Chemical reactions breaking and making covalent bonds

Breaking covalent bonds requires an initial input of energy in some form — normally as heat, but in some cases also light or other radiation. This is the activation energy of the reaction. The process of breaking a bond requires activation of the electrons forming the bond — a temporary shift of electrons from orbitals in which they have a stable configuration to other orbitals, further from the nucleus. Electrons that have been excited in this way have an unstable configuration, and the covalent bonds they had contributed to are broken. Electrons cannot remain in this excited state for more than a fraction of a second. Sometimes they simply return to their original unexcited state, emitting the same energy as was taken up to excite them, but usually as a series of small steps, rather than as a single step. Overall there is no change when this occurs.

More commonly, the excited electrons may adopt a different stable configuration, by interacting with electrons associated with different atoms and molecules. The result is the formation of new covalent bonds, and hence the formation of new compounds. In this case, there are three possibilities (as shown in Figure 2.1):

  • There may be an output of energy equal to the activation energy of the reaction, so that the energy level of the products is the same as that of the starting materials. Such a reaction is energetically neutral (thermoneutral).
  • There may be an output of energy greater than the activation energy of the reaction, so that the energy level of the products is lower than that of the starting materials. This is an exothermic reaction — it proceeds with the output of heat. An exothermic reaction will proceed spontaneously once the initial activation energy has been provided.
final

Figure 2.1 Energy changes in chemical reactions: thermoneutral, endothermic and exothermic reactions.

excited excited final final

Figure 2.1 Energy changes in chemical reactions: thermoneutral, endothermic and exothermic reactions.

• There may be an output of energy less than the activation energy, so that the energy level of the products is higher than that of the starting materials. The solution will take up heat from its surroundings and will have to be heated for the reaction to proceed. This is an endothermic reaction.

In general, reactions in which relatively large complex molecules are broken down to smaller molecules are exothermic, whereas reactions that involve the synthesis of larger molecules from smaller ones are endothermic.

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