Endergonic vs Exergonic Reactions and Processes

Endergonic and exergonic are two types of chemical reactions, or processes, in thermochemistry or physical chemistry. The names describe what happens to energy during the reaction. The classifications are related to endothermic and exothermic reactions, except endergonic and exergonic describe what happens with any form of energy, while endothermic and exothermic relate only to heat or thermal energy.

Endergonic Reactions

• Endergonic reactions may also be called an unfavorable reaction or nonspontaneous reaction. The reaction requires more energy than you get from it.
• Endergonic reactions absorb energy from their surroundings.
• The chemical bonds that are formed from the reaction are weaker than the chemical bonds that were broken.
• The free energy of the system increases. The change in the standard Gibbs Free Energy (G) of an endergonic reaction is positive (greater than 0).
• The change in entropy (S) decreases.
• Endergonic reactions are not spontaneous.
• Examples of endergonic reactions include endothermic reactions, such as photosynthesis and the melting of ice into liquid water.
• If the temperature of the surroundings decreases, the reaction is endothermic.

Exergonic Reactions

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Exothermic Reaction Definition

By Anne Marie Helmenstine, Ph.D.

• An exergonic reaction may be called a spontaneous reaction or a favorable reaction.
• Exergonic reactions release energy to the surroundings.
• The chemical bonds formed from the reaction are stronger than those that were broken in the reactants.
• The free energy of the system decreases. The change in the standard Gibbs Free Energy (G) of an exergonic reaction is negative (less than 0).
• The change in entropy (S) increases. Another way to look at it is that the disorder or randomness of the system increases.
• Exergonic reactions occur spontaneously (no outside energy is required to start them).
• Examples of exergonic reactions include exothermic reactions, such as mixing sodium and chlorine to make table salt, combustion, and chemiluminescence (light is the energy that is released).
• If the temperature of the surroundings increases, the reaction is exothermic.

Notes About the Reactions

• You cannot tell how quickly a reaction will occur based on whether it is endergonic or exergonic. Catalysts may be needed to cause the reaction to proceed at an observable rate. For example, rust formation (oxidation of iron) is an exergonic and exothermic reaction, yet it proceeds so slowly it's difficult to notice the release of heat to the environment.
• In biochemical systems, endergonic and exergonic reactions often are coupled, so the energy from one reaction can power another reaction.
• Endergonic reactions always require energy to start. Some exergonic reactions also have activation energy, but more energy is released by the reaction than what is required to initiate it. For example, it takes energy to start a fire, but once combustion starts, the reaction releases more light and heat than it took to get it started.
• Endergonic reactions and exergonic reactions are sometimes called reversible reactions. The quantity of the energy change is the same for both reactions, although the energy is absorbed by the endergonic reaction and released by the exergonic reaction. Whether the reverse reaction actually can occur is not a consideration when defining reversibility. For example, while burning wood is a reversible reaction theoretically, it doesn't actually occur in real life.

Perform Simple Endergonic and Exergonic Reactions

In an endergonic reaction, energy is absorbed from the surroundings. Endothermic reactions offer good examples, as they absorb heat. Mix together baking soda (sodium carbonate) and citric acid in water. The liquid will get cold, but not cold enough to cause frostbite.

An exergonic reaction releases energy to the surroundings. Exothermic reactions are good examples of this type of reaction because they release heat. The next time you do laundry, put some laundry detergent in your hand and add a small amount of water. Do you feel the heat? This is a safe and simple example of an exothermic and thus exergonic reaction.

A more spectacular exergonic reaction is produced by dropping a small piece of an alkali metal in water. For example, lithium metal in water burns and produces a pink flame.

A glow stick is an excellent example of a reaction that is exergonic, yet not exothermic. The chemical reaction releases energy in the form of light, yet it doesn't produce heat.