Enthalpy Change of Formation and Allotropes of Carbon

Key Definitions

• Allotropes: Different forms of the same element.
• Enthalpy Change of Formation: The enthalpy change when one mole of a substance is produced from its elements in their standard state at standard ambient temperature and pressure.
  • Standard state of carbon: Graphite

Enthalpy Change for Carbon Allotropes

• Graphite:

  • Standard state of carbon
  • Enthalpy change of formation is zero when graphite is formed from itself (no energy change).

• Diamond and Buckminsterfullerene (C60):

  • Endothermic reactions
  • More energy is required to break the bonds than is released when forming them.
  • Endothermic Characteristics:
    • Products have higher energy than reactants.
    • Higher energy products are less stable.
  • Stability:
    • Graphite is more stable (lower energy).
    • Diamond and Buckminsterfullerene are less stable (higher energy).

Specifics of Instability

• Buckminsterfullerene (C60):

  • Carbon atoms have SP2 hybridization.
  • Spherical shape causes bond strain.
  • Bond angle is less than 120°, causing bonded electrons to be closer together.
  • Requires significant energy to overcome repulsive forces.

• Diamond:

  • Different number and type of bonds, along with different atomic packing.
  • Requires a small amount of energy to form from graphite.
  • Over time, at high temperatures, diamond can convert back to graphite.

Summary

• Graphite is the most stable form of carbon under standard conditions.
• Diamond and Buckminsterfullerene require energy input to form and are less stable due to their structural differences and bond strain.
• The reaction of converting graphite to diamond or C60 is endothermic due to the higher energy of the products.
• Over time, or under certain conditions, less stable forms may revert to more stable ones (e.g., diamond to graphite).