Hesss Law and Thermochemistry

1
Hesss Law and Thermochemistry

• If a reaction is carried out in a series of
  steps, ?H for the reaction will equal the sum of
  the enthalpy changes for the individual steps.

2
Tabulated Enthalpies

• Vaporization
• Fusion
• Combustion
• Formation
• Appendix C - ?Hf?, ?Gf?, S?.

3
Enthalpies of Formation

• ?Hf is the change in enthalpy when a compound is
  formed from its elements in their standard
  states.
• E.g., standard state of oxygen is O2(g) at
  atmospheric pressure and 298K.
• ?Hf? is the change in enthalpy when both
  reactants and products are in their standard
  states.

4
Enthalpies of Formation, cont

• Stablest form of element is the reference for the
  standard enthalpy of formation.
• E.g., ?Hf? for diamond is for its formation from
  graphite, not C60.
• ?Hf? for ethanol (C2H5OH) is from O2(g), not from
  O3, and graphite (s), not diamond.