As chemists, we are used to writing down chemical equations such as 2H₂(g)+ O₂(g) -> 2H₂O(g). This reaction doesn't include some important information however. We don't know if this reaction gives off heat or requires heat from the surroundings, and we don't know how much heat is generated.

In a thermochemical equation, we include the value of DH, the change in enthalpy for the reaction. For example, the above equation showing the combustion of hydrogen would be written as

2H₂(g)+ O₂(g) -> 2H₂O(g), DH = -571.6 kJ

The sign and magnitude of the change in enthalpy tell us that this reaction is exothermic and gives off a lot of heat. Some rules for writing thermochemical equations are:

1. The sign of DH indicates if the reaction is exothermic (DH negative) or endothermic (DH positive)
2. The coefficients in front of the species represent number of moles
3. The phases of each reactant and product must be indicated: the enthalpy of liquid water at 25^oC is different from gaseous water at the same temperature, for example
4. The value of DH is computed with all reactants and products at 25^oC and 1 atm pressure unless otherwise indicated.

Information provided by: http://learn.chem.vt.edu