44. Bond Energies & Energy Changes

1. Key Definitions & Formulas

  • Bond Energy: The amount of energy required to break one mole of a particular covalent bond.

  • Breaking Bonds: This is an endothermic process because it requires energy from the surroundings.

  • Forming Bonds: This is an exothermic process because it releases energy to the surroundings.

  • Overall Energy Change Formula:

    Total Energy = Energy required to break bonds - Energy released by forming bonds

2. Determining Reaction Type

  • Exothermic Reaction: The energy released when forming new bonds is greater than the energy needed to break the existing bonds. This results in a negative overall energy change.

  • Endothermic Reaction: The energy needed to break existing bonds is greater than the energy released when forming new bonds. This results in a positive overall energy change.

3. Step-by-Step Calculation Guide

  1. Draw the Molecules: Use displayed formulas to see every single bond involved in the reactants and products.

  2. List the Bonds to Break: Identify all bonds in the reactant molecules and multiply their energy values by the number of bonds present.

  3. List the Bonds to Form: Identify all bonds in the product molecules and multiply their energy values by the number of bonds present.

  4. Apply the Formula: Subtract the "formed" total from the "broken" total.

4. Worked Example: Formation of Ammonia

Reaction: N₂ + 3H₂ → 2NH₃

  • Bonds to Break (Reactants):

    • 1 × N≡N triple bond

    • 3 × H-H single bonds

  • Bonds to Form (Products):

    • 6 × N-H single bonds (Each NH₃ molecule has 3 bonds; there are 2 molecules).

  • Calculation: (Energy of N≡N + 3 × Energy of H-H) - (6 × Energy of N-H)

  • Result: A negative value (e.g., -97 kJ/mol), confirming the reaction is exothermic.