What is the missing factor? What do the N₂0, NH₄Cl, and evaporation processes have in common with most exothermic reactions that X makes them take place spontaneously, even though these reactions are endothermic?

The answer is that all of these reactions create disorder NO₂ and O₂ gas molecules are more disordered than crystals of N₂0₆. Hydrated ammonium and chloride ions in solution are more disordered than the regular array of NH₄Cl ions in a crystal. H₂0 molecules moving about freely as water vapor are more disordered than the closely packed molecules of the liquid, or the frozen molecules of the solid .

Most explosions are destructive precisely because they convert solids or liquids into gases that push out against their surroundings. (The expansion of the gases when they are heated by the reaction is another destructive factor.) A decrease in energy or enthalpy certainly is an important component in determining spontaneity, but the other aspect is the production of disorder.

Disorder increases from ice to liquid to vapour, as more Hydrogen bonds are broken and as water molecules begin to move past one another.