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 t…
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.
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Q1: What is bond dissociation energy and how does it relate to homolytic cleavage?
Bond dissociation energy (BDE) is the energy required to break a covalent bond through homolytic cleavage, producing two uncharged radicals with unpaired electrons. Measured at one-atmosphere pressure and denoted as delta-H naught, BDE values are typically expressed in kcal/mol or kJ/mol. This energy represents the bond's resistance to breaking and is fundamental to understanding organic chemistry reactions.
Q2: How does activation energy affect reaction rates?
Activation energy is the energy barrier between reactants and products that must be overcome for a reaction to occur. A large activation energy results in a slow reaction because fewer reactant molecules possess the threshold energy needed. Conversely, a small activation energy leads to faster reactions as more molecules have sufficient energy to cross the barrier and form products.
Q3: Why must reactant molecules collide in a specific orientation?
Reactant molecules must collide in a specific orientation to maximize the impact of the collision and successfully reach the transition state. The colliding molecules require suitable directional alignment along with threshold kinetic energy to transform into products. Without proper orientation, even energetic collisions may fail to produce a reaction.
Q4: What is the relationship between bond strength and bond type?
Bond strength varies by bond type: single bonds are weaker than double bonds, which are weaker than triple bonds. Hydrogen forms relatively strong bonds with carbon, nitrogen, and oxygen. Single bonds between identical atoms (except carbon-hydrogen) are generally weak, making bond strength a critical factor in predicting reaction pathways and molecular stability.
Q5: How do catalysts lower activation energy and speed up reactions?
Catalysts lower the activation energy required for reactants to reach the transition state, enabling more molecules to possess sufficient energy for reaction. For example, yeast catalyzes sugar fermentation to produce ethanol. Although fermentation is thermodynamically favorable, yeast reduces its activation energy, allowing the process to occur at industrially economical rates without being consumed.
Q6: What happens during the transition state of a chemical reaction?
The transition state is a high-energy intermediate that reactants must pass through before forming products. Activation energy represents the free energy difference between the transition state and reactants, always yielding a positive value. Reactants must be supplied with sufficient kinetic energy from molecular collisions to reach this unstable state and proceed toward product formation.
Q7: Why is understanding bond strength essential in organic chemistry?
Bond strength determines a molecule's resistance to breaking and directly influences reaction mechanisms and rates. Since organic reactions involve making and breaking bonds, knowing bond strengths helps predict which bonds will break first and how easily reactions proceed. This knowledge is fundamental for understanding reaction pathways, stability of hydrocarbons, and designing synthetic routes.