Core Concept

Bond enthalpy (or bond dissociation energy) is the amount of energy required to break one mole of a particular type of bond in the gas phase. Average bond enthalpies are used to estimate the enthalpy change of a reaction by considering the energy required to break bonds in the reactants and the energy released when new bonds form in the products.

Key Concepts

1. Bond Enthalpy (Bond Dissociation Energy):

   • The amount of energy needed to break 1 mole of bonds in a gaseous substance.

   • Represented in kJ/mol.

   • Always positive because energy is required to break bonds.

2. Average Bond Enthalpy:

   • Bond enthalpies can vary depending on the molecular environment, so the average bond enthalpy is used as an approximation.

   • For example, the C–H bond enthalpy in methane ($\text{CH}_4$​) is slightly different from that in ethane ($\text{C}_2\text{H}_6$​), so an average value is used for general calculations.

3. Bond Breaking and Bond Formation:

   • Bond Breaking: Endothermic process (ΔH > 0), energy is absorbed.

   • Bond Formation: Exothermic process (ΔH < 0), energy is released.

4. Using Bond Enthalpies to Estimate $\Delta H_{\text{reaction}}$​:

   • The enthalpy change of a reaction can be estimated by subtracting the total bond energies of the bonds formed from the total bond energies of the bonds broken.

   $\Delta H_{\text{reaction}} \approx \sum \Delta H_{\text{bonds broken}} - \sum \Delta H_{\text{bonds formed}}$

Steps for Calculating Reaction Enthalpy Using Bond Enthalpies

1. Write the Balanced Equation:

   • Write the balanced chemical equation for the reaction to identify all bonds that will be broken and formed.

2. List Bonds Broken and Formed:

   • Identify and list all the bonds in the reactants that will be broken.

   • Identify and list all the bonds in the products that will be formed.

3. Find Average Bond Enthalpies:

   • Use a bond enthalpy table to find the average bond enthalpies for each type of bond involved.

4. Calculate the Total Energy for Bonds Broken and Formed:

   • Multiply each bond enthalpy by the number of that bond type in the molecule, and sum for all bonds broken and all bonds formed.

5. Calculate $\Delta H_{\text{reaction}}$​:

   • Use the formula: $\Delta H_{\text{reaction}} \approx \sum \Delta H_{\text{bonds broken}} - \sum \Delta H_{\text{bonds formed}}$

   • This gives an estimate of the enthalpy change for the reaction.