ENTHALPY

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ENTHALPY

• The change in energy (associated with heat flow)
  in a chemical reaction.
• ?H

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UNITS OF ENERGY

• One Joule the SI unit of heat. 1 J .239 cal
  1 kcal 1000 cal 4186 J.
• One calorie the quantity of heat that raises
  the temperature of 1 g of pure water by 1oC.
• One dietary calorie ? Calorie 1000 calories.

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THERMOCHEMICAL EQUATIONS

• Indicate reactants, products, and heat.
• Exothermic reactions in which heat is released
  (hreactants gt hproducts)
• Endothermic reactions in which heat is absorbed
  (hreactants lt hproducts)

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EXOTHERMIC REACTIONS

• Heat is released during the reaction
• ?hreaction ?hproducts ?hreactants lt 0
• Example
• CaO(s) H2(l) ? Ca(OH)2(s)
• ?hreaction -65.2kJ
• CaO(s) H2(l) ? Ca(OH)2(s) 65.2 kJ

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ENDOTHERMIC REACTIONS

• Heat is absorbed during the reaction
• ?hreaction ?hproducts ?hreactants gt 0
• Example
• 2NaHCO3(s) ? Na2CO3(s) H2O(g) CO2(g)
  ?hreaction 129kJ
• 2NaHCO3(s) 129kJ ? Na2CO3(s) H2O(g) CO2(g)

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Standard Heats of Formation

• ?Hfo the change in enthalpy that accompanies
  the formation of one mole of a compound from its
  elements with all substances in their standard
  states at 25oC
• Values are tabulated on page 787 in text
• Capital H when enthalpy is on a per-mole basis

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Standard Heats of Formation

• For CO2(g), ?Hfo -395 kg/mol
• This means, when one mole of CO2 gas is made from
  O2 and C, 395 the enthalpy of the reactants
  products decrease by 395 kJ.

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CHEMICAL EQUILIBRIUM

• Reversible Reactions reactions that proceed in
  both directions simultaneously
• Chemical Equilibrium condition in which both
  the forward and reverse reactions are occurring
  at the same time AND THE SAME RATE
• Equilibrium Position of a reaction is given by
  the relative concentrations of the components of
  the system at equilibrium.

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EXAMPLE OF EQUILIBRIUM

• After clicking on the link below, go to e-media
  activities listed on the screen.
• For a physical process select the applet named
  Dynamic Equilibrium
• For a chemical process select the applet named
  Concentration Versus Time
• http//wps.prenhall.com/esm_hillpetrucci_genchem_4
  /0,8603,1080784-,00.html)

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Le Châteliers Principle

• When a change (a concentration, temperature,
  pressure, or volume) is imposed on a system at
  equilibrium, the system responds by attaining a
  new equilibrium condition that minimizes the
  impact of the imposed change.

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In which direction will the equilibrium shift if

• 4NH3(g) 3O2(g) ? 2N2(g) 6H2O(l)
• ?h -1530.4 kJ
• pressure is decreased at constant temperature?
• nitrogen gas is added?
• heat is removed from the system?
• NH3 gas is removed?