Lecture Notes: Enthalpy as a State Function and Hess's Law

Key Concepts

• Enthalpy as a State Function

  • Enthalpy is a state function, which means it is independent of the path taken to reach a particular state.
  • This characteristic allows us to add enthalpy changes of multiple reactions together.

• Hess's Law

  • States that the total enthalpy change for a chemical reaction is the sum of all changes.
  • Useful for calculating the enthalpy change of complex reactions by breaking them down into simpler steps.

Example: Enthalpy of Formation of Iron(III) Chloride

• Objective: Calculate the enthalpy of formation for iron(III) chloride.

• Steps

  1. Write the reaction for the formation of iron(III) chloride (FeCl₃) from its elements in their standard states:
     • Iron (Fe) as a solid.
     • Chlorine (Cl₂) as a diatomic gas.
  2. Determine the amount of chlorine needed:
     • Since FeCl₃ forms, 1 mole of Fe will react with 1.5 moles of Cl₂.
  3. Represent this as the formation reaction:
     • [ \Delta H^\circ_{formation} ] is calculated from the given steps.

• Calculation Process

  • Given two fundamental steps in the reaction process:
    • The equations provided involve Fe and Cl₂ in different forms.
  • Add the two equations to achieve the final formation reaction:
    • Notice products and reactants cancel out appropriately.
    • Fe and Cl₂ react to form FeCl₃ after cancellation.

• Final Calculation of Enthalpy

  • By adding the enthalpy changes from the two given steps, the enthalpy of formation is calculated.
  • Final result: [ \Delta H^\circ = -399.5 \text{ kJ} ]_

Conclusion

• The example illustrates a straightforward application of Hess's Law.
• The key challenge was correctly writing the enthalpy of formation.
• The next discussion will focus on more complex scenarios involving Hess’s Law.