Ice Ih
Ice Ih (hexagonal ice crystal) (pronounced: ice one h, also known as ice-phase-one) is the hexagonal crystal form of ordinary
The crystal structure is characterized by the oxygen atoms forming
Physical properties
The density of ice Ih is 0.917 g/cm3 which is less than that of
The latent
Crystal structure
The accepted
Hydrogen disorder
The hydrogen atoms in the crystal lattice lie very nearly along the hydrogen bonds, and in such a way that each water molecule is preserved. This means that each oxygen atom in the lattice has two hydrogens adjacent to it, at about 101 pm along the 275 pm length of the bond. The crystal lattice allows a substantial amount of disorder in the positions of the hydrogen atoms frozen into the structure as it cools to absolute zero. As a result, the crystal structure contains some residual entropy inherent to the lattice and determined by the number of possible configurations of hydrogen positions that can be formed while still maintaining the requirement for each oxygen atom to have only two hydrogens in closest proximity, and each H-bond joining two oxygen atoms having only one hydrogen atom.[7] This residual entropy S0 is equal to 3.4±0.1 J mol−1 K−1 .[8]
By contrast, the structure of ice II is hydrogen-ordered, which helps to explain the entropy change of 3.22 J/mol when the crystal structure changes to that of ice I. Also, ice XI, an orthorhombic, hydrogen-ordered form of ice Ih, is considered the most stable form at low temperatures.
Theoretical calculation
There are various ways of approximating this number from first principles. The following is the one used by Linus Pauling.[9][10]
Suppose there are a given number N of water molecules in an ice lattice. To compute its residual entropy, we need to count the number of configurations that the lattice can assume. The oxygen atoms are fixed at the lattice points, but the hydrogen atoms are located on the lattice edges. The problem is to pick one end of each lattice edge for the hydrogen to bond to, in a way that still makes sure each oxygen atom is bond to two hydrogen atoms.
The oxygen atoms can be divided into two sets in a checkerboard pattern, shown in the picture as black and white balls. Focus attention on the oxygen atoms in one set: there are N/2 of them. Each has four hydrogen bonds, with two hydrogens close to it and two far away. This means there are allowed configurations of hydrogens for this oxygen atom (see Binomial coefficient). Thus, there are 6N/2 configurations that satisfy these N/2 atoms. But now, consider the remaining N/2 oxygen atoms: in general they won't be satisfied (i.e., they will not have precisely two hydrogen atoms near them). For each of those, there are 24 = 16 possible placements of the hydrogen atoms along their hydrogen bonds, of which 6 are allowed. So, naively, we would expect the total number of configurations to be
Using Boltzmann's entropy formula, we conclude that
The same answer can be found in another way. First orient each water molecule randomly in each of the 6 possible configurations, then check that each lattice edge contains exactly one hydrogen atom. Assuming that the lattice edges are independent, then the probability that a single edge contains exactly one hydrogen atom is 1/2, and since there are 2N edges in total, we obtain a total configuration count , as before.
Refinements
This estimate is 'naive', as it assumes the six out of 16 hydrogen configurations for oxygen atoms in the second set can be independently chosen, which is false. More complex methods can be employed to better approximate the exact number of possible configurations, and achieve results closer to measured values. Nagle (1966) used a series summation to obtain .[11]
As an illustrative example of refinement, consider the following way to refine the second estimation method given above. According to it, six water molecules in a hexagonal ring would allow configurations. However, by explicit enumeration, there are actually 730 configurations. Now in the lattice, each oxygen atom participates in 12 hexagonal rings, so there are 2N rings in total for N oxygen atoms, or 2 rings for each oxygen atom, giving a refined result of .[12]
See also
- Ice, for other crystalline forms of ice
References
- ^ Norman Anderson. "The Many Phases of Ice" (PDF). Iowa State University. Archived from the original (PDF) on 7 October 2009.
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Further reading
- Fletcher, N. H. (2009-06-04). The Chemical Physics of Ice. ISBN 9780521112307.
- Petrenko, Victor F.; Whitworth, Robert W. (1999-08-19). Physics of Ice. ISBN 9780191581342.
- Chaplin, Martin (2007-11-11). "Hexagonal ice structure". Water Structure and Science. Retrieved 2008-01-02.