Heat of reaction

This refers to the amount of heat that enters and leaves during a chemical reaction. When heat of reaction is released, it is called an exothermic reaction, and when it is absorbed, it is called an endothermic reaction. For example, when one mole of hydrogen molecule is burned, 57.8 kilocalories of heat is generated (usually expressed as the amount of heat generated under standard conditions of room temperature and atmospheric pressure). For example, when gaseous hydrogen and oxygen react to produce water, the heat of reaction is expressed as the change in enthalpy (heat content) of the system, as follows:

(The value of ΔH is often taken to be positive based on the amount of heat absorbed by the system). Reaction heat is usually measured under atmospheric pressure, so it is also called constant pressure reaction heat, but when it is measured at a constant volume, it is called constant volume reaction heat. Also, when a reaction is carried out in two or several stages, the sum of the reaction heats is the total reaction heat. This is called Hess's law.

Depending on the type of reaction, reaction heat can be called many names, such as heat of combustion, heat of neutralization, heat of formation, heat of solution, and heat of vaporization.

[Toda Genjiro and Nakahara Katsunori]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

化学反応に伴って出入りする熱量をいう。反応熱が出る場合を発熱反応、入る場合を吸熱反応という。たとえば、水素分子1モルが燃焼する際には、57.8キロカロリーの熱量を発生する（普通、室温・大気圧の標準状態下の発生熱量で示す）。たとえば気体の水素と酸素が反応して水を生成するときの反応熱は、系のエンタルピー（熱含量）変化として、

のように表す（ΔHの値は系の吸熱量をもって正とする）ことが多い。反応熱は普通、大気圧のもとで測るため、定圧反応熱ともいうが、これに対し、一定容積で測った場合を定容反応熱という。また、一つの反応を2段階あるいは数段階に分けて行ったとき、その反応熱の合計は全体の反応熱になる。これをヘスの法則という。

　反応熱は反応の種類によって、燃焼熱、中和熱、生成熱、溶解熱、蒸発熱など多くの呼び名がある。

［戸田源治郎・中原勝儼］

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