CHAPTER _7_Thermochem_FALL_2006_FINAL

CHAPTER _7_Thermochem_FALL_2006_FINAL - CHAPTER 6...

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CHAPTER 6: THERMOCHEMISTRY Chemists can write equations for lots of reactions -- key questions: WILL THE REACTION GO? IF IT GOES, HOW FAST DOES IT GO? This chapter deals primarily with the first of these questions. THERMODYNAMICS  == heat in motion . .. understanding the transfer  of energy in chemical reactions THERMOCHEMISTRY  == measurement and prediction of the effects  of heat Key concept: heat is a stoichiometric quantity Important point of view: SYSTEM vs. SURROUNDINGS ****************************** TYPICAL SITUATION: Burn a mole of methane (CH 4  (g) ) . .. 890 kJ energy released as heat process is EXOTHERMIC . ..  H = -890 kJ What a surprise . .. methane is a fuel . .. a material burned to provide  HEAT.
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SOURCES OF HEAT: Buckminster Fuller -- ". .. the sunlight unwinding."         HEAT IS STOICHIOMETRIC!    Burn 2 moles? … ½ mole? … 100  grams?   How do you determine the heat of a reaction?   (1) Calculations (these are estimates )     (2) Experiments  (A) ONE WAY TO CALCULATE: (and it’s only an approximation) –  (NOTE: additional value for bonds in CO 2 in footnote (b)) a chemical reaction = the making and breaking of bonds . .. it all involves ENERGY
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bond energies can be used to approximate energies of reaction Think of it like this: reactants ==(break apart) ==> atoms ==(come together) ==> products costs energy releases energy An example:  CH 4 (g)  +  O 2  (g)     CO 2  (g)  +  2 H 2 O (g) Just tally up the energies in the bonds …  H rxn   =   Σ BE (bonds broken) –  Σ BE (bonds formed) (B) use STANDARD STATE ENTHALPIES … enthalpy (heat)  depends on temperature and pressure Convention: tabulate enthalpies for specific conditions ==  **** STANDARD STATE ****
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pressure = 1 bar         temp = 25 o C (NOTE: 1 bar is 1.01325 atm) Define: STANDARD STATE = the most stable form of a substance in  the physical state in which it exists under standard conditions EX. hydrogen? . .. gas       mercury? . .. liquid       carbon? . .. GRAPHITE (most stable of the allotropes) Define: STANDARD ENTHALPY CHANGE OF A REACTION H = the observed or calculated enthalpy change for a reaction            occurring when all reactants and products are in their            standard states under standard conditions The standard enthalpy change for a reaction involving the formation  of  one mole  of a compound directly from  elements  is called the  STANDARD MOLAR ENTHALPY OF FORMATION = 
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CHAPTER _7_Thermochem_FALL_2006_FINAL - CHAPTER 6...

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