Will a reaction with a negative ΔH° be spontaneous?
Answers
When ΔH is positive and ΔS is negative, the sign of ΔG will always be positive, and the reaction can never be spontaneous. ... It is the entropy term that favors the reaction. Therefore, as the temperature increases, the TΔS term in the Gibbs free energy equation will begin to predominate and ΔG will become negative.
An exothermic reaction (negative ΔH ) 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 (
Δ
H
)
- Change in Entropy (ΔS )
Spontaneity in a reaction is rigorously determined by the sign of the change in Gibbs Free Energy (ΔG ) whereΔG=ΔH−TΔS
If ΔG
is negative, the reaction is spontaneous (but the rate of the reaction might be very slow). If ΔG
is positive, the reaction is non-spontaneous and cannot occur at all because the reverse reaction is spontaneous.
For most reactions, the ΔH term is larger than the TΔS
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 ΔS ).
A common example of a spontaneous endothermic reaction is dissolving ammonium chloride in water