Bond Enthalpies

Chemistry

Bond Enthalpies

Introduction:

• Bond enthalpies provide valuable information about the energy changes in chemical reactions.
• Understanding bond breaking and making is crucial in thermochemistry.

Bond Breaking and Bond Making

• Breaking a covalent bond requires energy, making it an endothermic process.
• Making new chemical bonds releases energy, making it an exothermic process.
• The energy required to break or make the same chemical bond is the same.

Mean Bond Enthalpies

• Mean bond enthalpies are found in data books.
• They represent an average energy value because bond enthalpies may vary slightly in different molecules.
• Mean bond enthalpies are typically quoted for bonds in the gaseous state.

Example

• Let's consider the carbon-carbon single bond (C-C) as an example:
  • The energy needed to break one mole of C-C bonds in ethane (C₂H₆) is different from that in decane (C₁₀H₂₂).
• Data books provide mean bond enthalpies to estimate the energy required to break or make specific bonds.

Calculating Enthalpy Change

• Bond enthalpies can be used to estimate the enthalpy change in a reaction.
• The enthalpy change (ΔH) for a reaction is the sum of the bond energies broken minus the sum of the bond energies formed:
  • ΔH = (Energy required to break bonds) - (Energy released when bonds are formed)
• Bond enthalpies help approximate this calculation, providing insights into the energy changes in chemical reactions.

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Bond Enthalpies

Conclusion

• Bond enthalpies represent the energy required to break or make specific chemical bonds.
• They are valuable for estimating enthalpy changes in reactions.
• Bond enthalpies are typically given as mean values for bonds in the gaseous state, allowing chemists to make useful approximations in thermochemical calculations.