高等水文地球化学备课稿2010.ppt

谢 谢！ * * The water molecule has the shape of the letter “V”. Because each O-H bond is polar (i.e., is a dipole), the overall V-shaped water molecule is also polar. This means that there is a positive and a negative end to the molecule. The negative end is the oxygen atom at the vertex of the “V”, and the positive end are the two hydrogen atoms on the “wingtips” of the “V”. This contrasts with a linear molecule such as CO2 (O=C=O) which is not polar. Because it is polar, the positive end of one water molecule can attract the negative end of a second water molecule, leading to hydrogen bonding. * The hydrogen bonding of water is responsible for many anomalous properties of water. For example, the boiling points of H2O, CH4 and CCl4 are 100°C, -161.5°C and 76.54°C, respectively. Clearly, water has an anomalously high boiling point because of H-bonding. The melting points of these three substances are 0°C, -182.5°C and -22.99°C, respectively. Once again, water has the highest melting point, reflecting the H-bonding. Water also has a higher heat of fusion, heat of vaporization, heat capacity and dielectric constant than CH4 or H2O. If we compare H2O, H2S, H2Se and H2Te, all of which are V-shaped molecules formed between hydrogen and elements in the same column in the periodic table, we see that water, the molecule with the lowest mass, is the only one which occurs as a liquid at standard temperature and pressure (STP: 25°C and 1 bar). Water is the only compound in which H-bonding is possible. The others are gases at STP. The behavior of water is anomalous because we would expect the least massive molecule to have the lowest boiling point. Because life as we know it requires the presence of liquid water, hydrogen-bonding is what makes water capable of supporting life on Earth! * In the drawing shown here, the big, light yellow circles represent oxygen atoms, and the smaller, orange circles represent hydrogen atoms. Cations tend to attract the negative ends (O)