Bond Energies and Bond Lengths

Every chemical reaction is associated with a change in enthalpy, which helps to determine whether energy is released or required during the reaction. This enthalpy change can be estimated using average bond energies.

The energy required to break a specific bond in 1 mole of a gaseous chemical compound is called bond energy and is expressed in kJ/mol. This energy depends on the type of bonded atoms and the number of shared pairs of electrons.

Bond energy is usually expressed as an average of bond energies of the same bond in different compounds. The average bond energy can be used to determine whether a chemical reaction is exothermic or endothermic.

Consider ethene and bromine reacting to form 1,2-dibromoethane. Initially, the carbon double bond and the bromine single bond break — a process requiring energy input, which increases the potential energy of atoms. Therefore, bond-breaking is an endothermic process with a positive change in enthalpy.

Subsequently, new bonds are formed between carbon and bromine atoms yielding the product. Bond formation increases the stability of the molecule by reducing the potential energy. Therefore, bond formation is an exothermic process, causing a negative change in enthalpy.

According to Hess’s law, the sum of enthalpy changes of reactants and products is equal to the overall enthalpy change of the reaction. The enthalpy change of reactants is the sum of enthalpy of bonds broken, whereas the enthalpy change of products is the sum of enthalpy of new bonds formed.

Therefore, the formation of 1,2-dibromoethane, with an enthalpy of +255 kJ/mol, is an endothermic reaction.

In addition to bond enthalpies, the type of bonds and bonded atoms also influence the bond length, which is the average distance between the nuclei of two bonded atoms.

Consider the different bonds between two nitrogen atoms and two carbon atoms. Atoms with multiple bonds, such as triple bonds, are held more tightly together, leading to shorter and stronger bonds. Consequently, the molecule is more stable and requires higher energy to dissociate.

Generally, the bond strength is indirectly proportional to the bond length, with some exceptions.