UNIT 29 Water, Acids, Bases and Aqueous Reactions

Water is the most common substance on earth, and it is one of only two liquids which occur naturally in appreciable quantities, the other being petroleum. About 60% of the weight of the human body is water, it being present in the interior of every cell. It also accounts for the bulk of such specialized media as blood and mucous. It is an almost universal solvent, and when it provides the medium for acid-base reactions, it is not just a bystander but an active participant.

The structure of the H₂O molecule gives water its special properties. The O—H bond length is = 0.095718 nm and the H—O—H angle is 104.52°. The former number is simply explained because it is essentially the sum of the Pauling radii for hydrogen (0,030 nm) and oxygen (0.066 nm). But the angle might seem more problematic. The electronic configuration of oxygen is 1s²2s²2p⁴, with the full complement of two electrons in the 2p_x orbital, but only one electron in each of the 2p_y and 2p_z orbitals. As first suggested by Linus Pauling, these latter become hybridized with the two 2s electrons to produce an electron probability distribution having four lobes which point roughly toward the corners of a slightly distorted tetrahedron, the oxygen nucleus lying at its centre. (The corresponding angle in a perfect tetrahedron is 109.5°.) Two of these lobes combine with the respective 1s orbitals of the two hydrogen atoms, to produce the O—H bonds, the hydrogen atoms becoming positive electrical poles because of the consequent incomplete screening of their proton nuclei. The remaining two lobes do not quite reach to the remaining two corners of the distorted tetrahedron. They are negatively charged, as would be expected from the overall neutrality of the molecule. The net charge on each of the four poles is approximately 20% of that of an electron. It is the existence of these poles which gives the water molecule its electrical dipole moment, the magnitude of which can be determined by a method devised by Peter Debye. The dielectric constant of water, at 20°C, is approximately 80 times that of the vacuum value.