Exothermic

The sign convention behind the definition is worth stating precisely: enthalpy change is measured for the system, so heat leaving the system toward the surroundings makes the change negative. Combustion, neutralisation, most oxidations and the setting of cement are all exothermic, and the released energy appears as a temperature rise in whatever surrounds the reaction vessel. A negative enthalpy change does not by itself guarantee that a process occurs, since spontaneity is governed by Gibbs free energy, which combines enthalpy with the entropy change scaled by temperature; a strongly exothermic reaction accompanied by a large decrease in entropy can be non-spontaneous at high temperature. Nor does it govern speed, an exothermic reaction with a high activation barrier proceeding imperceptibly without a catalyst.