Question

(a) Draw a reaction-energy diagram for the following reaction:

The activation energy is 4 kJ/mol (1 kcal/mol), and the overall for the reaction is -110 kJ/mol (-27 kcal/mol).

(b) Give the equation for the reverse reaction.

(c) What is the activation energy for the reverse reaction?

Short answer

(a)

Reaction-energy diagram

(b)

(c) E_a= 114 kJ/mol

Step-by-step solution

Activation energy (Ea)

The activation energy may be defined as the extra energy which the molecules of the 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. Transition state is most unstable. The transition state or activated complex gets converted into product molecules. During the formation of this complex, old bonds start breaking and the new bonds start making.

Intermediate

It may be defined as a species that exists for a finite amount of time, although it might be short but it has some stability.

Reaction energy diagrams

To understand the concepts of activation energy and transition state graphically, reaction energy diagram is used. 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.

Drawing the graph and calculation

(a)

Reaction-energy diagram

(b) The equation for the reverse reaction will just be reverse of the given equation in part (a).

(c)

Reaction-energy diagram for reverse reaction

Activation energy of reverse reaction (E_a) = 110kJ/mol + 4 kJ/mol = 114 kJ/mol