Bond enthalpy uses bond energy values to find the change in enthalpy of a chemical reaction (∆Hᵣₓₙ). If you need a refresher on bond energy, look back at the “Bond Information” section.
Recall that breaking bonds is an endothermic process. The bonds absorb energy until they are too weak to hold the atoms together.
On the other hand, forming bonds is an exothermic process. When atoms form bonds, they become more stable. When atoms become stable, they release energy.
So, in a chemical reaction, energy is absorbed to break the bonds of the reactants, then released to form the new bonds of the products. To find the total change of enthalpy (∆Hᵣₓₙ) of a chemical process, we use the equation ↴
∆Hᵣₓₙ = Σ(Reactants) – Σ(Products)
– Σ(Reactants) is the sum of the bond energies of the reactants
– Σ(Products) is the sum of the bond energies of the products
In the “Thermochemistry” unit, we calculated ∆Hᵣₓₙ using standard enthalpy of formation values (∆H˚). With standard enthalpy of formation, we did Σproducts – Σreactants. However, for bond enthalpies, we do Σreactants – Σproducts.
Remember, breaking the bonds of the reactants is endothermic. The system gains energy, so Σ(reactants) is positive in the equation. Forming the products is exothermic, so the system loses energy. This is why we subtract Σ(products).
Similar to enthalpy of formation problems, you will be given a table of bond energy values.
Let’s do an example problem with bond enthalpy ↴
Example Problem
Use bond enthalpy values to find ∆H˚ᵣₓₙ of the following reaction (in kJ/mol):
2CO + O₂ –> 2CO₂
| Bond | Energy (kJ/mol) |
|---|---|
| C – O | 358 |
| C = O | 745 |
| C ≡ O | 1,070 |
| O – O | 204 |
| O = O | 498 |
Answer
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