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Bond Enthalpies

Bond Enthalpies. Unit 03 - Thermodynamics. Bond Enthalpy. Bond Enthalpy is the amount of potential energy stored in the chemical bonds between two atoms in a molecule . These values can be found on an ‘Average Bond Enthalpy’ reference sheet. Bond Enthalpy.

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Bond Enthalpies

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  1. Bond Enthalpies Unit 03 - Thermodynamics

  2. Bond Enthalpy • Bond Enthalpy is the amount of potential energy stored in the chemical bonds between two atoms in a molecule. • These values can be found on an ‘Average Bond Enthalpy’ reference sheet.

  3. Bond Enthalpy • Recall that when a chemical reaction takes place, there is always an energy change. • This energy change is equal to the difference between the energy needed to break the bonds in the reactants and the energy released when bonds are formed in the products. • Therefore, ΔH = Σ E(bonds broken in reactants) – ΣE(bonds formed in products)

  4. Bond Enthalpy

  5. Bond Enthalpy ΔH = Σ E(bonds broken in reactants) – ΣE(bonds formed in products) • This is similar to Hess’s Law equation, but note that it is reactants - products, NOT products - reactants. • So, if we know how much energy is stored in the bonds, we can calculate the energy change (or the heat of the reaction).

  6. Using Lewis Structures • To do this section, we need to know how molecules are bonded together. • Therefore, Lewis structures may be required before we do any calculations. • A quick recap of Lewis structures (or electron dot diagrams): • A Lewis structure shows how the valence shells of atoms in a molecule are either transferred or shared to form a bond.

  7. Using Lewis Structures • Ex) Draw the Lewis structure for the following compounds: • a) H2O • b) CO2 • c) O2 • d) N2

  8. Steps for Bond Enthalpy Calculations • 1. Know the types of bonds within all molecules present. This will require a Lewis structure. • 2. For each unique bond in a molecule, multiply the bond enthalpy by the number of bonds in the molecule for both the reactants and products. • 3. Multiply each value from step two by the respective balancing coefficient. • 4. Use the equation in this section. Remember that the equation is reactants – products for bond enthalpies. • Note: These values are only approximate because we are using average bond enthalpies.

  9. Example 1: • Using bond enthalpies, calculate the heat of reaction for: ½ H2(g) + ½ Cl2(g) HCl(l)

  10. Example 2: • Using bond enthalpies, calculate ΔH for the following: C2H4(g) + H2(g)  C2H6(g)

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