# Clarification on Photodissociation

Can someone please explain what photodissociation is and if it is more efficient than electrolysis while splitting water into its components?

Thanks in advance.

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## 1 Answer

Photodissociation is a unimolecular reaction triggered by absorption of light. You start with one molecule, a bond (or more) breaks and you end up with two (or more) products. In water you can get $\ce{OH}$ and $\ce{H}$ (etc), usually in excited electronic states. You can not get the products of the electrolysis.

Water electrolysis is a more complex reaction. The final balance is: $\ce{2H2O -> 2H2 + O2}$, so you break four bonds and make three bonds.

Both reactions are not spontaneous (in the electronic ground state of water). The net balance is not energetically favorable (you start with more energy in bonds than what you end up). However, the way you add this energy is also different. In photodissociation the energy is given by light; in electrolysis the energy is given by forcing an electron current.

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