Ionization Reactions

• Formation of ions is seen throughout chemistry

• The enthalpy change to remove a single electron at \(0K\) from a species is defined as the ionization potential
  \[{X(g)}\rightarrow X^+{(g)}+e^-\]

• The removal of subsequent electros requires energies called the 2\(^{nd}\) ionization potential, 3\(^{rd}\) ionization potential, etc.

• Electron affinity is the enthalpy associated with the formation of negative ions \[{X(g)}+e^-\rightarrow X^-{(g)}\] with \(-\Delta {H(0K)}\equiv 1^{st}\) electron affinity (\(EA\))

Example:

• \(\Delta H_1 : M(g) \rightarrow M^+(g) + e^-\) \(\leftarrow 1^{st}\) Ionization Potential
• \(\Delta H_2 : M^+(g) \rightarrow M^{2^+}(g) + e^-\) \(\leftarrow 2^{nd}\) Ionization Potential

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Average Bond Enthalpies

• Without standard formation enthalpies, we can estimate reaction enthalpies using average bond enthalpies - though this is not perfect
• A bond dissociation energy \(D\) is defined as \[ \begin{align} XY(g) \rightarrow X(g)+Y(g) & & \Delta H \equiv D(X-Y) \end{align} \]
• Using this we can estimate the reaction enthalpy \[\Delta H_{rxn}=\sum{(bonds \; broken)} - \sum{(bonds \; formed)}\] \[\;\]

Example 3.12

Estimate the reaction enthalpy for the combustion of ethanol \[C_2H_5OH(g)+O_2(g) \rightarrow 2CO_2(g)+H_2O(g)\] using

Bond	Average Bond Energy (kJ/mol)
C-H	413
C-C	348
C-O	358
O=O	495
C=O	799
O-H	463