Overview
This lecture explains bond energies and how they determine whether a chemical reaction is exothermic or endothermic, using specific reaction examples and calculations.
Bond Energies: Concepts
- Bond energy is the energy required to break one mole of a specific covalent bond.
- Breaking bonds is an endothermic process because it absorbs energy from the surroundings.
- Forming bonds is an exothermic process because it releases energy to the surroundings.
- One mole refers to 6.02 × 10²³ bonds (Avogadro's constant).
Calculating Energy Changes in Reactions
- To determine if a reaction is exothermic or endothermic, compare energy needed to break bonds with energy released by forming bonds.
- The formula is: total energy to break bonds − total energy released forming bonds.
- If the result is negative, the reaction is exothermic; if positive, it is endothermic.
Example: Hydrogen and Chlorine Reaction
- Reaction: H₂ + Cl₂ → 2 HCl.
- Bonds broken: one H–H (436 kJ/mol), one Cl–Cl (242 kJ/mol).
- Bonds formed: two H–Cl (2 × 431 kJ/mol).
- Calculation: (436 + 242) − (2 × 431) = 678 − 862 = −184 kJ/mol; thus, exothermic.
Example: Nitrogen and Hydrogen Reaction
- Reaction: N₂ + 3 H₂ → 2 NH₃.
- Bonds broken: one N≡N (941 kJ/mol), three H–H (3 × 436 kJ/mol).
- Bonds formed: six N–H (6 × 391 kJ/mol).
- Calculation: (941 + 1308) − 2346 = 2249 − 2346 = −97 kJ/mol (corrected calculation from transcript).
- Negative value means the reaction is exothermic.
Key Terms & Definitions
- Bond Energy — Energy needed to break one mole of a particular covalent bond.
- Endothermic — A process that absorbs energy from its surroundings.
- Exothermic — A process that releases energy to its surroundings.
- Avogadro's Constant — 6.02 × 10²³, the number of particles in one mole.
Action Items / Next Steps
- Practice calculating energy changes for other reactions using provided bond energy tables.
- Draw displayed formulas to visualize bonds before calculating.