5.3 - Bond Enthalpy

• Bond Enthalpy (Bond Energy)

  The energy required to break one mole of a bond in a gaseous molecule under standard conditions, producing gaseous atoms. Bond enthalpies are average values since the energy required to break a specific bond varies with the molecule's environment.
• Use of Bond Enthalpies in Calculating Reaction Enthalpies

  The enthalpy change of a reaction ($ΔH _{rxn }​$) can be estimated by summing the bond enthalpies of bonds broken (input energy) and subtracting the bond enthalpies of bonds formed (released energy).

  Equation: $\Delta H_{\text{rxn}} =$$\Sigma \Delta H_{\text{broken}} - \Sigma \Delta H_{\text{formed}}$
• Ozone Formation Process

  Ozone ($O_3$) is formed in the stratosphere when an oxygen molecule ($O_2$​) is split by UV light into two oxygen atoms ($O$), which then each combine with another oxygen molecule to form ozone.
• Ozone Formation Equations

  $O_2 +$$\text{UV light} \rightarrow 2O$

  $O +$$O_2 \rightarrow O_3$
• Ozone Decomposition Process

  Ozone can be decomposed back into oxygen molecules and oxygen atoms by UV light or by collision with another oxygen atom.
• Ozone Decomposition Equations

  $O_3 +$$\text{UV light} \rightarrow O_2 +$$O$

  $O_3 +$$O \rightarrow 2O_2$