Q16.63P - Chemistry: Molecular Nature Of Matter And Change

Question

For the reaction ${\mathbf{A}}_{\mathbf{2}}\mathbf{+}{\mathbf{B}}_{\mathbf{2}}\mathbf{\to }\mathbf{2}\mathbf{A}\mathbf{B}$, $E_{a\left(fwd\right)}=125kJ/mol$ and $E_{a\left(rev\right)}=85kJ/mol$ . Assuming the reaction occurs in one step, 

(a) Draw a reaction energy diagram; 

(b) Calculate $\mathbf{∆}{{\mathbf{H}}^{\mathbf{0}}}_{\mathbf{r}\mathbf{\times }\mathbf{n}}$ ; and 

(c) Sketch a possible transition state.

Step-by-Step Solution

Answer

(a) A reaction energy diagram is

Reaction Energy Diagram

(b) the $∆{H^0}_{r\times n}$ is equal to 40 kJ/mol.

(c) A transition represents the weakening of old bonds (A-A) and (B-B) along with the formation of new bonds (A-B). Draw the transition state for the reaction:

Step 1: Draw a reaction energy diagram

Since the reaction is endothermic, the products’ energy will be higher than that of the reactants. Draw the reaction's energy diagram:

Step 2: Calculate ∆ H 0 r × n

Determine the $∆{H^0}_{r\times n}$  as follows:

$$\begin{gathered} ∆{H^0}_{r\times n}=E_{a\left(fwd\right)}-E_{a\left(rev\right)} \\ =125kJ/mol-85kJ/mol \\ =40kJ/mol \end{gathered}$$

Thus, it $∆{H^0}_{r\times n}$ is equal to 40 kJ/mol.

Step 3: Sketch a possible transition state

A transition represents the weakening of old bonds (A-A) and (B-B) along with the formation of a new bond (A-B). Draw the transition state for the reaction: