To determine whether the reaction is exothermic or endothermic, we will need to calculate the total bond energies for both the reactants and the products.
For the reactants:
- The bonds are:
- 4 H–C bonds (411 kJ/mol each)
- 4 Cl–Cl bonds (not directly involved in the reaction)
- 4 Cl–H bonds (not in the reactants)
Since only H–C and Cl–Cl present in the reactants will be broken:
Total bond energy of reactants = (4 H–C bonds × 411 kJ/mol) + (4 Cl–Cl bonds (not broken))
Total bond energy of 4 H–C bonds = 4 × 411 kJ/mol = 1644 kJ/mol
For the products:
- The bonds are:
- 4 C–Cl bonds
- 4 H–Cl bonds
Total bond energy of products = (4 C–Cl bonds × 327 kJ/mol) + (4 H–Cl bonds × 427 kJ/mol)
Total bond energy of products = (4 × 327 kJ/mol) + (4 × 427 kJ/mol) = 1308 kJ/mol + 1708 kJ/mol = 3016 kJ/mol
Now compare the total bond energies of reactants and products:
- Reactants: 1644 kJ/mol (only considering broken bonds)
- Products: 3016 kJ/mol (only considering formed bonds)
Since the total energy of the bonds formed in the products (3016 kJ) is greater than the energy of the bonds broken in the reactants (1644 kJ), the reaction requires energy, indicating that the reaction is endothermic.
Thus the correct answer is:
The reaction is endothermic because the total bond energy of the reactants is less than the total bond energy of the products.