Sodium chloride formation from elements

For example, look at the formation of sodium chloride (NaCl) from its elements, sodium (Na) and chlorine (Cl2) ... 

Let us consider the formation of sodium chloride from its elements. An energy (enthalpy) diagram (called a Born-Haber cycle) for the reaction of sodium and chlorine is given in Figure 3.7. (As in the energy diagram for the formation of hydrogen chloride, an upward arrow represents an endothermic process and a downward arrow an exothermic process.)... 

Sodium chloride is formed from the elements in their standard states with a heat of formation of -411 kj mol-1 ... 

The enthalpy of formation of a compound is a so-called thermodynamic state function, which means that the value depends only on the initial and final states of the system. When the formation of crystalline NaCl from the elements is considered, it is possible to consider the process as if it occurred in a series of steps that can be summarized in a thermochemical cycle known as a Born-Haber cycle. In this cycle, the overall heat change is the same regardless of the pathway that is followed between the initial and final states. Although the rate of a reaction depends on the pathway, the enthalpy change is a function of initial and final states only, not the pathway between them. The Born-Haber cycle for the formation of sodium chloride is shown as follows ... 

Ionic compounds consist of positive ions (cations) and negative ions (anions) hence, ionic compounds often consist of a metal and nonmetal. The electrostatic attraction between a cation and anion results in an ionic bond that results in compound formation. Binary ionic compounds form from two elements. Sodium chloride (NaCl) and sodium fluoride (NaF) are examples of binary ionic compounds. Three elements can form ternary ionic compounds. Ternary compounds result when polyatomic ions such as carbonate (C032 ), hydroxide (OH-), ammonium (NH4+), form compounds. For example, a calcium ion, Ca2+, combines with the carbonate ion to form the ternary ionic compound calcium carbonate, CaC03. Molecular compounds form discrete molecular units and often consist of a combination of two nonmetals. Compounds such as water (H20), carbon dioxide (C02), and nitric oxide (NO) represent simple binary molecular compounds. Ternary molecular compounds contain three elements. Glucose ( 12 ) is a ternary molecular compound. There are several distinct differences between ionic and molecular compounds, as summarized in Table 1.2. 

The atomisation enthalpy of elemental sodium Afl%tom, the first ionisation energy of atomic sodium Iu the dissociation enthalpy D of gaseous chlorine, the electron attachment energy Ex of atomic chlorine and the enthalpy of formation A//)1 of crystalline sodium chloride can all be taken from standard tabulations of experimental data. An experimental lattice energy UL is thus given by ... 

The electron affinities of elements (Chap, 7) that form negative ions may be calculated by considering the formations of compounds containing such negative ions. The formation of such a compound from the elements (the heat of such a reaction being directly measurable) may be broken down into a series of simpler steps. The treatment is again called a Born-Haber cycle and is analogous to the treatment of the conversion of an alkali metal to its hydrated ion (discussed in Chap. 6). Consider the formation of sodium chloride from the elements ... 

For the heat of formation of sodium chloride from its elements, Thomsen6 gives 97 69 Cal., and from the interaction of aqueous solutions of sodium hydroxide and hydrogen chloride 13 745 Cal. 

Sodium selenite, Na2Se03.—The selenite is formed by heating a mixture of selenious acid and sodium chloride.11 At ordinary temperature the aqueous solution deposits the pentahydrate, above 60° C. the anhydrous salt.12 For the heat of formation from its elements in aqueous solution Thomsen gives 238-4 Cal. 

Another method makes use of the heat of formation of a dilute solution of a salt of the anion. Once again the procedure will be explained by reference to the chloride ion. The heat of formation AH from its elements of sodium chloride in dilute solution, i.e.,... 

As we saw in Chapter 3, this transfer of electrons from metal atoms to nonmetal atoms is the general process for the formation of any binary ionic compound from its elements. For example, when sodium chloride is formed from the reaction of metallic sodium with gaseous chlorine, each sodium atom loses an electron, and each chlorine atom gains one. 

