Describing a Reaction Bond Dissociation Energies

HBr to yield bromoethane, which has A5° = -0.332 kJ/(K mol), is an example Table 5.2 describes the thermodynamic terms more fully. 

AGr Gibbs free-cnergy change The energy difference between reactants and products. When AG° is negative, the reaction is exergonic, has a favorable equilibrium constant, and can occur spontaneously. When AGC is positive, the reaction is endergonic, has an unfavorable equilibrium constant, and cannot occur spontaneously. 

Air Enthalpy change T he heat of reaction, or difference in strength between the bonds broken in a reaction and tire bonds formed. When All is negative, the reaction releases heat and is exothermic. When A IT is positive, the reaction absorbs heat and is endothermic. 

A S Entropy change The change in molecular randomness during a reaction. When AS° is negative, randomness decreases when AS° is positive, randomness increases. 

Equilibrium ----- In what direction does the reaction proceed  

Problem 5.10 Which reaction is more energeticalh- favored, one with AG = —44 kj/mol or one 

Problem 5.11 Which reaction is likely to be more exergonic, one with = 1000 or one with 

Problem 5.8 Which reaction is more favored, one with AG° = -44kJ/mol or one with AG° = +44 kJ/mol  

We ve just seen that heat is released (negative AH) when a bond is formed and absorbed (positive AH) when a bond is broken. The measure of the heat change that occurs on bond breaking is a quantity called the bond dissociation energy (D), defined as the amount of energy required to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25°C. 

Each specific bond has its own characteristic strength, and extensive tables of data are available. For example, a C—H bond in methane has a bond dissociation energy D = 438.4 kJ/mol (104.8 kcal/mol), meaning that 438.4 kJ/mol must be added to break a C-H bond of methane to give the 

Problem 6.10 Which reaction is more energetically favored, one with AG° = -44 kJ/mol or one with AG° = +44 kJ/mol Problem 6.11 Which reaction is likely to be more exergonic, one with /Ceq = 1000 or one with Keq = 0.001  

If enough bond dissociation energies were known, it would seem possible to calculate for any reaction of interest and thus be able to get a rough idea about whether the reaction is favorable. To take the radical substitution reaction of chlorine with methane (Section 5.3) as an example, the bonds formed in this gas-phase reaction (783 kj/mol) are stronger than the bonds broken (681 kJ/mol), so a net release of heat occurs and we calculate that the reaction is exothermic by about -102 kJ/mol (-24 kcal/mol). 

Unfortunately, there are several problems with this calculation that limit its value. First, the calculation say.s nothing about the entropy change S° for the reaction and thus nothing about the free-energy change AG . Furthermore, the calculation gives no information about the rate of reaction even if AG is favorable. And finally, bond dissociation energies refer to molecules in the gas phase and aren t directly relevant to chemistry in solutions. 

In practice, most organic reactions are carried out in solution, where solvent molecules can surround and interact with dissolved reactants, a phenomenon called solvation. Solvation can weaken bonds and cause large deviations from the gas-phase value of AH° for a reaction. In addition, the entropy term, AS , also can be different in solution because the solvation of a polar reactant by a polar solvent causes a certain amount of orientation in the solvent and thereby reduces the amount of disorder. Although we can often use bond-strength data to get a rough idea of how thermodynamically favorable a given reaction might be, we have to keep in mind that the answer is only approximate. 

---