Chemical reactions involve the breaking of bonds in reactants to give transient or intermediate chemical structures, and the formation of bonds within or between these structures to give the products (see diagram below).
For a reaction system that is not isolated from its surroundings, energy is absorbed by the reaction system from the surroundings during the breaking of bonds and is stored as potential energy, which is then released from the reaction system to the surroundings during the formation of bonds. Hence, energy is neither created or destroyed but is changed from one form to another, and flows from the surroundings to the system or vice versa.
Question
Why is energy released when bonds are formed to give products?
Answer
The fact that products are formed from the transient or intermediate chemical structures implies that products are more stable (lower energy) than the transient or intermediate structures (higher energy). Hence, the net energy change (final energy minus initial energy) is negative, i.e. energy is released during the bond formation process. The initial energy in this case is with reference to the transient or immediate structures and not to the reactants.
Potential energy profile diagrams, with the vertical axis representing potential energy and the horizontal axis representing the progress of a reaction, are used to illustrate energy changes during a reaction. The amounts of energy involved in the processes of bond breaking and bond formation do not exactly match. Consider the case of a reaction depicted in the diagram below.
The energy change in the bond breaking process, q1 = Ei – Ert (i.e. energy with respect to the intermediate structures minus the energy with respect to the reactants), is positive. The energy change in the bond formation process, q2 = Ep – Ei (i.e. energy with respect to the products minus the energy with respect to the intermediate structures), is negative. The total change in energy of the reaction is the energy with respect to the products minus the energy with respect to the reactants:
Substituting q2 = Ep – Ei and q1 = Ei – Ert in eq1
For the reaction depicted in the above diagram, qnet > 0. In general, a reaction that requires more energy in the process of bond breaking (energy absorbed from surroundings) than in the process of bond formation (energy released to the surroundings), resulting in the system having a net positive energy (net energy absorbed from surroundings), is called an endothermic reaction.
Conversely, a reaction that requires more energy in the process of bond formation (energy released to the surroundings) than in the process of bond breaking (energy absorbed from surroundings), resulting in a system having a net negative energy (net energy released to the surroundings, qnet < 0), is called an exothermic reaction (see diagram below).
