Hydrogen Bonds in Water and Ice

[mgkii]

Hi

I've read sufficient literature on Ice/Water expansion to understand that when water freezes into a lattice, the hydrogen bonds between molecules are longer than they are in liquid water, hence ice expands. However, everything I've read seems to skip over the reason why with statements like "because the hydrogen bonds are shorter in water the molecules can pack closer together".

It seems doubtful that the length of a hydrogen bond is related to temperature; super cooled water is still denser than ice, and I think I've read that water is at it's densest at 4 degrees centigrade.

Does anyone have an explanation / link to an article that explains why the hydrogen bonds in water are shorter than in the lattice of ice?

 

[Bystander]

Stillinger; Felix Franks, Water: A Comprehensive Treatise.

 

[mgkii]

Wow...! I was hoping for something a little shorter than 495 pages! Either my question is way harder than I imagined, or I'm going to hope there's a shorter article I can read before I have to resort to a book for the holidays :-)

Is the suggestion from the link that there isn't a simple answer to what I may naively think is a simple question? Thanks.

 

[Bystander]

Water is an "interesting" compound.

 

[mgkii]

I would agree; absolutely fascinating. Hence the interest in the Hydrogen bond length; but really struggling to find an answer as to why the bonds are shorter in liquid than in solid?

 

[Bystander]

The "party line" many moons ago was that they aren't --- you're looking at a "bulk" phenomenon of phase behavior. Current thinking? Dunno.

 

[mgkii]

Hi. Can you expand on that please? Everything I've read to date suggests that the length of the hydrogen bonds in liquid are shorter than in the ice lattice. I understand bulk phenomenon in relation to gas behavior, but how would it relate to seeing an overall shortening of the Bond length in liquid? Many thanks.

 

[Bystander]

Caveat emptor!
"Bond length" is a rather "flexible" term to use when discussing hydrogen-bonded networks (which atom is bound to what others?), and measureable spacing in a regular "frozen" lattice isn't easily compared to that of the liquid phase. That said, see http://www.pnas.org/content/104/23/9570.full for something a little more recent than Franks, that suggests "no difference."

 

[mgkii]

Ok. So to rephrase my original question in light of the information above...

If there is no discernable difference in length in hydrogen bonds between water molecules in liquid and in ice, then why is Ice less dense than water?

(Ps. Fab paper link. Never realized heavy water Ice sinks!)

 

[Bystander]

Think of the Ice I (one) lattice as being "fluffier" than liquid water.

 

[mgkii]

So you read something once and understand one thing... then you read it again and understand something different. That's what I've just done with the sources I read that started this quest for an answer! I'd appreciate knowing if the following matches anyone else's understanding:

1. In liquid water, the angle the hydrogen atoms of a water molecule make with it's central Oxygen atom (H - O - H) is approx 104.5^o.

2. In the tetrahedral structure of Ice, the angles made are approx 109.5^o.

3. It would appear from the various sources (including the ever-correct and incontrovertible wiki) that we still don't have a nailed on understanding of how molecules behave in a liquid that turns this steeper angle into a denser packing structure, but quite clearly they do... because water is denser than ice! And this would appear there there is a temperature related element to this mechanism, because the max density for water occurs at 4^oC.

4. Hydrogen bond length as far as I can tell from the sources I've read is shorter in liquid water than in ice. However, when water freezes and the crystalline structure forms 109.5^o angles, the hydrogen bonds must necessarily elongate. So, this elongation must be a consequence of the structure they are in, not the cause of that structure.

So I still don't know "why" water molecules in a liquid pack more densely than in ice; but it would seem I'm not alone in this and that sounds like a cue to stop worrying about it :-)Any thoughts / corrections will of course be appreciated.