Sodium chloride bond energy

Ionic compounds are high-melting solids because of their ionic bonds. As discussed previously in Section 6.6, a visible sample of sodium chloride consists, not of NaCI molecules, but of a vast three-dimensional network of ions in which each Na+ cation is attracted to many surrounding Cl- anions and each Cl- ion is attracted to many surrounding Na+ ions. For sodium chloride to melt or boil so that the ions break free of one another, every ionic attraction in the entire crystal must be overcome, a process that requires a large amount of energy. 

In those atomic reactions in which no energy of activation occurs, as in Na + Cl2 —> NaCl + Cl, and many reactions involving free radicals, the reaction mechanism is the simplest. In the corresponding reactions of sodium with organic chlorides an energy of activation is, however, already present dependent on the nature of the C—Cl bond. 

Covalent bonds are formed when two nonmetal atoms share electrons in order to satisfy their need to have a full outermost principal energy level. Covalent bonds are not as strong as the bonds formed between ions. For example, it would take a high flame and a temperature of almost 800 degrees Celsius to break the bonds between the sodium and chlorine in sodium chloride. The covalent bonds found in methane can be broken instantly with the introduction of a lit match. 

Extensive kinetic investigations have also indicated that the activation energy of the overall thermal-decomposition process is substantially lowered by the addition of sodium chloride and sodium carbonate. Madorsky and coworkers have, therefore, proposed that these salts catalyze the dehydration of cellulose by scission of the C —O bonds (bonds a, b, and c in 1 see p. 438), and that this results in destruction of the hexose units and increases the yield of water and char at the expense of levoglucosan. This theory has found substantial support in subsequent experiments and publications however, it may be noted here that Golova and associates" consider that inorganic salts promote the cleavage of C—C, rather than C—O, bonds in the macromolecule. 

In Chapter 3 the covalent bond has been discussed and the question now arises whether this is the only possible type of bond between atoms. Let us consider the gaseous molecule of sodium chloride. The sodium and chlorine atoms each have one unpaired electron, 35 in sodium and 3/> in chlorine, so that in principle the formation of a covalent bond is possible. The calculation of bond energies presents considerable difficulties even in a simple molecule such as hydrogen and the calculation for more complicated molecules is impossible except by an approximate method such as that introduced by Pauling In this method, it is assumed that the energy of the covalent bond A—B, is equal to one half of the sum of the bond energies of the homopolar molecules A—A and B—B, i.e. 

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