Disassociation of Water Molecules in an Electric Field

[KAM1]

Question for chemistry experts out there:

Say you have two electrically charged plates, covered by an insulating material, and separated by a narrow gap occupied only by relatively sparse water molecules, i.e. water vapor. Now say you apply a relatively high voltage to those plates, something on the order of thousands of volts or more, creating a very strong electric field between the insulated plates, across the area containing the water vapor.

1. What will happen to the water molecules between the plates? Will the molecules disassociate into hydrogen and oxygen ions?

2. If the molecules disassociate into ions, how is energy conserved in this scenario, since no electrical current would actually flow between the plates, yet work has been done on the molecules?

3. If the molecules do not disassociate into ions, what prevents the disassociation from occurring?

The same questions could be asked of hydrogen gas molecules, which are known to ionize under high voltage electrical fields.

Thanks,
KM

[Martin Brown]

Post by KAM1
Say you have two electrically charged plates, covered by an
insulating material, and separated by a narrow gap occupied only by
relatively sparse water molecules, i.e. water vapor. Now say you
apply a relatively high voltage to those plates, something on the
order of thousands of volts or more, creating a very strong electric
field between the insulated plates, across the area containing the
water vapor.

It is actually a physics question about electrical permittivity and
dielectric strength. See:

https://en.wikipedia.org/wiki/Dielectric_strength

If the insulators were perfect then the field gradient across them would
be huge and the electric field across the free space tiny.

Post by KAM1
1. What will happen to the water molecules between the plates? Will
the molecules disassociate into hydrogen and oxygen ions?

The best you might hope for is that they turn around so that the free
electron pairs face the positive plate. See fig 1 here:

https://cds.cern.ch/record/1342701/files/335.pdf

Post by KAM1
2. If the molecules disassociate into ions, how is energy conserved
in this scenario, since no electrical current would actually flow
between the plates, yet work has been done on the molecules?

To make them ionise you need the plates uninsulated and the gas pressure
low enough that particle mean free path is comparable with the
dimensions of the gap (or voltage above the breakdown for an arc).

Post by KAM1
3. If the molecules do not disassociate into ions, what prevents the
disassociation from occurring?

No current is flowing.
Unless you push the voltage so high that your insulator fails.

Post by KAM1
The same questions could be asked of hydrogen gas molecules, which
are known to ionize under high voltage electrical fields.