Light Heavy and Semiheavy Water Equilibrium

  • Thread starter Vanadium 50
  • Start date
  • Tags
    Light
  • Featured
  • #1
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2023 Award
33,017
19,456
If I start with a mix of half H2O and half D2O, when it equilibrates it will be half HDO, a quarter H2O and a quarter D2O. My question is "how long does this take?". Ballpark is fine - microsecodnds? Days? Centuries?
 
Chemistry news on Phys.org
  • #2
Vanadium 50 said:
If I start with a mix of half H2O and half D2O, when it equilibrates it will be half HDO, a quarter H2O and a quarter D2O. My question is "how long does this take?". Ballpark is fine - microsecodnds? Days? Centuries?
What temperature/phase(solid, liquid, vapor)? Liquid kinetics are O(m) different from solid/vapor.
 
  • #3
Liquid water, room temperature. Maybe a little cooler.
 
  • #5
  • Like
Likes Vanadium 50
  • #6
Mixing limited, technically this is not much different from acid/base reactions and these are quite fast, in nanosecond range if memory serves me well.
 
  • Like
Likes Vanadium 50, Frabjous and Bystander
  • #7
Thanks. I guess I could think of it as an acid-base reaction, between some very week acids and bases. I don't know the pH of D2O, but imagine it's around 7.3.

Calculating from 1st principles looks like a nightmare, since you have a 6-way equilibrium between H2O, HDO, D2O, H+, D+, OH-, and OD-. (Plus the complication of whether H+ is really H3O+) But "a tint fraction oif a second" is a good enough answer for me.
 
  • #9
I am not sure what that link is meant to say. It's just there.

It dies not mention a time, which is my original question. It does say, indirectly, that water (H2O) has a pH of 7. which is not news. (D2O I looked up and it is 7.4, I estimated 7.3)

I think @Borek answered my question. "As fast as they physically can mix"
 
  • #10
1696750889339.png


Handwavy, but more detailed answer. If you put several water molecules side by side, they will get linked by hydrogen bonds. Resulting four membered ring is highly symmetrical (even if not flat) and the bonds will start oscillating, resulting in a very quick exchange of H and D between water molecules. In the presence of H+ from water autodissociation this is made even easier, as H+ will attach itself to one of the lone electron pairs of a molecule (red outlined part). Charge will delocalize to all hydrogens three hydrogens (as in H3O+), which makes them even more eager to bond to neighbor water molecules, which further speeds up bond oscillations, to the point where charge easily jumps between water molecules, rearranging which hydrogen is attached to each oxygen on the way. That's actually why limiting ion conductivity of H+ is anomalously high, several times higher than that of any other ion - H+ doesn't have to travel by itself, it is charge that jumps (not the case of, say, Na+, which has to meticulously navigate between water molecules).

This explanation is far from being strict, but gives good intuition, and shows why individual water molecules in liquid water are not as "separate" as molecules in other liquids.
 
  • Like
Likes hutchphd, 256bits, DeBangis21 and 1 other person

Similar threads

  • Other Physics Topics
Replies
3
Views
2K
Replies
3
Views
6K
Replies
4
Views
1K
Replies
4
Views
1K
Replies
2
Views
2K
Replies
7
Views
4K
Replies
5
Views
4K
  • Chemistry
Replies
1
Views
3K
  • Sci-Fi Writing and World Building
Replies
0
Views
92
Back
Top