A negative sign represents an exothermic reaction. The compound has less enthalpy than the elements from which it was formed. A positive sign represents an endothermic reaction. The compound has more enthalpy than the elements from which it was formed. If the reaction forming a compound is exothermic, the decomposition of the compound would be endothermic.
Most enthalpies of formation are negative.
Compounds that have large negative enthalpies of formation require large amounts of energy to decompose and are considered thermodynamically stable . A table with all the thermodynamic properties needed for the problems on this page. Values on this table have been rounded for easy calculation. Refer to page 859 in your textbook or the CRC Handbook for more information. The NIST Chemistry WebBook has more information.