What makes a reaction exergonic

After learning that chemical reactions release energy when energy-storing bonds are broken, an important next question is the following: How is the energy associated with these chemical reactions quantified and expressed?

How can the energy released from one reaction be compared to that of another reaction? A measurement of free energy is used to quantify these energy transfers. Recall that according to the second law of thermodynamics, all energy transfers involve the loss of some amount of energy in an unusable form such as heat. Free energy specifically refers to the energy associated with a chemical reaction that is available after the losses are accounted for.

In other words, free energy is usable energy, or energy that is available to do work. Looking at this concept in a biological sense, free energy is the energy within a molecule that can be used to perform work. Glucose has a lot of free energy because there is a lot of energy stored within the bonds of the glucose molecule.

Carbon dioxide has a much lower free energy because there is much less energy stored in its bonds. In a chemical reaction, breaking and forming bonds between atoms is a form of energy. However, some exergonic reactions do not occur spontaneously and require a small input of energy to start the reaction. This input of energy is called activation energy. Once the activation energy requirement is fulfilled by an outside source, the reaction proceeds to break bonds and form new bonds and energy is released as the reaction takes place.

Note that while an exergonic reaction is spontaneous, it may not proceed quickly without the aid of a catalyst. For example, the rusting of iron is exergonic, but very slow. Examples of exergonic reactions include cellular respiration, the decomposition of hydrogen peroxide , and combustion. Endothermic and exothermic reactions are types of endergonic and exergonic reactions, respectively. The difference is the the energy absorbed by an endothermic reaction or released by an exothermic reaction is heat.

Endergonic and exergonic reactions may release other kinds of energy besides heat, such as light or even sound. For example, a glow stick is an exergonic reaction that releases light.

It is not an exothermic reaction because it does not release heat. If a reaction is endergonic in one direction, it is exergonic in the other direction and vice versa. What Happens to an Exothermic Reaction if the Temperature Role of Enzymes in Chemical Reactions. How to Calculate Entropy Change. How to Burn Potassium Nitrate. The Effect of Temperature on Activation Energy. How to Convert Joule to Mol. How to Convert an Alkane to an Alkene.

References Argonne National Laboratory: Endergonic vs.