AP Chemistry 6.7: Bond Enthalpy

Practice AP Chemistry questions on estimating reaction enthalpy from average bond energies by comparing the energy of bonds broken with bonds formed.

Exam points

Explain how redox reactions transfer electrons and change oxidation states in chemical systems.
Apply half-reactions, oxidation numbers and cell potentials to analyze electrochemical processes.

[Maximum number: 1]

Answer the following questions relating to the chemistry of the halogens.

Calculate the bond energy of the Br-Cl bond, in kJ/mol, using $\Delta H^{\circ}$ for the reaction $\left(1.6 \mathrm{~kJ} / \mathrm{mol}_{\text {rxn }}\right)$ and the information in the following table.

Table for Question 6.7.A—Calculate the enthalpy change of a reaction based on the average bond energies of bonds broken and formed in the… question 1 — AP Chemistry

\begin{aligned} & \Delta H^{\circ}=\sum(\text { bond energies })_{\text {broken }}-\sum(\text { bond energies })_{\text {formed }} \\ & 1.6 \mathrm{~kJ} / \mathrm{mol}=2(\mathrm{Br}-\mathrm{Cl} \text { bond energy })-(193 \mathrm{~kJ} / \mathrm{mol}+243 \mathrm{~kJ} / \mathrm{mol}) \\ & (436+1.6) \mathrm{kJ} / \mathrm{mol}=2(\mathrm{Br}-\mathrm{Cl} \text { bond energy }) \\ & \mathrm{Br}-\mathrm{Cl} \text { bond energy }=219 \mathrm{~kJ} / \mathrm{mol} \end{aligned}

1 point is earned for a correct calculation of the Br-Cl bond energy.

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6.7.A—Calculate the enthalpy change of a reaction based on the average bond energies of bonds broken and formed in the… question 1