Bond Energy Calculations

• An exothermic reaction has a negative overall energy change ($\Delta H < 0$). This happens because the energy released when making new bonds is greater than the energy taken in to break the existing bonds.
• An endothermic reaction has a positive overall energy change ($\Delta H > 0$). This occurs because the energy taken in to break existing bonds is greater than the energy released when making the new bonds.

Calculating Energy Changes

We can calculate the exact energy change of a reaction using values called bond energies. A bond energy is the mean amount of energy required to break one mole of a specific covalent bond in its gaseous state.

To find the enthalpy change ($\Delta H$), which is the total energy change of the reaction, we use the stoichiometry (balancing numbers) from the chemical equation to multiply the specific bond energies. The standard unit for these calculations is kJ/mol.

The overall energy change is calculated using the following formula:

Worked Example: Formation of Hydrogen Chloride

Calculate the overall energy change for the reaction between hydrogen and chlorine and state whether the reaction is exothermic or endothermic.

Equation: $H_2 + Cl_2 \rightarrow 2HCl$

Bond energies: $H-H = 436 \text{ kJ/mol}$, $Cl-Cl = 242 \text{ kJ/mol}$, $H-Cl = 431 \text{ kJ/mol}$

Step 1: List and count all bonds broken in the reactants and calculate the total energy taken in.

• $1 \times H-H$ bond = $436 \text{ kJ/mol}$
• $1 \times Cl-Cl$ bond = $242 \text{ kJ/mol}$
• Total energy taken in = $436 + 242 = 678 \text{ kJ/mol}$

Step 2: List and count all bonds made in the products and calculate the total energy released.

• $2 \times H-Cl$ bonds = $2 \times 431 = 862 \text{ kJ/mol}$
• Total energy released = $862 \text{ kJ/mol}$

Step 3: Calculate the overall energy change using the formula.

• $\text{Total energy change} = \text{Sum of energy of bonds broken} - \text{Sum of energy of bonds formed}$
• $\text{Total energy change} = 678 - 862$
• $\text{Total energy change} = -184 \text{ kJ/mol}$

Step 4: Identify the reaction type based on the sign of the final answer.

• The sign is negative ($-$), meaning more energy is released making bonds than taken in breaking them, so this is an exothermic reaction.