Bond Enthalpies

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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.

ΔHreaction≈∑ΔHbonds broken−∑ΔHbonds formed

Practice Tips

  • Use Average Values Carefully: Remember, these values are averages and may differ slightly depending on molecular environment.

  • Focus on Bonds Changed: Only consider bonds that are broken in the reactants and formed in the products.

  • Account for All Bonds: Double-check that all bonds in each molecule are accounted for, especially in large molecules.

  • Sign Conventions: Energy required to break bonds is positive, while energy released from forming bonds is negative.

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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 ΔHreaction​:

    • Use the formula: ΔHreaction≈∑ΔHbonds broken−∑ΔHbonds formed

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

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 (CH4​) is slightly different from that in ethane (C2H6​), 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 ΔHreaction​:

    • 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.

    ΔHreaction≈∑ΔHbonds broken−∑ΔHbonds formed

Example Problem

Calculate the enthalpy change (ΔHrxn​) for the following reaction using the average bond energies:

CH4(g)+2O2(g)→CO2(g)+2H2O(g)

Average Bond Energies (kJ/mol):

  • C–H: 412

  • O=O: 498

  • C=O (in CO2​): 799

  • O–H: 463

Step-by-Step Solution:

  1. Write the formula for enthalpy of reaction using bond energies:

    ΔHrxn = Bonds Broken − Bonds Formed

  1. Identify bonds broken (reactants):

    • In CH4: 4 C–H bonds
      Energy: 4⋅412=1648 kJ

    • In 2 O2​: 2 O=O bonds
      Energy: 2⋅498=996 kJ

    Total Energy for Bonds Broken:

    1648+996=2644 kJ

  1. Identify bonds formed (products):

    • In CO2​: 2 C=O bonds
      Energy: 2⋅799=1598 kJ

    • In 2 H2O2: 4 O–H bonds
      Energy: 4⋅463=1852 kJ

    Total Energy for Bonds Formed:

    1598+1852=3450 kJ

  1. Calculate ΔHrxn​:

    Using the formula:

    ΔHrxn=Bonds Broken−Bonds Formed

    Substitute values:

    ΔHrxn=2644−3450

    Solve:

    ΔHrxn=−806 kJ

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