Factors Affecting the Position of an Equilibrium

Thus we can understand easily enough that the reaction between bromine and ethylene giving dibromoethane is exothermic—it replaces one n bond (C=C) with two a bonds (C—Br) at the expense of a weak a bond (Br—Br). However, it is not always obvious how strong the bonds will be when one molecule combines with another to form a single new molecule, or what happens to the energy if we exchange parts of one molecule with parts of another. A useful addition to understanding this sort of problem has been Pearson s concept of hard and soft acids and bases (HSAB). 

Molecular Orbitals and Organic Chemical Reactions Reference Edition Ian Fleming 2010 John Wiley Sons, Ltd 

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