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Exergonic vs. Exothermic: Different Energy Reactions

Various chemical reactions can occur naturally or with added energy, and exergonic and exothermic are the two types of energy-releasing reactions. These chemical reactions are key components of thermodynamics, as they involve the transfer of energy with heat as a product.

Even though they describe very similar reactions and processes, the terms are not synonymous, so what’s the difference?

The difference between exergonic reactions and exothermic reactions lies in the processes through which they release energy. During exergonic reactions, the breaking of weak bonds releases the energy scientists refer to as free energy. Exothermic reactions release energy to their surroundings, usually in the form of heat.

This article will discuss what exergonic and exothermic mean, as well as their counterparts, endergonic and endothermic reactions. We will then explore how the processes work and look in their environment.

We will close with the differences between these reactions, as well as the reactions which absorb energy. 

Understanding the Meanings of Exergonic and Exothermic

The two types of chemical reactions that release energy, exergonic and exothermic, have variances in their definitions as well as their processes. Reactions that transfer energy do so to release energy from breaking weaker bonds and creating stronger ones. 

Their opposites, endergonic and exothermic reactions, also have their place in helping to understand thermodynamics. Reactions that absorb energy do so to create stronger products.

Knowing how each reaction works will help with identifying how they are different. 

What Makes a Reaction Exergonic?

It is important to understand the term “free energy” before delving into the definition of an exergonic reaction. Most often called Gibbs free energy, the term refers to the amount of usable energy within a process or system. 

As defined in thermodynamics, Usable energy is the energy that can move from a system to its surroundings via a reaction.

During a reaction, the amount of Gibbs free energy within a system (ΔG) increases or decreases, depending on the reaction type.

In exergonic reactions, the amount of free energy within a system decreases to level the system’s energy after releasing added energy (source).

The bonds formed by exergonic reactions are stronger than the ones broken in the reaction. Since there is now a difference in energy, the amount of energy must balance by releasing the broken bonds’ excess energy.

The release of Gibbs free energy from the broken bonds means that the final state (after the reaction) contains less energy than the initial state (before the reaction) because the extra energy is released.

Since there is a loss in ΔG, scientists note the change as negative. 

They also note spontaneous reactions as negative changes in ΔG. Almost all exergonic reactions are spontaneous, meaning they need no outside energy for them to occur. Reactions must occur at a constant temperature to make them spontaneous.

Some exergonic reactions do not occur spontaneously and need an outside energy source instead. We call this outside energy activation energy, and once applied, the exergonic reaction process can start (source).  

What Makes a Reaction Exothermic?

Exothermic reactions release energy, usually in the form of heat, into their surroundings. These chemical reactions can also transfer light or sound energy. The system releases the energy in order to keep the energy the same within the system. 

They are spontaneous reactions that result in a lowered amount of stored energy or enthalpy (ΔH) of the system.

The amount of initial energy within a system is more than the amount left after energy transfer. Energy is released in the form of heat to keep the amount of energy the same (source).

Physicists write exothermic reactions as having a negative heat flow because heat transfers to the surroundings instead of being absorbed, resulting in a decrease in ΔH within the system (source).  

Endergonic and Endothermic

Exergonic and exothermic reactions have inverse chemical reactions. Both processes involve releasing energy, while their respective opposites, endergonic and endothermic, absorb energy instead.

Endergonic reactions absorb free energy instead of releasing it like exergonic reactions — the change in energy results in an increase in ΔG, which researchers note as positive. 

Unlike exergonic reactions, endergonic reactions are not spontaneous. They need to have activation energy transferred or applied in order to make the reaction begin.

The nonspontaneous nature of exergonic reactions is also noted as a positive change in ΔG. 

Endothermic reactions, the opposite of exothermic, occur when the system absorbs energy — usually in the form of heat. Because of the heat transfer, the resulting product feels colder. 

Both endergonic and endothermic reactions are nonspontaneous energy transfers, which result in a positive change in ΔG for the reactions.

Identifying Exergonic and Exothermic Reactions

Laboratory, Chemistry, Chemical, Compounds, Experiment
Image by bdyczewski via Pixabay

The idea of absorbing and releasing energy to maintain equilibrium within systems is a fairly abstract one. Being able to connect the definitions to pictures will make these reactions more concrete. 

Examples of Exergonic Reactions

Exergonic reactions, or those which occur spontaneously and transfer energy, occur naturally in many processes. On a larger scale, combustion is one such reaction.

The combination of oxygen and a fueling agent causes combustion to occur. Fueling agents could be propane, acetone, methanol, etc. The chemical reaction of the oxygen and fuel results in an increased amount of energy in the system.

After the exergonic reaction occurs, the extra energy releases as heat. 

There is usually smoke release with the heat as well. While combustion, or burning, is often accompanied by a flame, it is not necessarily a product of the reaction. 

Cellular respiration is another form of an exergonic reaction. Oxygen combines with glucose, which produces energy. This energy takes the form of carbon dioxide, water, and usable energy.

The cells store usable energy while they release carbon dioxide and water as extra energy. 

The cells then use this transferred usable energy to power a variety of cellular processes. Without the energy, cells would not be able to move themselves or molecules across cell membranes.  

Examples of Exothermic Reactions

Exothermic refers to spontaneous chemical reactions where something releases energy into the surroundings, usually in the form of heat. 

When oxygen combines with iron or a metal that contains iron, the result is extra energy. This energy then transfers to its surroundings in the form of rust.

