1Chpt. 3: Properties of Water and Seawater James W. Murray(10/01/01) Univ. WashingtonI. The Nature of pure waterSeawater is mostly water (H2O). In fact it is about 96.5 wt % water. Sediments arealso mostly water. Most fine grained surface sediments have a porosity ( Φ = volume ofpores to volume of solids) of greater than 90%. Almost every process we discuss willoccur and be affected by water. Thus, water has been called the universal solvent.Water has unique and unusual properties both in pure form and as a solvent.These properties influence chemical reactions.Structure of the water molecule: The structure ofH2O is shown in Fig 3.1. It consists of an O atom with 6e- that have the electronic configuration of: 1 s2 2s2 2pz2 2py 2pxwhich merge with two H atoms with 1 e- each resulting ina neutral molecule with 8 e- which form four pairs calledsp3 hybrids. The most stable configuration of these fourlobes is a tetrahedral arrangement, with two e- in eachlobe. Two lobes are used for O-H bonds (shared electrons) and two lobes have free lonepairs of electrons (Fig 3.1). The water molecule lackssymmetry (as for the linear O=C=O molecule, e.g. CO2)that would otherwise cancel out polarity.The H-O-H tetrahedral angle is 105° which is less thanthe ideal tetrahedral angle of 109° (Fig 3.2). The reasonthis is so is because of e- repulsion. The bent structureexplains why water has a dipole moment. There isseparation of charge, which means it is a polarmolecule.An important aspect of the water molecule structure is itspropensity to form hydrogen bonds (H-bonds). H-bondsoften occur in liquids made up of molecules in which H is bonded to a highlyelectronegative element (e.g. O, N, Cl or F).1. H-bonds can be thought of as resulting from "resonance" of an H between twomore polar atoms and necessitates that the three atoms be collinear (e.g. see Fig 3.3).2. The H-O----H bond is relatively strong and has a strength of about 4.5 kcal mol-1,compared to 10-20 kcal mol-1 for ionic bonds and ~0.5 kcal mol-1 for van der Waalsbonds.These H-bonds also lead to "cooperative bonding" in which water molecules linktogether to actually form regions with structure. What this means is that formation of oneH-bond makes it easier to form a stronger second bond because of the first. In some waysthese regions may have an ice like structure. One line of evidence is that when ice melts2only about 15% of the bonds are broken. In one modelthese regions are called "flickering clusters" becausethey form and reform at a rapid rate (the Frank-WenFlickering Cluster Model - Fig 3.4).The structure of the H2O molecule explainswhy it is easier to freeze water than convert it to a gas.It also explains why H2O is a good solvent,it attracts anions and cations. When a salt dissolves insolution, water molecules surround each ion in aprocess called hydration (see Fig 3.5). Such hydrationnumbers are difficult to measure but the strength ofhydration is probably proportional to the charge to radius ratio (q/r) of the central ion.Thus Ca2+ hydrates more strongly than Na+ and Mg2+ hydrates more strongly than Ca2+ .Hydration isolates ions from each other and enhances solubilization relative to othersolvents (Fig 3.6).The waters around a central ion are called the hydration sphere. The watermolecules probably arrange themselves into an inner sphere of tightly bound waters andan outer sphere that is less tightly bound. The water molecules are oriented with theiroxygens pointed toward a cation. The process of hydration usually results in a decrease involume and is called electrostriction and influences the molal volumes occupied by ions(e.g. Vion in cm3/mol).Example:The effect of hydration and electrostriction can be illustrated using a simpleexample. If we make a 0.5m NaCl solution we can predict the volume of the solutionfrom the weights and densities of the recipe and compare the predicted volume with thatmeasured. Thus:Component density volume29.22 g NaCl ÷ 2.165 g/cm3 = 13.50 cm3970.78g H2O ÷ 0.997 g/cm3 = 973.70 cm31000.0g of 0.5m of NaCl = 987.2 cm3 (predicted) 983.0 cm3 (actual) Volume difference = 4.2 cm33Fig. 3.5 Fig. 3.6Fig 3.74The anomalous properties of water (Table 3.1) are primarily due to these H-bonds andthe structure of water. These properties (which were originally presented by Sverdrup,Johnson and Fleming in 1942) include:high heat capacity Heat Capacity (Cp) is the thermal energy it takes to raise 1gm of a substance by 1°C. Water has the highest heatcapacity of all solids and liquids except liquid NH3. This isbecause it takes a lot of energy to break the hydrogen bondsand change the structure of water. Thus water has a largethermal buffer capacity and acts as a climate buffer.high heat of evaporation mammals cool by sweating! - it takes energy (∆H = 540 calg-1) to break the hydrogen bonds. In fact water has thehighest heat of evaporation of all liquids.high boiling point The boiling points and freezing points of the group VIAhydrides (S, Se and Te) fall on a line of decreasingB.P./F.P. with decreasing molecular weight except water(see Fig. 3.7). The projected B.P. should be - 68°C whilethe real value is 100°C.high freezing point Easier to freeze than convert to a gas. The freezing point0°C is anomously high. The projected F.P. is -90°C (seeFig 3.7). The heat of freezing is only 1/7 that of evaporationimplying that there is a relative small difference in thenumber of bonds between water and ice.low heat of freezing Water structure can move easily to the ice structurehigh surface tension Water likes itself relative to most other surfaces andbecause of this water minimizes its surface area. When airbubbles break at the sea surface the high surface tensioncauses the surrounding water to snap back into thedepression left by the bubble resulting in injection of asmall droplet of surface seawater into the atmosphere. Thewater soon evaporates leaving a small aerosol of seasalt.This is the mechanism for seasalt transfer from ocean toland.high dielectric constant Water has a high dissolving power because water reducesthe forces of attraction between ions. You could considerthis as a result of ion hydration. The force between ions (F)is coulombic and the dielectric constant (ε) reduces thisforce according to: F = q1q2/r2ε . For water at 25°C, ε = 78.5The high heat capacity and heats of fusionand