Hesse's Law and Hesse Cycles

Understanding Enthalpy Change

• Definition: Amount of heat energy released or absorbed per mole of substance during a physical or chemical change.
  • Exothermic process: Gives out heat, increasing the temperature of surroundings.
  • Endothermic process: Takes in heat, decreasing the temperature of surroundings.
• Types of Enthalpy Changes:
  • Formation: Energy change when 1 mole of a compound is formed from its elements in standard states.
  • Combustion: Energy change when 1 mole of a substance is completely burnt in oxygen.
  • Bond Enthalpy: Energy change when 1 mole of gaseous covalent bonds is broken.

Hess's Law

• Statement: The total enthalpy change during a chemical change is independent of the path taken.

Hesse Cycles

• Visual representation of Hess's Law.
• Used to calculate unknown enthalpy changes, commonly using:
  • Formation enthalpies
  • Combustion enthalpies
  • Bond enthalpies

Examples of Using Hesse Cycles

Example 1: Using Formation Enthalpies

• Calculate enthalpy of combustion for methane given formation enthalpies.

Methodology for Hesse Cycles

• Find two pathways that follow the direction of the arrows to simplify calculations and avoid sign errors.

Example 2: Using Combustion Enthalpies

• Calculate enthalpy of formation for benzene given combustion enthalpies.

Example 3: Using Bond Enthalpies

• Calculate enthalpy of combustion for ethanol using bond enthalpies.

Summary and Tips

• Minimize sign error risk by choosing pathways that follow arrow directions.
• Simplifies calculations, making Hesse cycles easier to solve.

Conclusion

• Using Hesse cycles strategically can greatly simplify the process of calculating unknown enthalpy changes.