[Studies in Environmental Science] Chemistry and Biology of Water, Air and Soil - Environmental Aspects Volume 53 || 3 The chemistry of water

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  • 3 The chemistry of water

    3.1 Composition and structure of pure water

    Water is a chemical compound whose molecules each contain two hydrogen atoms and one oxygen atom. Since natural hydrogen and oxygen consist of several isotopes, water consequently consists of more than one type of molecule. The quantitative ratio of molecules of different isotope composi- tion is determined by the ratio of the individual isotopes in the individual elements. The existence of three hydrogen isotopes (light hydrogen H, heavy hydrogen - deuterium 2H, or alternatively D , tritium 3H or T) and 6 oxygen isotopes ( 1 4 0 , I 5 O , l60, 1 7 0 , l 8 0 , " 0 ) allows altogether 36 possibilities for the structure of a water molecule, 9 of which contain only stable nuclides. The water molecules formed by stable nuclides are present in natural water in the concentrations as follows (mol.%): H 2 1 6 0 - 99.73; 'H2170 - 0.04; 'H2l8O - 0.20; 'HD160 - 0.03; 'HD170 - 1.2 x 'HD180 - 5.7 x D2170 - 0.9 x lo-' and D2"0 - 4.4 x lo-'.

    Differences in the isotope composition of molecules are apparent from different saturated vapour pressure, temperature and heat of phase con- version, heat capacity, and in the thermal dependence of thermodynamic quantities, etc. Some physical constants of l H 2 l 6 0 and D2160 are shown in Table 3.1 [l].

    The chemical properties of deuterium oxide do not differ greatly from the chemical properties of H2"0. However, its reactivity is generally lower, and the reactions are usually slower; similarly to H 2 l 6 0 , it reacts with a number of compounds. For example, the reaction with respective anhydrides gives a "heavy" sulphuric acid (D,SO,) and phosphoric acid (D,PO,). From the chemical viewpoint the differences in the solubility of isotope compounds are of great importance. The solubility of some inorganic compounds in pure and heavy waters are shown in Table 3.2 [2].

    1

    1

    D2160 - 2.3 x

    14

  • Table 3.1. Physical constants of H20 and D2O [l]

    Mailing Boiling Density Temperature Latent heat Latent heat Specific heat Water point point (kg m-3) of maximum of melting of boiling heat capacity

    ("C) ("C) density (kJ kg-') (kJ kg-') (kJ kg-' K - I ) ("C)

    0 100 998.203 (101.325 kPa) (101.325 kPa) (2OOC) 3.98 , 333.5 2259 4.1819 (20C) I

    3.82 101.42 1105.34 (101.325 kPa) (101.325 kPa) (2OOC) 11.185 332.4 2072 4.27 (20'C) I D2'60

  • Table 3.2. Solubilities of some inorganic compounds in HzO and D2O [2]

    A 100 mol of dissolv. species

    mol of solvent 1 . - LH2O Compound Temperature Solubility [ I

  • hydrogenHz molecule

    oxygen molecule

    02

    hydrogen

    dipole water molecule fi

    .- molecule

    water molecule

    Fig. 3.1. A water molecule [3]

    ... 1 8 - H ... 1 8 - H ... 1 8 - H ... I I I H H H

    Hydrogen is covalently bonded to one oxygen atom, and electrostatically to the other one. Intramolecular hydrogen bonding causes the high boiling temperature of water (100C) as compared to the boiling temperature of a similar compound - hydrogen sulphide, H,S (-61C). In the gaseous phase, water associates only slightly at normal temperatures [3].

    In nature, water occurs in the gaseous, liquid and solid states. The structure of liquid water (Fig. 3.2) was determined by X-ray struc-

    tural analysis and infrared spectroscopy. Each molecule of water is sur-

    Fig. 3.2. A scheme cf an octahedral molecule of Lquid water structure [4]

    17

  • rounded on average by six other molecules forming a deformed octahedron around it. At the same time, a similar arrangement of molecules can be found in small amounts of liquid water as is the arrangement in common ice, particularly at temperatures approaching the melting point of ice.

    Water has seven crystal modifications in the solid state. At a normal pressure only the hexagonal modification - ice - is stable. Oxygen atoms form layers consisting of six-component chair-like bent circles (Fig. 3.3). Each oxygen atom is surrounded by other oxygen atoms in a tetrahedral arrangement, three of which belong to the same layer and one to the neigh- bouring upper or lower layer. The internuclear distance of the neighbouring oxygen atoms is 0.276 nm. Hydrogen atoms are located along the connect- ing lines of the oxygen atoms. Each oxygen atom has two hydrogen atoms in close vicinity (0.10 nm), forming covalent bonds with them, whereas two other hydrogen atoms are connected by hydrogen bonding at a greater distance (Fig. 3.4) [l, 4-61.

    T - -

    \ \ .

