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

No. A reaction is spontaneous when its Gibbs free-energy change is negative: ΔG = ΔH − TΔS. Because entropy and temperature also affect ΔG, enthalpy alone cannot reliably indicate whether a reaction is spontaneous; the fact that these reactions are spontaneous does not change that.

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