The chemistry of water deals with the fundamental chemical property and information about water. Water chemistry is discussed in the following subtitles.
|Abundances (% or halflife) of hydrogen and oxygen isotopes|
|70.6 s||122 s||99.762%||0.038%||0.200%|
|Relative abundance of isotopic water|
The predominant water molecules H216O have a mass of 18 amu, but molecules with mass 19 and 20 occur significantly. Because the isotopic abundances are not always the same due to their astronomical origin, The isotopic distribution of water molecules depends on its source and age. Its study is linked to other sciences. (See Dojlido, J.R. & Best, G.A. (1993) Chemistry of Water and Water Pollution, Ellis Harwood for isotopic distribution of water.)
In particular, D216O is called heavy water, and it is produced by enrichment from natural water. Properties of heavy water are particularly interesting due to its application in nuclear technology.
Lewis Dot Structures H H | | " " H--C--H H--N : H--O : H--F : | | | H H H " CH3 NH3 H2O HF Bondlength /pm C-H N-H O-H H-F 109 101 96 92
There are six valance electrons on the oxygen, and one each from the hydrogen atom in the water molecule. The eight electrons form two H-O bonds, and left two lone pairs. The long pairs and bonds stay away from each other and they extend towards the corners of a tetrahedron. Such an ideal structure should give H-O-H bond angle of 109.5°, but the lone pairs repel each other more than they repel the O-H bonds. Thus, the O-H bonds are pushed closer, making the H-O-H angle less than 109°.
After the introduction of
quantum mechanics, the electronic
configuration for the valence electron of oxygen are
Since the energy levels of 2s and 2p are close,
valence electrons have characters of both s and p.
The mixture is called sp3 hybridization.
These hybridized orbitals are shown on the right.
The structures of CH4, NH3, and H2O
can all explained by these
hybrid orbitals of the central atoms.
The above approach is the valence bond theory, and both the
C-H bonds and lone electron pairs are counted as VSPER pairs
in the Valence-shell Electron-Pair Repulsion (VSEPR) model, according
to which, the four groups point to the corners of a tetrahedron.
For triatomic molecules such as water,
molecular orbital (MO) approach can also be
applied to discuss the bonding. The result however is similar to
the valence bond approach, but the MO theory gives the energy
levels of the electron for further exploration.
Molecular vibration of water
Atoms in a molecule are never at rest, and for each type of molecule,
there are some normal vibration modes. For the water molecule,
the three normal modes of vibrations are symmetric stretching, bending
and assymmetric stretching.
For triatomic molecules such as water, molecular orbital (MO) approach can also be applied to discuss the bonding. The result however is similar to the valence bond approach, but the MO theory gives the energy levels of the electron for further exploration.
|Basic modes of vibration for H2O|
O O O / \ / \ / \ / \ / \ H \ H H HH HH H H H H H symmetroc bending assymmetric stretching stretchinng v1 v2 v3
|Transition bands of D2O, H2O, and HDO|
of upper state
|Absorption wavenumbers |
of bands /cm-1
|Data from Eisenberg, D. and Kauzmann, W. |
(1969) Structure and properties of water,
Oxford University press.
Many more less intense absorption bands extend into the green part of the visible spectrum. The absorption spectrum of water may contribute to the blue color for lake, river and ocean waters.
A point group has a definite number of symmetry elements arranged in certain fashion. Molecules can be classified according to their point groups. Molecules of the same point group have similar spectroscopic characters. Other molecules of C2v point group are CH2=O, CH2Cl2, the bent O3 etc.
|Comparison of melting and boiling|
points for a few substances
|m.p.||b.p. /° C|
In this table, the melting and boiling points for water are particular high for its small molecular mass. This is usually attributed to the formation of hydrogen bonds. The small electronegative atoms F, O and N are somewhat negatively charged when they are bonded to hydrogen atoms. The negative charges on F, O and N attract the slightly positive hydrogen atoms, forming a strong interaction called hydrogen bond.
|Hydrogen bonds among water molecules|
H \ / O . . . . H-O H . . . .O / \ / \ H H . . . .O \ H H-O . . . H | | H . . . O--HDimer
Based on the observed absorption at 3546 and 3691 cm-1, Van Thiel, Becker, and Pinmentel (1957, J. Chem. Phys. 27 386) suggested the formation of water dimer when trapped in a matrix of nitrogen.
Due to hydrogen bonding, water molecules form dimers, trimers, polymers, and clusters. The hydrogen bonds are not necessarily liner.
The density of ice is dramatically smaller than that of water, due to the regular arrangement of water molecule via hydrogen bonds. In an idealized structure of ice, every hydrogen atom is involved in hydrogen bond. Every oxygen atom is surrounded by four hydrogen bonds.
This diagram from caltech.edu, shows the structure of hexagonal ice in (a) and cubic ice in (b). A rod here represents a hydrogen bond. Since the hydrogen bonds are not linear, the real structure is a little more complicated.
