Majority of the exothermic reactions are spontaneous because there is decrease in energy.
Burning of a substance is spontaneous reaction.
C (*) + O2 (g) → CO2 (g); ΔH = -394 kJ mol-1
Neutralization of an acid with a base is a spontaneous reaction.
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O; ΔH = -57 kJ mol-1
However, many spontaneous reactions proceed with the absorption of heat. Conversion of liquid water into its vapour by absorption of heat is an endothermic spontaneous change. Therefore, change in enthalpy is not the only criterion for deciding the spontaneity of a reaction.
