2.10:
Bond Dissociation Energy and Activation Energy
The breaking of a covalent bond is associated with the exchange of energy, delta-H, between the system and its surroundings.
When covalent bonds break via homolytic cleavage, they produce two uncharged radicals, each of which bears an unpaired electron.
The energy required to break a bond along homolytic cleavage is called the bond dissociation energy, BDE or D. The BDE measured at one-atmosphere pressure is denoted by delta-H naught.
During a chemical reaction, reactants go through a high-energy transition state before the formation of products. This energy barrier between the reactants and products is called the activation energy, denoted by the symbols Ea or delta-G double dagger.
Activation energy is expressed as the difference between the free energies of the transition state and the reactants. Since the energy of reactants is lower than the transition state, the value of activation energy is always positive.
If a collision between reactants does not cross the activation energy barrier, they will not react with each other to form products. The number of successful collisions depends on the number of reactant molecules with a certain threshold value of activation energy.
The size of the activation energy controls the reaction rate. A large value leads to a slow reaction, whereas a small value leads to a faster reaction, as a large number of reactant molecules possess the threshold energy necessary to produce a reaction.
Sometimes, a catalyst is used to lower the activation energy and speed up the reaction rate.
For example, yeast is used as a catalyst to brew beer, made by the fermentation of sugars to produce ethanol. Although the process is thermodynamically favorable, it has a large activation energy.
Adding yeast to the mixture lowers the activation energy, and the process takes place at a faster rate, which is industrially economical.
2.10:
Bond Dissociation Energy and Activation Energy
Bond energy is the energy required to break a bond homolytically. These values are usually expressed in units of kcal/mol or kJ/mol and are referred to as bond dissociation energies when given for specific bonds or average bond energies when indicated for a given type of bond over many compounds. Firstly, the bond dissociation energy for a single bond is weaker than that of a double bond, which in turn is weaker than that of a triple bond. Secondly, hydrogen forms relatively strong bonds with carbon, nitrogen, and oxygen. Finally, with the exception of carbon and hydrogen, single bonds between atoms of the same element are relatively weak. Reactions between organic compounds involve the making and breaking of bonds. Hence, the strengths of bonds and their resistance to breaking are essential concepts in organic chemistry.
Reactions in which bonds are broken pass through a high-energy transition state before transforming into products. In order to reach this transition state, the reactant molecules must be oriented in a suitable direction and must be supplied with certain threshold energy. The activation energy, ΔG‡, is the energy provided to the reactants to raise them to the transition state. Overall, for a reaction to occur, the reacting molecules must collide or otherwise interact. The necessary activation energy for the reactant–transition-state jump is provided by the kinetic energy of the colliding particles. Lastly, the colliding molecules must collide in a specific orientation so as to maximize the impact of the collision.