To determine whether the reaction is exothermic or endothermic, we need to calculate the total bond energies of the reactants and the products.
Step 1: Calculate the total energy of the bonds broken (reactants)
- In the reaction:
- 4 H–C bonds: 4 × 411 kJ/mol = 1644 kJ/mol
- 4 Cl–Cl bonds: 4 × 242 kJ/mol = 968 kJ/mol
Thus, the total energy of bonds broken in the reactants is: \[ 1644 , \text{kJ/mol} + 968 , \text{kJ/mol} = 2612 , \text{kJ/mol} \]
Step 2: Calculate the total energy of the bonds formed (products)
- In the products:
- 4 C–Cl bonds: 4 × 327 kJ/mol = 1308 kJ/mol
- 4 H–Cl bonds: 4 × 427 kJ/mol = 1708 kJ/mol
Thus, the total energy of bonds formed in the products is: \[ 1308 , \text{kJ/mol} + 1708 , \text{kJ/mol} = 3016 , \text{kJ/mol} \]
Step 3: Comparison of energies
- Energy of bonds broken (reactants): 2612 kJ/mol
- Energy of bonds formed (products): 3016 kJ/mol
Step 4: Determine the overall energy change
In an exothermic reaction, the energy of the products is lower than that of the reactants, meaning that the total bond energy of the reactants is greater than that of the products. Conversely, in an endothermic reaction, the energy of the products is higher than that of the reactants.
Since we have:
- Total energy of bonds broken (reactants) = 2612 kJ/mol
- Total energy of bonds formed (products) = 3016 kJ/mol
The energy of the bonds in the products is greater than that in the reactants. This indicates that the total bond energy of the reactants is less than that of the products, meaning energy was absorbed.
Thus, the correct statement is: The reaction is endothermic because the total bond energy of the reactants is less than the total bond energy of the products.