Question

These reactions are all spontaneous. Based on this information, is enthalpy of reaction a reliable indicator of whether a reaction is spontaneous?

Answer 1

No. Enthalpy (ΔH) by itself is not a reliable indicator of spontaneity.

Why: spontaneity is determined by the Gibbs free energy change
ΔG = ΔH − TΔS.
A reaction is thermodynamically spontaneous only when ΔG < 0. That depends on both the enthalpy change ΔH and the entropy change ΔS (and on the temperature T).

Practical consequences:
- Some endothermic reactions (ΔH > 0) are spontaneous if ΔS is sufficiently positive and TΔS > ΔH (e.g., dissolution of ammonium nitrate, melting of ice above 0 °C).
- Some exothermic reactions (ΔH < 0) are nonspontaneous if they cause a large entropy decrease so that −TΔS outweighs ΔH at that temperature.
- If ΔH < 0 and ΔS > 0 the reaction is always spontaneous; if ΔH > 0 and ΔS < 0 it is never spontaneous; the other two sign combinations are temperature dependent.
- Even if ΔG < 0 (thermodynamically spontaneous), a reaction may not proceed at an observable rate because of kinetic barriers (activation energy).

Conclusion: You need ΔG (or both ΔH and ΔS and the temperature) to judge spontaneity; ΔH alone is not sufficient.