Enthalpy Changes Also Accompany Physical Transformations

This result is only an estimate because the bond enthalpy of an O—H bond is an average quantity. Alternatively, we can use Equation 7.40 and the data in Appendix 2 

Check The estimated value based on average bond enthalpies is elose, but not exactly equal, to the value calculated using A//f data. In general. Equation 7.42 works best for reactions that are either quite endothermic or quite exothermic, that is, reactions for which A// 100 kJ mol or for which AH — lOOkJ mol 

Enthalpy Changes Also Accompany Physical Transformations 

Although we have focused so far primarily on the thermal energy effects resulting from chemical reactions, many physical processes also involve the absorption or release of heat. Examples include phase transformations, such as the melting of ice or the condensation of water vapor. Enthalpy changes occur as well when a solute dissolves in a solvent or when a solntion is diluted. 

Vaporization is the process by which a liquid transforms into a vapor (gas). The molar enthalpy (or heat) of vaporization (A//vap) is the energy required to vaporize 1 mole 

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Enthalpy Changes Also Accompany Physical Transformations 405... 

Enthalpy changes also accompany physical transformations such as vaporization, freezing, and sublimation. 

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