The tetrahedral coordination opens up the space between molecules. On each hydrogen bond, shown by a rod joining the oxygen atoms, lies one proton in an asymmetric position (not shown). Bond lengths, 275 pm, are indicated. Ordinary ice is hexagonal. and the hexagonal c axis is labelled 732 pm, and one of the hexagonal a axes is labelled 450 pm. If water vapor condenses on very cold substrate at 143-193 K (-130 to -80ºC) a cubic phase is formed. In (b) the cubic unit cell is outlined with dashed lines; dimensions are in pm determined at 110 K.
These diagrams can also be used to represent the two forms of diamond, and in this case, the rods joining the atoms represent C-C bonds. Each C-C bondlength is 154 pm. Silicon and germanium crystals have the same structure, but their bondlengths are longer. The two diamond types of structure are related to the packing of spheres. The hexagonal type has the ABABAB... sequence, whereas the cubic type has the ABCABC... sequence. In both cases, half of the tetrahedral sites are occupied by tetrahedrally bonded carbon atoms. Hexagonal diamonds have been observed in meteorites.
The four hydrogen bonds around an oxygen atom form a tetrahedron in a fashion found in the two types of diamonds. Thus, ice, diamond, and close packing of spheres are somewhat topologically related.
A phase diagram of water shows 9 different solid phases (ices). Ice Ih is the ordinary ice. In addition to ice Ic from vapor deposition, conditions for nine phases are shown. Aside from ice I, other phases are formed and observed under high pressure generated by machines built by scientists. So far, ten different forms of ice have been observed, and some ice forms exist at very high pressure. The pressure deep under the polar (Antarctic) ice cap is very high, but we are not able to make any direct observation or study.
There is a report of the 11th ice, and the ice phase diagram and drawings of ice structures given here is extremely interesting.
For example, HCl is a stronger acid than H2O, and the reaction takes place as HCl dissolves in water.
On the other hand, a stong base also react with water to give the stong base species, OH-.
If several acids and bases are dissolved in water, all equilibria must be considered. To estimate the pH of these solutions requires the exact treatment of several equilibrium constants. For example, many species dissolve in rain water, and many equilibria must be considered. Detail consideration and examples are given in Acid-Base Reactions
Carbon dioxide in the air dissolve in rain water, lakes and rivers.
A solution of CO2 involves the following reaction:
|Reaction||K formula||K value|
|H2O(l) + CO2(g) = H2CO3(l)||1/PCO2||?|
|H2CO3 = HCO3- + H+||[HCO3-] [H+] / [H2CO3]||5e-7|
|HCO3- = CO3-2 + H+||[CO3-2] [H+] / [HCO3-]||5e-11|
|HOH(l) + HOH(l) = H3O+ + OH-||[H3O+] [OH-]||1e-14|
From the given data, we have the following five equations and five unknowns:
|H2CO3 « HCO3- + H+||[HCO3-] [H+]|
---------------- = 5e-7
|HCO3- « CO32- + H+||[CO32-] [H+]|
-------------- = 5e-11
|2 H2O « H3O+ + OH-||[H3O+] [OH-] = 1e-14||(3)|
= [HCO3-] + [OH-] + 2 [CO32-]
|All species containing C||[H2CO3] + [HCO3-]
= 8.0e-4 M
|[H+], [OH-], [H2CO3], [HCO3-], [CO32-]|
Solving these equations for the 5 unknowns can be done using Maple, Mathcad,
spread sheet, or approximation. In any case, we are interested in the pH,
and we can make the following approximations or assumptions
|Assume H+ mostly|
comes from (1)
|[H+] = [HCO3-]|
|H2CO3 is a weak acid|
|[H2CO3] = 8.0e-4 M||(6)|
|Let x = [HCO3-] = [H+]||[HCO3-] [H+] / [H2CO3]|
= x2 / [H2CO3]
Generally speaking, rain water has a pH about 5, rather acidic. It dissolves limestone and marble readily. Due to the dissolved carbon dioxide, rain water is a buffer solution.
Increased carbon dioxide level forces an increase in dissolved carbon dioxide. Would this causes pH of rain water to decrease or increase? Justify your answer by giving the reasons.
Since [H+] = 2.0e-5, [OH-] = 5e-9, the amount of H+ from ionization of water is also 5.0e-9, small with respect to 2.0e-5 from ionization of H2CO3. Similarly, the ionization from
H2CO3 is a weak acid, its ionization is small indeed.
Now, you may proceed to evaluate other concentrations: [OH-], [HCO3-], and [CO32-]
Many metals displace H+ ions in acidic solutions. This is often seen as a property of acids.
The Gibb's energy is the energy released other than pressure-volume work. This redox reaction to form water can be engineered to proceed in a Daniel cell. In this case, the energy is converted into electric energy according to this equation.
= 1.23 V
The excess energy can also be evaluated using
Evaluate this value please!