1. The reaction is N₂ + 3H₂ → 2NH₃.
- Bonds broken: 1 N≡N bond (941 kJ/mol) and 3 H-H bonds (3 × 436 = 1308 kJ/mol). Total energy absorbed = 941 + 1308 = 2249 kJ/mol.
- Bonds formed: 6 N-H bonds (6 × 391 = 2346 kJ/mol). Total energy released = 2346 kJ/mol.
- Energy change (ΔH) = energy absorbed - energy released = 2249 - 2346 = -97 kJ/mol.
2. The reaction is H₂(g) + Cl₂(g) → 2HCl(g).
- Bonds broken: 1 H-H bond (436 kJ/mol) and 1 Cl-Cl bond (242 kJ/mol). Total energy absorbed = 436 + 242 = 678 kJ/mol.
- Bonds formed: 2 H-Cl bonds (2 × 431 = 862 kJ/mol). Total energy released = 862 kJ/mol.
- Energy change (ΔH) = 678 - 862 = -184 kJ/mol.
3. The reaction is C₂H₄(g) + F₂(g) → C₂H₄F₂(g).
- Bonds broken: 1 C=C bond (614 kJ/mol) and 1 F-F bond (155 kJ/mol). Total energy absorbed = 614 + 155 = 769 kJ/mol.
- Bonds formed: 2 C-F bonds (2 × 485 = 970 kJ/mol). Total energy released = 970 kJ/mol.
- Energy change (ΔH) = 769 - 970 = -201 kJ/mol.
- What you notice: In all three reactions, the energy change is negative, meaning they are exothermic. This occurs because the total energy required to break the bonds in the reactants is less than the total energy released when new bonds form in the products.
Parent Tip: Review the logic above to help your child master the concept of bond energy worksheet.
