Standard State

  • Standard enthalpy (ΔH): 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)

       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