Lecture 24 - Bond Energies and Phase Transitions

Bond Energies

  • Bond Strength: Covalent bonds involve breaking and re-making.
  • Endothermic: +ΔH+ \Delta H (energy required to break bonds).
  • Exothermic: −ΔH- \Delta H (energy released when bonds form).
  • Bond Dissociation Energy: Energy required to break bonds of 1 mol of gaseous substance.

Selected Bond Energies (kJ/mol)

  • Examples of bond energies for various bonds (H-O, H-F, H-Si, etc.) are provided in a table.
  • Observe the patterns in bond energies; do not memorize all values.

Bonds and Enthalpy of a Reaction

  • \Delta H = \Sigma (\text{Bonds broken}) - \Sigma (\text{Bonds formed})

Ionic Bonds – Lattice Energy

  • Definition: Energy released when respective gaseous ions form one mole of ionic solid.
    +(g)+−(g)→(s)ΔH+ (g) + - (g) \rightarrow (s) \qquad \Delta H

Born-Haber Cycle

  • Combining energy as income (endothermic) and expenditure (exothermic) to establish unknown energies and overall change.

Phase Transitions

  • Heating and Cooling Curves: Illustrates phase changes with heat addition (e.g., ice to water to steam).
  • Calculations for heat needed during phase transitions involve different formulas for different stages (heating ice, melting ice, heating water, evaporating water, heating steam).

Phase Diagrams

  • Representing physical state and phase-transition temperatures graphically.