Free Science Lessons: Calculating Energy Changes Using Bond Energies

Key Concepts

• Exothermic Reactions: Energy is transferred from chemicals to surroundings. Energy change is negative.
• Endothermic Reactions: Chemicals gain energy from surroundings. Energy change is positive.
• Bond Energies:
  • Breaking bonds requires energy (endothermic).
  • Making bonds releases energy (exothermic).

Calculating Energy Changes

Steps for Calculation

1. Identify the Reaction

   • Example: Hydrogen + Chlorine → Hydrogen Chloride
   • Write the balanced symbol equation.

2. Gather Information

   • Structures of Molecules: Identify all different bonds.
   • Bond Energy Values: Given in exams for each bond.

3. Analyze the Reaction

   • Left Side (Reactants): Breaking chemical bonds requires energy (endothermic).
   • Right Side (Products): Making bonds releases energy (exothermic).

Example Calculation

• Breaking Bonds (Reactants):

  • H-H Bond: Requires 436 KJ/mol (endothermic, + sign)
  • Cl-Cl Bond: Requires 242 KJ/mol (endothermic, + sign)
  • Total Energy Needed: 678 KJ

• Making Bonds (Products):

  • H-Cl Bond: Releases 431 KJ/mol (exothermic, - sign)
  • Making 2 H-Cl Bonds: Multiply by 2 → 862 KJ
  • Total Energy Released: -862 KJ

4. Calculate Final Energy Change
   • Formula: Sum of energy changes
   • Example Calculation: 678 KJ (breaking) - 862 KJ (making) = -184 KJ
   • The result is negative, indicating an exothermic reaction.

Conclusion

• The method shows that calculating energy changes is straightforward with practice.
• The next video will provide more examples to practice.