1. Lesson # 3: Bond Energies
Thermochemistry
Lesson # 3:
Bond Energies

2. Measuring Bond Energies
If you want to know how much energy is absorbed or released in chemical reactions, you can estimate it using the energy associated with the chemical bonds of both reactants and products.
Bond dissociation energy is the quantity of energy required to break a chemical bond.
For example, a single C-H covalent bond has an average bond energy of 413 kJ/mol. This means it takes 413 kJ of energy to break 1 mol of C-H bonds in 1 mol of C atoms and 1 mol of H atoms.
Conversely, when C-H bonds form, 413 kJ of energy is released.

3. Bond Lengths & Energies
Multiple bonds (double and triple) have larger bond energies than single bonds.
This is because the bond length in multiple bonds is shorter, meaning the atoms are closer together.
This means that multiple bonds are stronger than single bonds.

4. Enthalpy & Bond Energies
For bonds to be broken, energy must be added – an endothermic process. The enthalpy is positive.
Making new bonds releases energy – an exothermic process. The enthalpy is negative.
The enthalpy change for a reaction is ΔH = Σ n x D (bonds broken) – Σ n x D (bonds formed)
Σ = sum of
n = moles
D = bond energy per mol of bonds

5. Example 1
Calculate the enthalpy change for the reaction in which hydrogen gas is combined with fluorine gas to produce hydrogen fluoride gas.

6. Example 2
Calculate the enthalpy change for the reaction in which methane gas is combined with chlorine gas and fluorine gas to produce Freon-12 gas (CF2Cl2).