TWEED RIVER HIGH SCHOOL 2006 PRELIMINARY CHEMISTRY Unit 3 Water Part 2 The wide distribution and importance of water on Earth is a consequence of its molecular structure and hydrogen bonding. Construct Lewis electron dot structures of water, ammonia and hydrogen sulfide to identify the distribution of electrons. Compare the molecular structure of water, ammonia, and hydrogen sulfide, the differences in their moleculare shapes and in their melting and boiling points. Process information from secondary sources to graph and compare the boiling and melting points of water with other similar sized molecules. Copy Table 13.5, p233 text and plot a suitable graph from this data. New edition table 11.1 1
Describe hydrogen bonding between molecules. Identify water as a polar molecule Describe the attractive forces between polar molecules as dipoledipole forces Electronegativity The electronegativity of an element is a measure of the ability of the atom of that element to attract bonding electrons towards itself when it forms compounds. If the electronegativities of two elements differ by more than about 1.5, then the elements will tend to form ionic compounds. Electronegativities increase from left to right across any given period and they increase from bottom to top in any particular group. Fluorine is the most electronegative element. Francium is the least electronegative element. 2
Electronegativities for some common elements. Element Electronegativity (Pauling Scale) H 2.2 He 0 Li 1.0 Be 1.6 B 2.0 C 2.6 N 3.0 O 3.4 F 4.0 Ne 0 Na 0.9 Mg 1.3 Al 1.6 Si 1.9 P 2.2 S 2.6 Cl 3.2 Ar 0 K 0.8 Ca 1.0 Fr 0.7 3
Intermolecular Forces Forces exist between molecules that are not chemical bonds. 1. Dipole-Dipole interactions Molecules in which the bonding electrons are unevenly shared between the bonded atoms are called polar molecules and the bonds are called polar (covalent) bonds). A pair of equal and opposite charges separated in space is called a dipole. The positive and negative charges interact between molecules form an intermolecular attraction. Take notes and diagrams from class discussions. 4
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Explain the following properties of water in terms of its intermolecular forces: - surface tension - viscosity - boiling and melting points Surface Tension Water molecules at the surface of a beaker, are not surrounded by other water molecules in the same way as those molecules in the centre of the beaker. Surface tension results from the molecules on the surface of water (or any liquid) having an overall attractive force downwards into the rest of the water. This downwards force creates a tension on the surface of the water, so that it behaves like a tightly stretched skin. Water has a high surface tension because of its ability to form many hydrogen bonds. Draw Fig 13.19, p232 text 7
Viscosity The viscosity of a liquid is a measure of its resistance to flow. When a liquid flows the molecules slide over one another. If the molecules have strong intermolecular attractions and are long and thin and easily tangle the liquid will have a high viscosity, e.g. tar and honey. If the molecules have weak intermolecular attractions and are smaller then liquid has a low viscosity. Viscosity decreases as temperature rises. Gases have the lowest viscosity. Viscosity relates directly to the strength of the forces between molecules and the size of the molecules. These forces determine how easily the molecules move past each other. The viscosity of water is greater than many other similar liquids, e.g. petrol, because the intermolecular forces are much stronger in water than those in other liquids. Melting and Boiling Points The melting and boiling points of water are much higher than molecules of similar size. This is because of the strong hydrogen bonds in water and the fact that each water molecule is hydrogen bonded to four other water molecules. It takes a greater amount of heat energy to increase the kinetic energy of the molecules, which is required to break the relatively strong hydrogen bonds. Identify data and process information from secondary sources to model the structure of the water molecule and the effects of forces between water molecules. (You can refer to diagram from Part 1). 8