Overview

If energy releases during a chemical reaction, then the resulting value will be a negative number. In other words, reactions that release energy have a ∆G < 0. A negative ∆G also means that the reaction's products have less free energy than the reactants because they gave off some free energy during the reaction. Scientists call reactions with a negative ∆G, and which consequently release free energy, exergonic reactions. Exergonic means energy is exiting the system. We also refer to these reactions as spontaneous reactions, because they can occur without adding energy into the system. Understanding which chemical reactions are spontaneous and release free energy is extremely useful for biologists because these reactions can be harnessed to perform work inside the cell. Contrary to the everyday use of the term, a spontaneous reaction is not one that suddenly or quickly occurs. Rusting iron is an example of a spontaneous reaction that occurs slowly, little by little, over time.

If a chemical reaction requires an energy input, then the ∆G for that reaction will be a positive value. In this case, the products have more free energy than the reactants. Thus, we can think of the reactions' products as energy-storing molecules. We call these chemical reactions endergonic reactions, and they are non-spontaneous. An endergonic reaction will not take place on its own without adding free energy.

For example, building complex molecules, such as sugars, from simpler ones is an anabolic process and requires energy. Therefore, the chemical reactions involved in anabolic processes are endergonic reactions. Alternatively, the catabolic process of breaking sugar down into simpler molecules releases energy in a series of exergonic reactions. 

This text is adapted from Openstax, Biology 2e, Section 6.2: Potential, Kinetic, Free, and Activation Energy.

Procedure

If the free energy change is less than zero, the process is spontaneous and is called an exergonic reaction. In contrast, if the free energy change is greater than zero, the process is not spontaneous, and the reaction is endergonic.

To compare the energy changes of different reactions, the free energy change is usually reported under a standard set of conditions written as ΔG₀.

The hydration of fumarate to malate in the presence of fumarase is an exergonic reaction because the standard free energy change during the process is −3.8 kJ/mol.

For endergonic reactions to occur, energy needs to be added to the system. In cells, endergonic reactions are coupled with exergonic reactions.

For instance, the endergonic conversion of glucose to glucose-6-phosphate has a standard free energy change of 13.8 kJ/mol, and the exergonic hydrolysis of ATP has a standard free-energy change of  −30.5 kJ/mol.

These reactions are coupled inside the cell, and the entire process is exergonic, with a standard free-energy change of −16.7 kJ/mol, a total of the standard free energy of both processes.