6 - Enthalpy

6 - Enthalpy - Enthalpy Calorimetry of Chemistry Reaction...

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Click to edit Master subtitle style Enthalpy Calorimetry of Chemistry
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Reaction Energies In our earlier discussions of calorimetry, we used  physical  sources of heat (hot metal slug).  It is  also possible to use  chemical  sources of heat (like  hot packs and cold packs). The energy change associated with a chemical  reaction is called the  enthalpy of reaction  and  abbreviated ΔH.
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Enthalpy of Reactions There are actually a number of different types of  enthalpies because enthalpy depends on  conditions.  THEY ARE ALL JUST SPECIFIC  TYPES OF A GENERAL CONCEPT CALLED  “ENTHALPY”. ΔH = Hfinal - Hinitial
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General Reaction Scheme Reaction Coordinate Ener gy Reactant s Product s E a Δ H
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Reaction Coordinate The “reaction coordinate” is actually a complicated to determine, but  easy to understand. The actual energy profile of a reaction is a multi-dimensional curve with  lots of different paths from reactants to products. The  reaction coordinate  is simply the most common path that  averages all of the different parameters: bond length, bond angle,  collision frequency, etc. 
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Exothermic Reaction – “hot pack” Reaction Coordinate Ener gy Reactant s Product s E a Δ H
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Endothermic Reaction – “cold pack” Reaction Coordinate Energy Reactant s Products E a ΔH
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Where does the Energy go? In the case of a chemical reaction, you need to  keep the different types of energy separate in your  mind: Bond energy – energy INSIDE the molecules Thermal energy (heat) – kinetic energy of the  molecules Energy of the “bath” – kinetic energy of solvent or  other molecules in the system
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Energy changes ΔH represents the change in INTERNAL  MOLECULAR ENERGY. ΔH = Hfinal - Hinitial
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Exothermic Reaction – “hot pack” Reaction Coordinate Ener gy Reactant s Product s E a Δ H
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Exothermic energy changes ΔH = Hfinal – Hinitial < 0 Hinitial>Hfinal This energy is internal to the molecule. The excess gets absorbed by the rest of the  system as heat causing the molecules to move  faster (more kinetic energy) and the temperature to  increase.
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Endothermic Reaction – “cold pack” Reaction Coordinate Energy Reactant s Products E a ΔH
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Endothermic energy changes ΔH = Hfinal – Hinitial > 0 Hinitial<Hfinal This energy is internal to the molecule and must come from  somewhere. The additional energy required by the system gets  absorbed from the rest of the system as heat causing the  molecules to move slower (less kinetic energy) and the  temperature to decrease.
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Reaction
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This note was uploaded on 04/17/2008 for the course CHEM 1011.216.0 taught by Professor Joelanzafame during the Winter '07 term at RIT.

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6 - Enthalpy - Enthalpy Calorimetry of Chemistry Reaction...

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