The addition of water can increase the amount of produced rust. The water molecules contained in cracks or surfaces of metal can combine with other elements to cause the exposure of more metal.

The oxygen is then able to combine with a larger area of iron-containing metals.

Exothermic reactions also occur as the body metabolizes food. This chemical reaction, commonly known as digestion, involves the process of breaking down food substances like proteins into sugars and amino acids. 

Enzymes within the digestive organs, such as the stomach, combine with proteins.

The additional energy within the system then releases into its surroundings in the energy units that can do work for the body. Eventually, the system releases the end byproducts as carbon dioxide and water (source). 

Examples of Endergonic Reactions

Endergonic reactions are nonspontaneous chemical reactions that absorb energy. They are the opposite of exergonic reactions, which usually don’t need activation energy and transfer energy instead.

Photosynthesis is an example of an endergonic reaction. Water and carbon dioxide molecules combine with help from the sun. The light from the sun acts as an activation agent to convert the water and carbon dioxide into glucose and oxygen.

The plant cells store energy from the glucose while releasing oxygen as a waste product. 

Here, light, water, and carbon dioxide are the reactants. These reactants are weaker than products of glucose and oxygen. 

Melting an ice cube is another example of endergonic reactions. When adding heat to an ice cube, it begins to melt. Eventually, it will change forms from a solid to a liquid. 

The heat acts as activation energy to turn the solid ice cube into the liquid form of water. The ice cube will continue to melt as it absorbs the heat energy because its temperature is rising.

DNA replication is also an example of an endergonic reaction. The two strands of the double helix are copied during the replication process.

In order to copy the strands individually, energy must be absorbed. Our article on DNA replication has more information. 

Examples of Endothermic Reactions

Endothermic chemical reactions are ones that need an activation energy to begin the absorption of energy. Their opposites, exothermic reactions, are spontaneous and involve the process of transferring energy.

One example of endothermic reactions is baking bread. When adding heat to bread ingredients — usually a combination of at least bread flour, water, yeast, and salt — as activation energy, the ingredients absorb the heat. 

When the reactants combine and absorb the heat, the synthesis of the bread ingredients begins, creating bread as the final product.

Sweating is another example of an endothermic reaction. When a person exposes their skin to heat for a long period of time, they will begin to feel warmer.

The source of the heat could be the sun, overexertion, exercise, etc. As the body temperature increases, a person will begin to sweat. 

The sweat then absorbs the added heat, which has increased the body temperature, and it evaporates. The goal of sweating is to cool one’s body, achieved because, as the sweat absorbs the heat, the body temperature reduces through evaporation. 

Differences in Reactions that Release or Absorb Energy

Even though exergonic and exothermic reactions seem very similar, they do have their differences as well. They not only have different qualities from their counterparts but from their opposite reactions as well. 

Exergonic reactions are spontaneous chemical reactions that create stronger bonds from breaking weaker ones.

The energy from the weaker bonds then releases or transfers to the system’s surroundings. Endergonic reactions (absorbing energy) are the opposite of exergonic reactions. 

Exothermic reactions are examples of exergonic reactions. They do not need activation energy to start the reaction that transfers energy from a system to its surroundings.

Exothermic reactions usually release this energy in the form of heat. Endothermic reactions have opposing characteristics to exothermic ones.

As for chemical reactions that absorb energy, there are endergonic and endothermic reactions. Endothermic reactions are examples of endergonic reactions.

Endergonic reactions require activation energy to start the process of absorbing energy. The addition of this extra energy helps the weaker reactants combine, and the resulting product has much more energy than the reactants. 

Endothermic reactions are also nonspontaneous and refer to an energy-absorbing process. The activation energy, again, usually in the form of heat, is absorbed to kickstart the reaction (source).

Types of Reactions

ExergonicExothermic
Prefix ex- means “out of” or “outside”Prefix exo- means “outside” or “turning out”
Spontaneous reactionsSpontaneous reactions
Transfers energy from the system to surroundingsTransfers energy from the system to surroundings (usually in the form of heat or light)
Stronger reactants than productStronger reactants than product
Weak bonds break to create stronger ones. The extra energy needs to transfer out of the system.Energy from broken, weak bonds transfer or release to the surroundings, usually in the form of heat.
CombustionCellular respirationRustMetabolizing foodCombustionCellular respirationRustMetabolizing food
EndergonicEndothermic
Prefix end- means “inside” or “taking in”Prefix endo- means “inside” or “taking in”
Nonspontaneous reactionsNonspontaneous reactions 
Absorbs energy from surroundingsAbsorbs energy from surroundings (usually from heat or light)
Stronger products than reactantsStronger products than reactants
Uses an outside energy source to start the reaction where energy is absorbed into a system to create a stronger product from weaker reactants. The weaker products, combined with the activation energy, resulting in a stronger product. The sun is most often the activation energy. 
PhotosynthesisMelting an ice cubeBreaking breadSweatingPhotosynthesisMelting an ice cubeBreaking breadSweating

Final Thoughts

Exergonic and exothermic reactions may seem exactly the same, but, in reality, they have their own distinct properties. Exothermic reactions are exergonic reactions, but exergonic refers more to a process and exothermic to a more specific reaction. 

There are also differences between reactions that transfer energy and reactions that absorb it. Understanding exergonic and exothermic reactions and endergonic and endothermic reactions will help to deepen one’s knowledge of thermodynamics.