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An exothermic reaction is a chemical reaction that releases energy by light or heat. It is the opposite of an endothermic reaction.〔Article written by Anne Marie Helmenstine, Ph.D on exothermic and endothermic reactions http://chemistry.about.com/cs/generalchemistry/a/aa051903a.htm〕 Expressed in a chemical equation: reactants → products + energy ==Overview== An exothermic reaction is a chemical or physical reaction that releases heat. It gives net energy to its surroundings. That is, the energy needed to initiate the reaction is less than the energy that is subsequently released.〔Article written by Anne Marie Helmenstine, Ph.D on exothermic and endothermic reactions http://chemistry.about.com/cs/generalchemistry/a/aa051903a.htm〕 When the medium in which the reaction is taking place gains heat, the reaction is exothermic. When using a calorimeter, the total amount of heat that flows into (or through) the calorimeter is the negative of the net change in energy of the system. The absolute amount of energy in a chemical system is difficult to measure or calculate. The enthalpy change, ΔH, of a chemical reaction is much easier to work with. The enthalpy change equals the change in internal energy of the system plus the work needed to change the volume of the system against constant ambient pressure. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a combustion reaction. Measured and calculated ΔH values are related to bond energies by: :ΔH = energy used in bond breaking reactions − energy released in bond making products by definition the enthalpy change has a negative value: :ΔH < 0 in an exothermic reaction, gives a negative value for ΔH, since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction). For example, when hydrogen burns: :2H2 (g) + O2 (g) → 2H2O (g) :ΔH = −483.6 kJ/mol of O2 〔http://chemistry.osu.edu/~woodward/ch121/ch5_enthalpy.htm〕 The most commonly available hand warmers make use of the oxidation of iron to achieve an exothermic reaction: :4Fe(s) + 3O2(g) → 2Fe2O3(s). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Exothermic reaction」の詳細全文を読む スポンサード リンク
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