Thermochemical equation

A thermochemical equation is the balanced chemical equation in which the enthalpy change, the physical states and the number of moles of reactants and products have been specified. Here follow the guidelines for writing thermochemical equations:

i. Consider the balanced equation for reactants and products.

ii. The value and appropriate sign of the enthalpy change are given on the right-hand side. This value is $\Delta_r H^0$.

iii. The physical states of reactants and products are specified by letter: s (solid), $l$ (liquid), g (gas) and aq (aqueous). The $\Delta_r H^0$ value refers to the physical states of the substances as they appear in the equation.

iv. The given value of $\Delta_r H^0$ assumes that the reaction occurs in the given direction. $\Delta_r H^0$ for the reverse reaction is equal in magnitude, and opposite in sign, to that of the forward reaction. An exothermic reaction on reversal becomes endothermic, and vice versa.

v. When the coefficients indicating the number of moles of all the substances in a thermochemical equation are multiplied or divided by a certain numerical factor, the corresponding $\Delta_r H^0$ needs to be multiplied or divided by the same.

Example of thermochemical equation …