Write a balanced equation and draw an approximate enthalpy diagram for each of the following (a) the formation of 1 mol of sodium chloride from its elements (heat is released) (b) the conversion of liquid benzene to gaseous benzene. 

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

There i.s a difference between the way substances—either elements or compounds—combine to form mixtures and the way elements combine to form compounds. Each substance in a mixture retains its chemical identity.The si ar molecules in the teaspoon of sugar in Figure 2.17, for example, arc identical to the. sugar molecules already in the tea. The only difference is that the sugar molecules in the tea are mixed with other. substances, mostly water. The formation of a mixture, therefore, is a physical change. As was discussed in Section 2.3, when elements join to form compounds, there is a change in chemical identity. Sodium chloride is not a mixture of sodium and chlorine atoms. Instead, sodium chloride is a compound, which means it is entirely diflerent from the elements used to make it. The formation of a compound is therefore a chemical change. 

We have seen examples of endothermic processes that are spontaneous, such as the dissolution of ammonium nitrate in water. (Section 13.1) We learned in our discussion of the solution process that a spontaneous process that is endothermic must be accompanied by an increase in the entropy of the system. However, we have also encountered processes that are spontaneous and yet proceed with a decrease in the entropy of the system, such as the highly exothermic formation of sodium chloride from its constituent elements. (Section 8.2) Spontaneous processes that result in a decrease in the system s entropy are always exothermic. Thus, the spontaneity of a reaction seems to involve two thermodynamic concepts, enthalpy and entropy. 

The periodic table places the most reactive metals in Group 1A and the most reactive nonmetals in Group 7A. When these elements are mixed, we predict a vigorous reaction to ensue, as evidenced by the formation of sodium chloride from sodium and chlorine. The models show Na metal, CI2 molecules, and NaCl. 

This lattice energy is 787 kJ/mol and is more than sufficient to make the overall process for formation of sodium chloride from the elements exothermic. Eorces between oppositely charged particles are called electrostatic, or Coulombic, and constitute an ionic bond when they are attractive. 

Chlorine and sodium hydroxide are the main products of the industrial chlor-alkali electrolysis that is described as a process example in Section 6.19. Hydrochloric acid is produced by reaction from the elements H2 and CI2 or by the reaction of chloride salts such as, for example, NaCl or CaCl2, with sulfuric acid. Other important sources of HCl are industrial chlorination processes using CI2 as chlorination agent (e.g., chlorination of benzene to form chlorobenzene and HCl or the chlorination of methane to give chloromethane and HCl) or industrial dehydrochlorination processes (e.g., production of vinyl chloride and HCl from 1,2-dichloroethane). The main uses of hydrochloric acid are addition reactions to unsaturated compounds (by hydrochlorination or oxychlorination), formation of chlorine in the Deacon process, production of chloride salts from amines and other organic bases, dissolution of metals, regeneration of ion exchange resins, and the neutralization of alkaline products. 

Other dissolved salts, e.g. chlorides and sulfates, increase the tendency of local corrosive attack. These salts stabilize the formation of local elements. The small area where chloride or sulfate ions are adsorbed become anodic (active corrosion) to the large cathodic surface (passive oxide). Sea water contains on average ca. 30000 mg L" , or 3%, dissolved salts, although salt content can vary considerably from one sea to another (e.g. Baltic Sea 0.7%, Dead Sea 21%). Approximately 70 to 80% of the salt content of sea water is sodium chloride (NaCl), which means that sea water is very corrosive. In coastal areas and estuaries, in particular, sea water is often heavily polluted (brackish water). Mention must made, for example, of hydrogen sulfide (H2S) and ammonium ions (NH4). Even at concentrations as low as a few milligrams per liter these elements can drastically intensify aggressiveness. 

M FIGURE 9.4 Born-Haber Cycle for Sodium Chloride The sum of the steps is the formation of NaCl from elemental Na and CI2. The enthalpy change of the last step is the lattice energy. 

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