Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 7

Big Idea #5: Laws of Thermodynamics and Changes in Matter

BOND ENERGY

Bond energy is the energy required to break a bond. Because the breaking of a bond is an endothermic process, bond energy is always a positive number. When a bond is formed, energy equal to the bond energy is released.

H° = Σ Bond energies of bonds broken − Σ Bond energies of bonds formed

The bonds broken will be the reactant bonds, and the bonds formed will be the product bonds.

The number of bonds broken and formed is affected by both the number of that particular type of bond within a molecule, as well as how many of those molecules there are in a balanced reaction. For instance, in the equation below, there are four O-H bonds being formed because each water molecule has two O-H bonds, and there are two water molecules present.

Let’s find ∆ for the following reaction.

2H2(g) + O2(g) → 2H2O(g)

Bond

Bond Energy (kJ/mol)

H–H

436

O–O

499

O–H

463

H° = Σ Bond energies of bonds broken − Σ Bond energies of bonds formed

H° = [(2)(H–H) + (1)(O=O)] − [(4)(O–H)]

H° = [(2)(436 kJ) + (1)(499 kJ)] − [(4)(463 kJ)]

H° = (1,371 kJ) − (1,852 kJ)

H° = −481 kJ/mol