AP Chemistry Unit 6.8 Enthalpies of Formation

Standard State

  • Standard enthalpy (ΔH⁰): the enthalpy change at standard conditions
    • ⁰ = recorded in standard state
  • Standard State conditions:
    • For a gas: pressure is 1 atm
    • For a pure substance in a condensed state (liquid or solid) = pure liquid or solid
    • For a solution, concentration is 1 molar
    • Temp is at exactly 25 C
  • Remember: of a pure element is 0
  • Standard heats of formation: the amount of heat needed to form 1 mole of a compound from its elements in their standard states (ie. how it exists in nature → oxygen is O2)
    • Unit: kJ
    • Have to make one mole of product to meet the definition
    • SS of an element is the form in which the element exists under 1 atm and 25 C
      • Not 0 = not how the element exists in nature
  • Ex: Write the equation for the formation of methanol (CH3OH)
    1. Step 1: write the elements (in standard state)
      • Note: The elements that exist as diatomic molecules are hydrogen (H2), nitrogen (N2), fluorine (F2), oxygen (O2), iodine (I2), chlorine (Cl2) and bromine (Br2
    1. Step 2: Balance
  • Formula:  (on RFS)
    • Use chart → add up all of heats of formation of products – all the heats of formation of reactants
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