# Will a reaction with a negative ΔH° be spontaneous?

Jun 17, 2014

An exothermic reaction (negative $\Delta H$) 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 ($\Delta H$)
- Change in Entropy ($\Delta S$)

Spontaneity in a reaction is rigorously determined by the sign of the change in Gibbs Free Energy ($\Delta G$) where
$\Delta G = \Delta H - T \Delta S$

If $\Delta G$ is negative, the reaction is spontaneous (but the rate of the reaction might be very slow). If $\Delta G$ is positive, the reaction is non-spontaneous and cannot occur at all because the reverse reaction is spontaneous.

For most reactions, the $\Delta H$ term is larger than the $T \Delta S$ 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 $\Delta S$).

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.