What Is a Chemical Reaction That Absorbs Energy in the Form of Heat Described As?

Endothermic Reactions

• Endothermic chemical reactions absorb energy from their surroundings. Usually these reactions cause the surroundings and the reaction mixture to decrease in temperature. Thermal energy is taken in from the surroundings and converted to chemical energy, which is stored in the chemical bonds of the products.

  Some endothermic reactions get their energy from sources other than heat. For example, in electrolysis, electrical energy is used to drive a chemical reaction; and in photosynthesis in plants, the additional energy is absorbed from sunlight by the plant's green leaves.

Exothermic Reactions

• Exothermic chemical reactions transfer energy to their surroundings. These reactions cause the surroundings and the reaction mixture to increase in temperature. In exothermic reactions, more chemical bonds are broken than made. Since chemical bonds store energy, this energy is released in the form of thermal energy -- heat -- to the surroundings.

  Examples of exothermic chemical reactions are combustion (e.g. in an auto engine) and the neutralization reactions that occur between acids and alkalis, such as vinegar (acetic acid) and bicarbonate of soda (sodium bicarbonate).

Reversible and Irreversible Reactions

• In addition to all chemical reactions being either exothermic or endothermic, chemical reactions can be classified as either "reversible" or "irreversible." In irreversible reactions the products cannot rereact to form the reactants.

  On the other hand, in a reversible reaction the products can rereact to produce the original reactants. However this has implications for whether chemical reaction is endothermic or exothermic: If the forward reaction of a reversible reaction is exothermic, then the reverse reaction is endothermic, and vice versa.

Exothermic and Endothermic Reactions in Industry

• In the laboratory, the knowledge of whether a given reaction is exothermic or endothermic can be useful. However on an industrial scale it is very important.

  For example, an endothermic reaction in the laboratory will probably be able to get enough energy from a small heat source, such as a Bunsen burner. However when the reaction is scaled up to industrial size, a much larger, more expensive and hazardous heat source is required.

  For exothermic reactions in the laboratory, the heat can usually be absorbed by the surroundings, but on an industrial scale a lot more thermal energy is released, and this is much harder to deal with safely.