6246lect03_S10

6246lect03_S10 - Advanced Environmental Geochemistry GLY...

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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 3 - Physical and Chemical Properties of Water Introduction In a 2008 paper in Nature, Philip Ball says, “No one really understands water. It’s embarrassing to admit it, but the stuff that covers two-thirds of our planet is still a mystery.” Much more work remains to be done on water, but we can examine the properties of water, especially as they affect the earth. The physical and chemical properties of water are anomalous with respect to almost all other known liquids, and the degree of this anomalous behavior is often large. We have seen that the melting and boiling points of water are much higher than would be expected, and have explained this behavior on the basis of hydrogen bonding. We have also seen that the increase of density upon melting may be explained by the rigid tetrahedral structure of H 2 O in the solid state resulting in a coordination number of IV. The rigidity of this structure is due at least in part to hydrogen bonding. In the liquid state it has been demonstrated that some hydrogen bonding persists and accounts for the loose chains of water molecules observed in x-ray studies. The lack of rigidity allows the coordination to increase to approximately 4.4 at 4°C. We will now examine some other physical and chemical properties of water to ascertain in what way the behavior of water is anomalous, and how such anomalies affect the geochemical and geological phenomena associated with water. Physical properties Lect03, slide 2 here Heat Capacity - Heat capacity is the amount of heat required to increase a given quantity of material by a given temperature at constant pressure and volume. This is usually expressed as calories per degree Celsius. A calorie is defined as the amount of heat required to raise one gram of water one degree Celsius at a
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 2 pressure of one atmosphere and a temperature of 15°C. The calorie is equivalent to 4.18400 joules (NBS standard). Using the absolute system of electrical units to define the conversion factor results in one calorie equaling 4.18674 joules, which is the international standard. One BTU (British Thermal Unit) is equal to 251.996 international calories. Molar heat capacity is defined as the quantity of heat necessary to raise the temperature of one molecular weight of a substance by one degree Celsius. Lect03, slides 3-4 here Water has one of the highest heat capacities of any known substance. Mercury has a heat capacity of 0.033382 calories/gram at 0°C. The value for bromine is 0.113 calories/gram at 25°C. Water has a heat capacity of 1.00 calories/gram. Only liquid ammonia, another substance in which hydrogen bonding is important, has a higher heat capacity (1.23 calories/gram). One explanation for this effect is that, as water is heated, the increased movement of water causes the hydrogen bonds to bend and break. Energy absorbed in bond bending and breaking processes is not available to increase the kinetic energy of the water, so it takes considerable heat to raise water's temperature.
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This note was uploaded on 05/04/2011 for the course GLY 6246 taught by Professor Warburton during the Spring '11 term at FAU.

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6246lect03_S10 - Advanced Environmental Geochemistry GLY...

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