Question

Use average bond energies to estimate Δh° for the following reaction?

Answer 1

To estimate the enthalpy change for a chemical reaction, identify the bonds broken and formed, use bond dissociation energies from a given table to calculate the respective energies, and apply the formula for enthalpy change.

Step-by-step explanation:

To estimate the Δh° or standard enthalpy change for a chemical reaction using average bond energies, we follow these steps:

1. Identify all the bonds broken in the reactants and calculate their total bond energies.
2. Identify all the bonds formed in the products and calculate their total bond energies.
3. Apply the formula ΔH = Σbond energies of broken bonds − Σbond energies of formed bonds to find the enthalpy change of the reaction.

As an example, if we have a reaction that breaks one mole of H-H bonds and forms one mole of H-Br bonds, we would use the bond dissociation energies to find the ΔH. Given that H-H bonds have an energy of 436 kJ/mol and H-Br bonds have a value of 366 kJ/mol, the calculation would be:

ΔH = (1 mole × 436 kJ/mol for H-H bond) − (1 mole × 366 kJ/mol for H-Br bond)

ΔH = 436 kJ/mol − 366 kJ/mol

ΔH = +70 kJ/mol

This means that the reaction is endothermic and requires an input of 70 kJ of energy per mole of reaction.