    Fig. 3.3. Structure of ice. a - scheme of the arrangement of oxygen atoms and layer marking, b - arrangement of atoms in a layer; lower situated oxygen atoms are shown

    dotted [l]

    18

  • Fig. 3.4.

    n

    Arrangement of hydrogen atoms along the lines connecting oxygen 0 - oxygen atom, o - hydrogen atom, 0 - ion pair electrons

    W Arrangement of hydrogen atoms along the lines connecting oxygen 0 - oxygen atom, o - hydrogen atom, 0 - ion pair electrons

    atoms

    References

    1. Gaio, J.: General and !norganic Chemistry. Alfa, Bratislava 1974 (in Slovak). 2. Tolgyessy, J., Varga, S. and Krivih, V.: Nuclear Analytical Chemistry, Vol. 1,

    Introduction to Nuclear Analytical Chemistry. University Park Press, Baltimore - Publishing House of the Slovak Academy of Sciences, Bratislava 1971.

    3. Kemmer, F. N.: Water: the Universal Solvent. Nalco Chem. Co., Oak Brook 1977. 4. Marton, J., Tolgyessy, J., Hygnek, C. and Piatrik, M.: Obtaining, Treatment, Pu-

    5 . Franks, F.: Water. A Comprehensive Treatise. Plenum Press, New York 1972. 6. Tchobanoglous, G. and Schroeder, E. D.: Water Quality. Addison-Wesley, Reading

    rification and Protection of Waters. Alfa, Bratislava 1991 (in Slovak).

    1985.

    3.2 Physical properties of water and aqueous solutions

    3.2.1 Changes of state of water - the triple point

    The changes of state of water can be schematically illustrated as follows:

    ice 2 water water = water vapour

    Corresponding quantities of heat required for the change of state are called specific latent heat of melting or solidification (333.3 kJ kg-') and evaporation or boiling (2257 kJ kg-').

    When water is transformed into ice (under normal conditions) the vo- lume increases by approximately 9.2%. Then changes in the volume of

    19

  • water which take place around the freezing point, intensify the erosive ac- tivity of water in nature.

    Water and ice evaporate (ice sublimes), therefore, they are always sur- rounded by water vapour. In a closed space a t a given temperature and pressure an equilibrium is achieved after a certain time interval, the space being saturated with water vapour. This saturated vapour exerts the same partial pressure by which the original pressure of air is reduced. The higher the temperature, the greater is the pressure of saturated vapour (Table 3.3).

    Table 3.3. Dependence of saturated water vapour pressure on temperature ~ ~

    0 5 10 15 20 25 30 50 100

    Pressure of saturated water vapour ( lo5 Pa) 6.1 8.7 12.3 17.0 23.37 31.65 42.42 123.3 1013.27

    When the saturated vapour pressure reaches the value of the atmospheric pressure, vapour can be generated inside the liquid, and boiling starts. The boiling temperature of water is quite high considering the relatively low molecular weight of water (100C at 101.325 kPa).

    If liquid water is heated, only part of the supplied heat causes a rise in the temperature, the rest is employed in breaking the hydrogen bonding. This results in a high value of the heat capacity of water (4.1819 kJ kg-' K-' a t 20C) which undergoes an anomalous transformation with temperature - it decreases with increasing temperature, and then it begins to increase. On account of the high thermal capacity of water, the land becomes relatively warmer in winter and cooler in summer. Thus, large water surfaces (lakes, seas, oceans) participate significantly in the temperature control over the entire earth's surface.

    As water evaporates under given conditions at any temperature, all three states can be studied at the same time. They can be in equilibrium only at a certain pressure and a certain temperature - this state is called the triple point. It is defined by the following values in the case of water:

    p = 6.1 x lo2 Pa and t = -0.0075"C ( 3 4

    Figure 3.5 shows a simplified phase diagram of water in which mutual relationships between the triple point (T), curves of evaporation (TK), melting and solidification (TA) and sublimation curve (OT), can be seen.

    20

  • A

    - TEMPERATURE Fig. 3.5. Phase diagram of water. T - triple point, OT - sublimation curve, T A - curve of melting and solidification, TK - curve of evaporation, QI - fluid zone,

    K - critical point

    The three curves divide the diagram field into three unequal parts: vapour region (I), liquid water (11) and ice (111). In these regions always only a single state can exist (vapour, water, or ice). Under the conditions of pressure and temperature given by the points lying on the curves of the phase diagram only two states can exist in equilibrium: - along the TK evaporation curve - water and vapour, - along the curve of melting and solidification TA - water and ice, - along the sublimation curve OT - ice and vapour.

    At the triple point T all three states can coexist. The curve TK is not an infinite curve, it is terminated by the point K

    corresponding to the so-called critical state. The temperature and pressure corresponding to this state are called the critical tempemture tc, and critical pressure per. In the critical states the densities of both states are equal and any differences b

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