4 Thermochemistry

4 Thermochemistry - Thermochemistry Chapter 2 of Atkins The...

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Thermochemistry Sections 2.7-2.9 of Atkins Standard Enthalpy Changes Enthalpies of Physical Change Enthalpies of Chemical Change Hess’ Law Standard Enthalpies of Formation Changes Reaction Enthalpy & Enthalpy of Formation Group Contributions Temperature Dependence of Reaction Enthalpies Chapter 2 of Atkins: The First Law: Concepts
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Thermochemistry Thermochemistry is the branch of thermodynamics which studies heats of reaction: heat produced by or required for a chemical reaction. In thermochemistry, chemical reactions are divided into two categories: exothermic reaction q rxn < 0; heat is produced by the reacting system (i.e., the temperature of the system is higher right after the reaction than initially; heat must be transferred from the system to the surroundings in order to return the system to its initial temperature) endothermic reaction q rxn > 0; heat is absorbed by the reacting system (i.e., the temperature of the system is lower right after the reaction than initially; heat must be transferred from the surroundings to the system in order to return the system to its initial temperature) These reactions can also be described in terms of enthalpy , ) H
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Standard Enthalpy Changes Numerical values describing ) U or ) H of a system when some chemical or physical change occurs depend upon the nature of the reaction, as well as the physical states of reactants and products It is useful to define a standard enthalpy change , ) H o , which is the change in enthalpy for a process (chemical or physical) where initial and final substances are in standard states standard states : pure form of a substance at specified temperature at pressure of 1 bar, e.g. standard state of liquid water at 298 K is pure liquid water at 298 K and 1 bar Examples: The standard enthalpy of vapourization , ) vap H o , is change in enthalpy per mole when pure liquid vapourizes to pure gas at pressure of 1 bar. The standard enthalpy of fusion , ) fus H o , is enthalpy change accompanying change from liquid to solid H 2 O(l) 6 H 2 O(g) ) vap H o (373 K) = +40.66 kJ mol -1 ) H o can be reported at any temperature, convention is at 298.15 K
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