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preface to fifteenth edition

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6.2 SECTION 6<br />

the partial molal enthalpy and the heat capacity of the solute are the same as in the infinitely dilute<br />

real solution.<br />

For some tables the uncertainty of entries is indicated within parentheses immediately following<br />

the value; viz., an entry 34.5(4) implies 34.50.4 and an entry 34.5(12) implies 34.51.2.<br />

References: D. D. Wagman, et al., The NBS Tables of Chemical Thermodynamic Properties, in<br />

J. Phys. Chem. Ref. Data, 11: 2, l982; M. W. Chase, et al., JANAF Thermochemical Tables, 3rd ed.,<br />

American Chemical Society and the American Institute of Physics, 1986 (supplements <strong>to</strong> JANAF<br />

appear in J. Phys. Chem. Ref. Data); Thermodynamic Research Center, TRC Thermodynamic Tables,<br />

Texas A&M University, College Station, Texas; I. Barin and O. Knacke, Thermochemical Properties<br />

of Inorganic Substances, Springer-Verlag, Berlin, 1973; J. B. Pedley, R. D. Naylor, and S. P. Kirby,<br />

Thermochemical Data of Organic Compounds, 2nd ed., Chapman and Hall, London, 1986; V. Majer<br />

and V. Svoboda, Enthalpies of Vaporization of Organic Compounds, International Union of Pure<br />

and Applied Chemistry, Chemical Data Series No. 32, Blackwell, Oxford, 1985.<br />

6.1.1 Some Thermodynamic Relations<br />

6.1.1.1 Enthalpy of Formation. Once standard enthalpies are assigned <strong>to</strong> the elements, it is<br />

possible <strong>to</strong> determine standard enthalpies for compounds. For the reaction:<br />

C(graphite) O (g) : CO (g) H 393.51 kJ (6.1)<br />

2 2<br />

Since the elements are in their standard states, the enthalpy change for the reaction is equal <strong>to</strong> the<br />

standard enthalpy of CO 2 less the standard enthalpies of Cand O 2 , which are zero in each instance.<br />

Thus,<br />

H 393.51 0 0 393.51 kJ (6.2)<br />

f<br />

Tables of enthalpies, such as Tables 6.1 and 6.3, can be used <strong>to</strong> determine the enthalpy for any<br />

reaction at 1 atm and 298.15 K involving the elements and any of the compounds appearing in the<br />

tables.<br />

The solution of 1 mole of HCl gas in a large amount of water (infinitely dilute real solution) is<br />

represented by:<br />

<br />

<br />

HCl(g) inf H2O : H (aq) Cl (aq) (6.3)<br />

The heat evolved in the reaction is H 74.84 kJ. With the value of f H from Table 6.3, one<br />

has for the reaction:<br />

<br />

<br />

fH fH[H (aq)] fH[Cl (aq)] fH[HCl(g)] (6.4)<br />

for the standard enthalpy of formation of the pair of ions H and Cl in aqueous solution (standard<br />

state, m 1). To obtain the f H values for individual ions, the enthalpy of formation of H (aq) is<br />

arbitrarily assigned the value zero at 298.15 K. Thus, from Eq. (6.4):<br />

<br />

fH[Cl (aq)] 74.84 (92.31) 167.15 kJ<br />

With similar data from Tables 6.1 and 6.3, the enthalpies of formation of other ions can be determined.<br />

Thus, from the f H[KCl(aq, std. state, m 1 or aq, ss)] of 419.53 kJ and the foregoing<br />

value for f H[Cl (aq, ss)]:<br />

<br />

<br />

fH[K (aq, ss)] fH[KCl(aq, ss)] fH[Cl (aq, ss)]<br />

419.53 (167.15) 252.38 kJ (6.5)

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