Endergonic reactions are processes in physical chemistry or thermochemistry. This type of reaction needs an input of energy to create the products, which have more free energy as a whole than the sum of free energy of each of the reactants. An endothermic reaction is an endergonic reaction that involves heat or thermal energy in the process.
Endergonic reactions are nonspontaneous, as they need an input of energy to occur. An example of this type of reaction in biology is photosynthesis. This process depends on the reaction to absorb energy in the form of sunshine from the surroundings for it to occur. Plants capture some of the sun's energy as sunlight and use it to produce glucose from water and carbon dioxide. Glucose has more free energy than the reactants of carbon dioxide and water. The chemical bonds formed in an endergonic reaction are weaker than the chemical bonds that are broken. For this reason, it may also be called an unfavorable reaction, as it requires more energy input than you get from the end product. Another example of an endergonic reaction occurs when ice as a solid is melted by heat into liquid water, which is also called endothermic because the results are driven by warmer temperatures.
An exergonic reaction is called a spontaneous or a favorable reaction, and it is the opposite of an endergonic reaction. This type of reaction releases energy into the surroundings and forms stronger chemical bonds in the process than those that were broken in the reactants to produce the product. The free energy of the system decreases in an exergonic reaction. Some examples include mixing chlorine and sodium to make ordinary table salt and chemiluminescence when visible light is the energy that is released in the process. When the temperature of the surroundings rises, the reaction is exothermic as well as exergonic.
What Are Endergonic and Exergonic Reactions?
An endergonic and endothermic reaction occurs when energy is absorbed from the surroundings. In endothermic reactions, heat is absorbed. If you mix sodium carbonate (baking soda) and citric acid in water, the liquid becomes cold, but not cold enough to cause frostbite.
An exergonic reaction releases energy to the surroundings, and when it does, it is in the form of heat—it is exothermic. An example of this can be seen when you do laundry. Put a small amount of laundry detergent in your hand and add a small amount of water to it. You feel warmth emitting from the mixture, as it is an exothermic and exergonic reaction.
An example of an exergonic reaction that is not exothermic is a glow stick. Instead of releasing heat into the surroundings, it emits light.
About the Author
Mary Lougee has been writing about chemistry, biology, algebra, geometry, trigonometry and calculus for more than 12 years. She gained the knowledge in these fields by taking accelerated classes throughout college while gaining her degree.