An exothermic reaction (negative #DeltaH#) will usually be spontaneous, but not always.
There are two thermodynamic driving forces that determine if a reaction is spontaneous at constant temperature and pressure:
- Change in Enthalpy (#DeltaH#)
- Change in Entropy (#DeltaS#)
Spontaneity in a reaction is rigorously determined by the sign of the change in Gibbs Free Energy (#DeltaG#) where
If #DeltaG# is negative, the reaction is spontaneous (but the rate of the reaction might be very slow). If #DeltaG# is positive, the reaction is non-spontaneous and cannot occur at all because the reverse reaction is spontaneous.
For most reactions, the #DeltaH# term is larger than the #TDeltaS# term, so the change in enthalpy dominates and most but not all exothermic reactions are spontaneous. However, if a reaction is only mildly endothermic and/or involves a big increase in entropy, it is possible that an endothermic reaction can be spontaneous. Also, a mildly exothermic reaction might be non-spontaneous if the reaction involves a big decrease in entropy (negative #DeltaS#).
A common example of a spontaneous endothermic reaction is dissolving ammonium chloride in water. This is the basis for chemical "cold packs" that are used to provide a portable source of cooling for wounds and injuries.