KEY2CHEM

Bond Energies

During a chemical reaction, bonds are broken and formed. These changes alter the potential energy of the system. Bond breaking requires input of energy to break up electrostatic interactions, while bond formation releases energy. This is carried out through energy transfer with the surroundings. Bond energies are tabulated as the energy required to break the bond, so they are positive values. The overall heat of reaction can be calculated using tabulated bond energy ($$BE$$) values

$$\\\Delta H_{rxn} = \sum BE_{reactants} - \sum BE_{products}\\$$

This is analogous to using heats of formation but bond energies are defined as positive quantities; since the products bonds are formed (which releases energy), these values are subtracted from the reactant values.

Example 1.

Using bond energies, determine the $$\Delta H_{rxn}$$

$$\require{mhchem}\ce{H2 + Cl2 -> 2 HCl}$$

\begin{align}BE_{H-H} &= 432 \text{ kJ/mol}\\ BE_{Cl-Cl} &= 240\text{ kJ/mol}\\ BE_{H-Cl} &= 428 \text{ kJ/mol}\end{align}

A. $$184\text{ kJ/mol}$$

B. $$-184\text{ kJ/mol}$$

C. $$244\text{ kJ/mol}$$

Solution

B. $$-184\text{ kJ/mol}$$

\begin{align}\Delta H_{rxn} &= \sum BE_{reactants} - \sum BE_{products}\\\Delta H_{rxn} &= [(1\text{ mol H}_2)(432\text{ kJ/mol} + (1\text{ mol Cl}_2)(240\text{ kJ/mol}) ] - [(2\text{ mol H-Cl})(428\text{ kJ/mol}]\\ \Delta H_{rxn} &= -184\text{ kJ/mol}\end{align}

Example 2.

Based on bond energies, which bond is the shortest?

A. $$\require{mhchem}\ce{N-N}$$$$BE = 167 \text{ kJ/mol}$$

B. $$\require{mhchem}\ce{N=N}$$$$BE = 418 \text{ kJ/mol}$$

C. $$\require{mhchem}\ce{N}\equiv{\text{N}}$$$$BE = 942 \text{ kJ/mol}$$

Solution

C. $$\require{mhchem}\ce{N}\equiv{\text{N}}$$$$BE = 942\text{ kJ/mol}$$

Generally, stronger bonds are shorter bonds due to the increased electrostatic attraction holding the atom pair together.

Example 3.

Bond breaking is __________ and bond formation is _____________.

A. endothermic, endothermic

B. exothermic, endothermic

C. endothermic, exothermic

Solution

C. endothermic, exothermic

Bond breaking is endothermic (requires input of energy) and bond formation is exothermic (releases energy).