO ₂ and O ₂, small molecules with strong bonds, form sugars, with ring structures, which are heavier, more solid, and melt at a temperature of around 186ºC.

Note that the CC, CH and CO bonds are weaker than those of O = C = O and O = O. And from a sugar unit, the plant can synthesize polysaccharides, such as cellulose.

Synthesis of biomolecules and macromolecules

Endergonic reactions are part of anabolic processes. Like carbohydrates, other biomolecules, such as proteins and lipids, require complex mechanisms that, without them, and coupling with the hydrolysis reaction of ATP, could not exist.

Likewise, metabolic processes such as cellular respiration, the diffusion of ions across cell membranes, and the transport of oxygen through the bloodstream are examples of endergonic reactions.

The formation of diamonds and heavy compounds from crude oil

Diamonds require enormous pressures and temperatures, so that their components can be compacted into a crystalline solid.

However, some crystallizations are spontaneous, although they occur at very slow speeds (spontaneity has no relation to the kinetics of the reaction).

Finally, crude oil alone represents a product of endergonic reactions, especially heavy hydrocarbons or the macromolecules called asphaltenes.

Its structures are very complex, and their synthesis takes a long time (millions of years), heat and bacterial action.

References

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4. Lougee, Mary. (May 18, 2018). What is an Endergonic Reaction? Sciencing. Recovered from: sciencing.com
5. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Endergonic vs Exergonic (With Examples). Recovered from: thoughtco.com
6. Arrington D. (2018). Endergonic reaction: definition & examples. Study. Recovered from: study.com
7. Audersirk Byers. (2009). Life on Earth. What is Energy?. Recovered from: hhh.gavilan.edu