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.