water molecules fall apart and come back together all the time in liquid water
Water molecules can dissociate into [H3O+] and [OH-] - sort of, they’re complexes with more water with partially-covalent hydrogen bonding. But you can’t just go directly from those to H2 and O2 - for good efficiency you need fancy catalysts and it’s a multistep process starting with OH or H bonding to a metal surface.
Nitrogen conversion generally starts with the end or side of N2 bonding to a metal ion. (Usually the end, IIRC.) The triple bond has electron density far enough away from the nuclei for that to happen. It’s more diffuse and bigger electron density than the orbital of N in NH3 that can get protonated, so it interacts better with positive metal ions bigger than H but smaller than molecules.
Water molecules can dissociate into [H3O+] and [OH-] - sort of, they’re complexes with more water with partially-covalent hydrogen bonding. But you can’t just go directly from those to H2 and O2 - for good efficiency you need fancy catalysts and it’s a multistep process starting with OH or H bonding to a metal surface.
Nitrogen conversion generally starts with the end or side of N2 bonding to a metal ion. (Usually the end, IIRC.) The triple bond has electron density far enough away from the nuclei for that to happen. It’s more diffuse and bigger electron density than the orbital of N in NH3 that can get protonated, so it interacts better with positive metal ions bigger than H but smaller than molecules.