Question

Calculate the energy change for the reaction

Answer 1

$\text{Enthalpy Change} \; \Delta H= E\text{(Bonds Broken)} - E\text{(Bonds Formed)}$

• Bonds broken: $1 \times \text{H-H}$ and $1 \times \text{Cl-Cl}$;
• Bonds formed: $2\times \text{H-Cl}$

$E(\text{Bonds Broken}) = 436 + 242 = 678 \; \text{kJ} \cdot \text{mol}^{-1}$

$E(\text{Bonds Formed}) = 2 \times 431 = 862 \; \text{kJ} \cdot \text{mol}^{-1}$

Therefore

$\Delta H = 678 - 862 = -184\; \text{kJ} \cdot \text{mol}^{-1} \; \text{Reaction}$

Answer 2

The change in energy is the energy after the reaction - the energy before the reaction. We can easily find these two energies because of the table above. The energy before the reaction is the energy of one H2 molecule + the energy of one Cl2 molecule. The energy after the reaction is the energy of TWO HCl molecules because you get 2 molecules after the reaction. The total energy change will then be (862-436-242=) 186kJ