Thermodynamics and Enthalpy Lecture

Learning Outcomes

• State the first law of thermodynamics.
• Define enthalpy and explain why it's a state function.
• Write thermochemical equations using stoichiometry.
• Calculate enthalpy changes for various chemical reactions.
• Explain Hess’s Law and use it to compute enthalpies.
• Apply enthalpy to calorimetry experiments.

First Law of Thermodynamics

• Total energy of an isolated system is constant.
  • Energy can be transferred but not created or destroyed.
• Internal Energy (U):
  • Calculated as the sum of heat plus work of the system.
  • Heat can be added or removed, work can be done on or by the system.
• Conservation of Energy:
  • Energy can only be transferred.
  • Internal energy involves only heat and work.

State Functions

• U (Internal Energy) is a state function:
  • Depends only on initial and final states, not the path.
  • Example: Driving to a destination where only start and end points matter.
• Q (Heat) and W (Work) are not state functions:
  • Depend on the path taken.

Enthalpy (H)

• Definition: Enthalpy is the internal energy plus a PV (pressure-volume) work component.
• PV Work:
  • Represented by PΔV, where work equals PΔV (negative of work).
• Relation to Internal Energy:
  • ΔU = Q + W
  • ΔH = ΔU + PΔV
• At Constant Pressure:
  • ΔH = Q (when W - W = 0)
  • Relevant to experiments like bomb and coffee cup calorimetry.

Calorimetry

• Types of Calorimeters:
  • Coffee cup calorimeter: Used under constant pressure.
  • Bomb calorimeter: Used under constant volume.
• Calculating Heat Exchange:
  • Q of the reaction equals the enthalpy change of the reaction in calorimetry.