13ThermoP1 - THERMODYNAMICS Review of Energy and Enthalpy...

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Bojan Chem 13 THERMODYNAMICS THERMODYNAMICS Review of Energy and Enthalpy Changes (Ch. 5) Energy Changes: Heat and Work Heat = q = energy transferred due to a difference in temperature. +q means heat is added to the system Work = w = action of force through a distance (often P ! V) +w means work is done on the system Energy Changes Energy Changes ! E = E final state ! E initial state ! E (kJ/mol) O 2 (g) " 2 O atoms +498.3 Water " ice at 25 o C ! 6.0 Water + salt " salty water ! 3.9 Si (s) + O 2 (g) " SiO 2 (s) ! 908 Bojan Chem 13 STATE FUNCTIONS STATE FUNCTIONS A state function is a quantity that does not depend on the process by which the system was prepared Example: Your altitude (height above sea level) does not depend on the route you took to class this morning. State functions are written as uppercase letters (E, H, P, V, T, S…) Changes in state functions are path-independent: q and w are not state functions but ! E (= q + w) is a state function reactants products ! E 1 2
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Bojan Chem 13 THERMOCHEMISTRY LAWS OF THERMODYNAMICS 1 st Law: The total energy in the universe is constant. # E universe = 0 # E universe = # E system + # E surroundings # E system = ! # E surroundings Heat and work: ! E system = q + w and for PV work at const. pressure, ! E system = q – P ! V Bojan Chem 13 Enthalpy (H) Enthalpy (H) ! H = heat transferred at const. P If (+), endothermic (need to add heat) If ( ! ), exothermic (heat is given off) Classify as endo- or exothermic: Ice melting Water boiling Wood burning H is a state function – changes are path- independent H = E + PV (sums and products of state functions are also state functions)
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Bojan Chem 13 Standard Enthalpy of Formation Standard Enthalpy of Formation ! H o f : ! H for making a compound from elements in
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13ThermoP1 - THERMODYNAMICS Review of Energy and Enthalpy...

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