Molecular Energetics Group

One of the more commonly studied reactions in a reaction-solution calorimeter is the ligand exchange reaction. On such a reaction two species react to give another two species by changing a ligand. As an example the reaction of an alkaline metal alkoxide (MOR) with water is presented:

MOR + H2O = MOH + ROH

(1)

During this process two chemical bonds are cleaved and two other formed. Representing a bond dissociation enthalpy (DH) as the enthalpy associated with a reaction like:

ROH = RO• + H•

(2)

the enthalpy change of the first reaction (ΔrH) can be expressed as a balance of bond enthalpies cleaved and formed:

ΔrH = DH(M-OR) + DH(HO-H) - DH(RO-H) - DH(M-OH)

(3)

If three of these BDEs are known, then the other one can be calculated.