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How to Find Bond Enthalpy?

Published in Chemical Bonds 3 mins read

You can find bond enthalpy by analyzing the energy changes during a chemical reaction, using the following steps:

Steps to Calculate Bond Enthalpy

Bond enthalpy calculations are estimations based on average bond energies. The actual enthalpy change of a reaction might differ slightly.

  1. Identify the Bonds Broken: Determine all the bonds that are broken in the reactants during the chemical reaction.

  2. Calculate the Total Energy Required to Break Bonds: For each bond broken, find its bond enthalpy value (usually provided in a table of average bond enthalpies). Multiply each bond enthalpy by the number of times that bond appears in the reactants. Sum up all these values. This gives you the total energy input to break the bonds. This value will be positive because energy is required to break bonds.

    • Example: If you are breaking two O-H bonds, and the bond enthalpy of an O-H bond is 463 kJ/mol, the total energy required to break the O-H bonds is 2 * 463 kJ/mol = 926 kJ/mol.

  3. Identify the Bonds Formed: Determine all the bonds that are formed in the products of the chemical reaction.

  4. Calculate the Total Energy Released When Bonds Form: Similar to step 2, find the bond enthalpy value for each bond formed and multiply it by the number of times that bond appears in the products. Sum up all these values. This gives you the total energy released when the bonds form. This value will be negative because energy is released when bonds form.

    • Example: If you are forming four C-H bonds, and the bond enthalpy of a C-H bond is 413 kJ/mol, the total energy released is - (4 * 413 kJ/mol) = -1652 kJ/mol. The negative sign indicates energy released.

  5. Calculate the Overall Enthalpy Change (ΔH): Use the following formula:

    ΔH = Σ(Bond Enthalpies of Bonds Broken) + Σ(Bond Enthalpies of Bonds Formed)

    Where:

    • ΔH is the enthalpy change of the reaction.
    • Σ(Bond Enthalpies of Bonds Broken) is the sum of the bond enthalpies of all bonds broken (a positive value).
    • Σ(Bond Enthalpies of Bonds Formed) is the sum of the bond enthalpies of all bonds formed (a negative value).

Example Calculation

Let's consider the reaction: H2(g) + Cl2(g) → 2HCl(g)

  1. Bonds Broken:

    • 1 H-H bond
    • 1 Cl-Cl bond

  2. Energy to Break Bonds:

    • H-H bond enthalpy: 436 kJ/mol
    • Cl-Cl bond enthalpy: 242 kJ/mol
    • Total energy to break bonds: 436 kJ/mol + 242 kJ/mol = 678 kJ/mol

  3. Bonds Formed:

    • 2 H-Cl bonds

  4. Energy Released When Bonds Form:

    • H-Cl bond enthalpy: 431 kJ/mol
    • Total energy released: -(2 * 431 kJ/mol) = -862 kJ/mol

  5. Overall Enthalpy Change:

    • ΔH = 678 kJ/mol + (-862 kJ/mol) = -184 kJ/mol

Therefore, the enthalpy change for the reaction H2(g) + Cl2(g) → 2HCl(g) is -184 kJ/mol. This means the reaction is exothermic (releases heat).

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