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A Thermochemical Equation is a balanced stoichiometric chemical equation that includes the enthalpy change, ΔH. In variable form, a thermochemical equation would look like this: :A + B → C :ΔH = (±) # Where are the usual agents of a chemical equation with coefficients and “(±) #” is a positive or negative numerical value, usually with units of kJ. ==Understanding Aspects of Thermochemical Equations== Enthalpy (H) is the transfer of energy in a reaction (for chemical reactions it is in the form of heat) and ΔH is the change in enthalpy. ΔH is a state function. Being a state function means that ΔH is independent of the processes between initial and final states. In other words, it does not matter what steps we take to get from initial reactants to final products—the ΔH will always be the same. ΔHrxn, or the change in enthalpy of a reaction, has the same value of ΔH as in a thermochemical equation, but is in units of kJ/mol being that it is the enthalpy change per moles of any particular substance in the equation. Values of ΔH are determined experimentally under standard conditions of 1atm and 25 °C (298.15K). As discussed earlier, ΔH can have a positive or negative sign. A positive sign means that the system uses heat and is endothermic. The negative value means that heat is produced and the system is exothermic. Endothermic: A + B + Heat → C, ΔH > 0 Exothermic: A + B → C + Heat, ΔH < 0 Since enthalpy is a state function, the ΔH given for a particular reaction is only true for that exact reaction. Physical states (of reactants or products) matter, as do molar concentrations. This matter of ΔH being dependent on physical state and molar concentration means that thermochemical equations must be stoichiometrically correct. If one agent of the equation is changed through multiplication, then all agents must be proportionally changed, including ΔH. (See Manipulating Thermochemical Equations, below.) Thermochemical equation’s multiplicative property is largely due to the First Law of Thermodynamics, which says that energy can be neither created nor destroyed, a concept commonly known as the conservation of energy. It holds true on a physical or molecular scale. 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「Thermochemical equation」の詳細全文を読む スポンサード リンク
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