Physical Chemistry of Water

 

            Water is the dominant component of living organisms on earth, and most processes of soil chemistry also take place in water. Water is a unique solvent in many ways. Compared to other liquids, water has a large heat capacity (so lakes and our bodies do not change temperature too rapidly as the environment changes), water has a viscosity that is reasonably constant with temperature, water has a fairly high surface tension (which lets water form drops and also to cohere into a water column that trees can raise from the ground), water has a high boiling point and a high heat of vaporization so it tends to remain in the liquid state, it is an excellent solvent for a wide range of salts and polar organic molecules, and it has the highest dielectric constant known so the solvent responds readily to any change in the electrostatic environment. What is it?

 

In the gas phase:

            The H-O bond length is about 0.96 ångström (Å).

The H-O-H bond angle is about 104.5 degrees.

            Vibrational normal modes are located at 3756 cm-1 (symmetric stretch), 3657 cm-1 (antisymmetric stretch), and 1595 cm-1 (bend).

            The dipole moment is –1.8546 ± 0.0006 debye. Water is a very polar molecule.

 

When two water molecules are brought together in the gas phase:

            The O···O separation distance is 2.98 Å.

            The association energy is about –22.8 kJ/mol, which is very large for a “nonbonded” interaction. That is why the hydrogen bond is given special attention.

 

In the solid phase (normal ice):

            Every O atom sits at the center of a tetrahedron of O atoms from other water molecules, and the H atoms are disordered. The O···O separation distance is 2.76 Å at 273 K. This is smaller than the gas-phase distance, mostly because of polarization in the condensed phase. The central O atom donates two H bonds to neighboring O atoms and also accepts two H-O bonds, again in a disordered manner. The density of ice is about 0.94 g cm-3 at 94 K.

            The O-H bonds stretch to about 1.01 Å in ice while the O-H···O hydrogen bond is about 1.75 Å long.

            The internal energy of ice (the energy required to bring a gas-phase water molecule to a central lattice position in the solid) is –58.9 kJ/mol. This is much more than twice the dimmer association energy, again because of polarization in the condensed phase. The effective dipole moment of a water molecule in ice is thus estimated to be larger than that in the gas phase, with estimates around 2.6 debye.

            As T increases, the dielectric constant of ice decreases from 99 at 243 K to that of the liquid.

In liquid water:

            The density is about 0.997 g cm-3 at 298 K.

            The internal energy is –41.5 kJ/mol.

            The dielectric constant is 78.4. Experiments that measure the “dielectric relaxation” estimate that a water molecule in the liquid, on average, can reorient itself within 10-11 s. For comparison.

The O-H bond length seems to be about 0.97 Å and the H-O-H angle is about 103 degrees, although there is some evidence that both are larger.

Vibrational normal modes are located near 3490 cm-1 (symmetric stretch), 3280 cm-1 (antisymmetric stretch), and 1645 cm-1 (bend). These stretching frequencies are somewhat lower than those in the gas phase, while the bending frequency is increased, which can be explained by hydrogen bonding interactions. As the liquid water temperature is raised, the vibrational spectrum approaches that of the gas. In the liquid, there are additional vibrational frequencies due to intermolecular modes observed at 730, 550, 430, 170, and 50 cm-1. Some of these slower, H-bonding modes have a period of vibration roughly 2 E-13 s long.

            The effective dipole moment of water molecules in the liquid is estimated at 2.4 debye.

            X-ray and neutron diffraction can be used to study the “structure” of water, including the distributions of interatomic distances (see attached radial distribution functions). These studies indicate that a given water molecule has about 4.5 neighbors in the liquid. This is consistent with the picture of a general, ice-like tetrahedral structure mixed with other fluctuating configurations that give water a higher density than ice.

            The molecular self-diffusion coefficient of water in the liquid is about 2.4 E-5 cm2 s-1. This means that a water molecule can diffuse from one side of a neighbor to the other in about 10-11 s.

           

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