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. 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. 2. Step 2: Balance

• Formula:  (on RFS)
  • Use chart → add up all of heats of formation of products – all the heats of formation of reactants