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Chemical reactions may be viewed in terms of the amount of energy rrequired by the reaction at various stages. A convenient way to do this is with an energy hill diagram. In these diagrams, the total amount of energy, both kinetic and potential in the chemicals involved in the reaction is plotted as a function of time. Chemical reactions can be classified into two types depending on whether or not the chemical products of the reaction have more or less energy than the chemical starting material. These two types or reaction are endergonic and exergonic reactions.
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Chemical reactions that lead to products or a product that has more chemical
energy than does the starting materials are called
endergonic reactions. Observe that the final product, C has more
chemical energy that the original starting materials. Endergonic reactions
typically involve the synthesis of complex molecules from simplier ones.
An example is the cell making proteins from amino acids or plant cells
carrying out photosynthesis in which the carbohydrate product has more
energy than the starting materials for photosynthesis, carbon dioxide
and water.
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Exergonic reactions are chemical reactions where the chemical products
of the reaction have less energy than the starting materrials. For example
when wood or a match burns in a fire, the wood which is mainly cellulaose
has a lot of chemical energy. But the products of burning, mainly carbon
dioxide and water have much less energy, the nbalance of the energy having
been given off as light and heat.