Chapter 4: Q20P (page 215)

Use the bond-dissociation enthalpies in Table 4-2 (page 203) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. Use this information to draw a reaction-energy diagram like Figure 4-8, comparing the activation energies of the two radicals.

Short Answer

Expert verified

-9KJ/mol (Propagation step I)

-29KJ/mol (Propagation step II)

Step by step solution

Free radicals

An atom or group of atoms containing odd or unpaired electrons is known as free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

Steps involved in free radical chain reaction

In a free-radical chain reaction, free radicals are generally created in the initiation step. A free radical and a reactant are combined in the propagation step. In the termination step, the product will be formed.

Bond dissociation enthalpy (BDE)

BDE is the amount of enthalpy required to break a bond homolytically in such a way that each bonded atom retains one of the bond’s two electrons.

Mathematically, $△ H^{0} = \sum (B D E o f b o n d s b r o k e n) - \sum (B D E o f b o n d s f o r m e d)$

Activation energy ( Ea )

Activation energy is the extra energy that the molecules of reactants have to absorb so that their energy becomes equal to the threshold energy.

Transition state or activated complex

The highest energy state in a molecular collision that leads to reaction is the transition state. During the formation of this complex, old bonds start breaking, and new bonds are formed.

Intermediate

Intermediate is a species that exists for a finite amount of time and has some stability, although its lifespan is short.

Reaction energy diagrams

A reaction energy diagram is used to understand the concepts of activation energy and transition state graphically. The vertical axis of the diagram represents the total potential energy of all the species present in the reaction. The horizontal axis represents the reaction coordinate that gives the progress of the reaction, proceeding from reactants on the left to products on the right. The highest point on the graph is the transition state, and the activation energy is the difference in energy between the reactants and the transition state.