Enthalpies of vaporization, fusion, and sublimation

The energy that must be supplied as heat at constant pressure per mole of molecules that are vaporized under standard conditions (that is, pure liquid at 1 bar changing to pure vapor at 1 bar) is called the standard enthalpy of vaporization of the liquid and is denoted (Table 1.2). For example, 44 kj of heat is 

Alternatively, we can report the same information by writing the thermochem-ical equation  

A note on good practice The attachment of the subscript vap to the A is the modern convention however, the older convention in which the subscript is attached to the H, as in AH is still widely used. All enthcdpies of vaporization cire positive, so the sign is not normally written exphdtly in tables of data. 

Substance Freezing point, Tfus/K (kJ mol ) Boiling point, Tb/K (kJ mol ) 

A thermochemical equation shows the standard enthalpy change (including the sign) that accompanies the conversion of an amount of reactant equal to its stoichiometric coefficient in the accompanying chemical equation (in this case, 1 mol H2O). If the stoichiometric coefficients in the chemical equation are multipHed through by 2, then the thermochemical equation would be written 

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Hougen et discuss the estimation of thermodynamic properties from molecular structure data. Gambill, in numerous articles to be found in the four volumes listed, deals with methods for the prediction of heat capacities of liquids and gases, enthalpies of vaporization, fusion and sublimation, critical temperature and pressure, and p, K, T data, including liquid densities. Dasent < presents methods for the estimation of the standard Gibbs energy of formation of non-existent compounds and compounds of low stability by procedures based on the use of ionic and covalent models. 

Describe the energy changes associated with sensible heat, latent heat, and chemical reaction on both a macroscopic and a molecular level. Calculate their enthalpy changes using available data such as heat capacity, enthalpies of vaporization, fusion and sublimation, and enthalpies of formation. 

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