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How do you calculate bond enthalpy?

Published in Chemical Enthalpy 3 mins read

Bond enthalpy is calculated using Hess's Law and bond dissociation energies to estimate the enthalpy change of a reaction. Here's a breakdown of the process:

Understanding Bond Enthalpy

Bond enthalpy (also known as bond dissociation energy) is the amount of energy required to break one mole of a particular bond in the gaseous phase. It's always a positive value because energy is absorbed to break a bond (endothermic process).

Steps to Calculate Enthalpy Change Using Bond Enthalpies

  1. Identify all the bonds broken in the reactants and all the bonds formed in the products. This often involves drawing Lewis structures to visualize the bonds.

  2. Determine the bond enthalpies for each type of bond. Bond enthalpy values are typically provided in tables. Keep in mind that these are average values.

  3. Calculate the total energy required to break all the bonds in the reactants. This involves multiplying the bond enthalpy of each bond by the number of those bonds present in the reactants and their stoichiometric coefficients.

    Total energy to break reactant bonds = Σ (number of bonds broken bond enthalpy of each broken bond stoichiometric coefficient of the reactant)

  4. Calculate the total energy released when all the bonds are formed in the products. This is a negative value because energy is released when bonds are formed (exothermic process). Multiply the bond enthalpy of each bond by the number of those bonds present in the products and their stoichiometric coefficients, and then multiply the result by -1 (to signify energy release).

    Total energy released from product bonds = - Σ (number of bonds formed bond enthalpy of each formed bond stoichiometric coefficient of the product)

  5. Apply Hess's Law. The enthalpy change of the reaction (ΔHrxn) is approximately equal to the sum of the energy required to break the bonds in the reactants plus the energy released when the bonds are formed in the products.

    ΔHrxn ≈ Σ (bond enthalpies of bonds broken) - Σ (bond enthalpies of bonds formed)

    Alternatively:

    ΔHrxn ≈ Σ (bond enthalpies of reactants) - Σ (bond enthalpies of products)

Example

Let's estimate the enthalpy change for the reaction:

H2(g) + Cl2(g) → 2HCl(g)

  1. Bonds broken: 1 H-H bond in H2 and 1 Cl-Cl bond in Cl2

  2. Bonds formed: 2 H-Cl bonds in 2HCl

  3. Bond enthalpies (approximate values):

    • H-H: 436 kJ/mol
    • Cl-Cl: 242 kJ/mol
    • H-Cl: 431 kJ/mol

  4. Calculation:

    • Energy to break bonds (reactants): (1 436 kJ/mol) + (1 242 kJ/mol) = 678 kJ/mol
    • Energy released upon bond formation (products): - (2 * 431 kJ/mol) = -862 kJ/mol
    • ΔHrxn ≈ 678 kJ/mol - 862 kJ/mol = -184 kJ/mol

Therefore, the estimated enthalpy change for the reaction is -184 kJ/mol.

Important Considerations

  • Average Values: Bond enthalpy values are average values, meaning they can vary slightly depending on the specific molecule. Therefore, calculations using bond enthalpies provide an estimate of the enthalpy change, not an exact value. Heats of formation provide more accurate values.
  • Gaseous Phase: Bond enthalpies are defined for substances in the gaseous phase. If reactants or products are in a different phase, additional energy considerations (e.g., heat of vaporization) are needed.

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