scan0011 - Energy of the Ionic Bond According to the...

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Unformatted text preview: Energy of the Ionic Bond According to the American Dental Association, the addition of fluoride to tooth- paste and public water systems has brought about a nationwide reduction in tooth decay. The Centers for Disease Control suggest a “safe, effective and inex— pensive” municipal waterway fluoride concentration of between 0.7 and 1.2 parts per million. The mineral portion of teeth is hydroxyapatite, Ca5(PO4)3(OH). \Vhen you drink fluoridated water or brush your teeth with toothpaste containing fluoride, some of the hydroxide anions in your teeth are replaced with fluoride. C215(PO4)3(OH) ~—-—> Ca5(PO4)3(OH,F) Hydroxyapatite Fluorapatite The new mineral, called fluorapatite, is much stronger than hydroxyapatite. What accounts for the added strength? Does the size of the fluoride and hydroxide ions have something to do with the strength of the mineral? The strength of the ionic bond is usually referred to as the lattice enthalpy of the ionic solid. The lattice enthalpy of a molecule is the amount of energy re— quired to separate 1 mol of a solid ionic crystalline compound into its gaseous ions (see the following equation). The lattice enthalpies of some common com- pounds can be found in Table 8.3. MX®—»[email protected][email protected] Although qualitative statements can be made about the relative size of the lattice enthalpy on the basis of ionic radii, most chemists use lattice enthalpy to make some quantitative statements about the strength of an ionic compound. Unfortu- nately, it is quite difficult to measure lattice enthalpy accurately in an ionic crys— talline solid. However, we can calculate the lattice enthalpy using Hess’s law (see Chapter 5) in a process known as the Born-Haber cycle, named after two Nobel Prize—winning German scientists (Max Born, 1882—1970, and Fritz Haber, 1368—1934) The Born—Haber cycle is a diagrammatic representation of the formation of an ionic crystalline solid. Figure 8.8 illustrates the Born—Haber cycle for the formation of sodium chloride from its elements in their standard states. Al— though chemistry is a process in which all kinds of things happen concurrently and continuously, for clarity we often break down the Born—Haber cycle into a series of steps. _ABLE 8.3 Lattice Enthalpies for Some Common Ionic Solids Values in the table are in kilojoules per mole for the simple ionic compounds (for example, the lattice energy of K20 is 2238 kl/mol). F‘ Cl‘ Br' I‘ OH‘ 02— Li+ 1030 834 788 757 1039 2799 Na+ 923 787 747 704 887 2481 K+ 821 701 682 649 789 2238 Rb+ 785 689 660 630 766 2163 Cs+ 740 659 631 604 721 — Mg1+ 2913 2326 2097 1944 2870 3795 Ca2+ 2609 2223 2132 1905 2506 3414 Ba2+ 2341 2033 1950 1831 2141 3029 363+ 5096 4874 4711 4640 5063 13,557 Al3+ 5924 5376 5247 5070 5627 15,916 8.2 |onicBonding% 313 .Application . CHEMICAL ENCOUNTERS: Fluoridated Water and Tooth Decay ...
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