Enthalpy

• Enthalpy: the stored chemical potential energy in a system/stored thermal energy
• can't measure enthalpy, so we measure the change in it
• H = Enthalpy = Ep
• Change in Enthalpy: the potential chemical energy change as a reaction proceeds from reactants to products. The change is a result of the bonds and IMF's breaking and forming
• System: the chemical reaction
• Surroundings: water or air surrounding the system
• Heat and energy flows between the system and surroundings
• Endothermic reaction: energy is absorbed
• Exothermic reaction: energy is released
• Endothermic:

  • feels cold
  • energy went into system so temperature decreases. Kinetic energy decreases, and the net energy in the system (Ep) increases
• surroundings = system

  • equal and opposite
• in an endothermic potential energy diagram, the reactants are more stable than the products. H is always positive
• Exothermic:

  • Feels hot
  • energy came out of the system, temperature increases. Kinetic energy increases, and net energy in system (Ep) decreases
• in an exothermic potential energy diagram, the products are more stable than the reactants. H is a negative value.
• 4 ways of communicating enthalpy changes:

  1. molar enthalpies of reaction
  2. Enthalpy changes
  3. energy terms in BCR
  4. Ep Diagram
• Exo: ∆rHm < 0

  Endo: ∆rHm > 0
• exo: reactants -> products

  ∆rH < 0
• endo: reactants -> products

  ∆rHm > 0
• Exo: reactants -> products + Energy
• Endo: reactants + Energy -> products
• Exo: Ep(reactants) > Ep(products)

  Endo: Ep(reactants) < Ep(products)