Chemistry 7.14 - Bond energy calculations

Higher tier bond-energy calculations

A bond energy in $\mathrm{kJ\,mol^{-1}}$ tells you the energy required to break one mole of that type of bond. Bond-energy tables list positive breaking values. When products form, the same type of energy is released, so the product-bond total is subtracted.

• A negative $\Delta E$ means more energy is released than required: the reaction is exothermic.
• A positive $\Delta E$ means more energy is required than released: the reaction is endothermic.
• The unit is $\mathrm{kJ\,mol^{-1}}$ for the reaction as written.

Use this sequence every time:

1. Read the balanced equation and the displayed structures.
2. Count each bond in the reactants. A coefficient multiplies every bond in that molecule.
3. Multiply each bond count by its supplied bond energy and add the reactant total.
4. Repeat for the bonds formed in the products.
5. Calculate $E_{\text{broken}}-E_{\text{formed}}$, then give the sign and unit.
6. Use the sign and the two totals to check the exothermic/endothermic classification.

Worked example: hydrogen reacting with chlorine

Calculate the energy change for:

$\ce{H-H + Cl-Cl -> 2 H-Cl}$

1. Bonds broken in the reactants

There is one $\ce{H-H}$ bond and one $\ce{Cl-Cl}$ bond.

$E_{\text{broken}}=436+243=679\,\mathrm{kJ\,mol^{-1}}$

2. Bonds formed in the products

The coefficient 2 means two $\ce{H-Cl}$ molecules, so two $\ce{H-Cl}$ bonds form.

$E_{\text{formed}}=2\times431=862\,\mathrm{kJ\,mol^{-1}}$

3. Overall energy change

$\Delta E=E_{\text{broken}}-E_{\text{formed}}$

$\Delta E = 679 - 862 = -183\,\mathrm{kJ\,mol^{-1}}$

4. Interpret and sense-check